Short Notes on Microbodies, DNA and RNA

MICROBODIES (GLYOXYSOMES AND PEROXYSOMES)

In plant cell they are of following two types:

(1) Peroxysome: They are single membrane bounded microbodies that contain enzymes for transferring hydrogen atom to oxygen forming hydrogen peroxide (H₂O₂), a toxic molecule that is immediately broken down t water by the enzyme catalase. Peroxysome are abundant in cells that are metabolizing alcohol. Peroxysome are believed to help in detoxification of alcohol.

(2) Glyoxysome: They are another type of microbodes. Each glyoxysome has single layered bounding membrane enclosing fine granular stroma. Glyoxysome contain enzymes that can metabolize some of the molecules involved in Photosynthesis process and respiration through oxidation of fatty acid.

DNA (DEOXYRIBO NUCLEIC ACID):

They are important constituents of cell. They occur in nuclear reticulum (chromosome). DNA has deoxyribose with one less oxygen atom in ints molecule. DNA is double strended molecule Biologists view that all secrets of life are embodied in DNA. It is the chemical basis of life. DNA is the controlling centre of all vital activities of the cell. DNA is sole genetic (hereditary) material migrating intact from generation to generation through the reproductive units or gametes and is responsible for the development of specific characters of a plant. It also controls biosynthetic process of cell including Protein synthesis. In 1953 Watson and Crick gave the model of DNA.

Each pair is made of two distinct nitrogenous bases Purines and Pyramidines. Altogether there are two Purnes adnine and guanine and two Pyrimidines thyamine and cytosine. It is a rule that a specific purine always pairs with a specific pyrimidine.

RNA (RIBONUCLEIC ACID):

RNA occurs in Nucleoli, chromosomes and cytoplasm (about 90% of the cells RNA occurs in the cytoplasm). RNA chemically consists of 5 carbon ribose sugar. RNA is single stranded molecule/ RNA is chemical messenger and plays a key role in the process of Protein synthesis,

RNA consists of sugars, bases and Phosphoric acid. Sugar is ribose against Deoxyribose in DNA. In RNA the bases are adenine, guanine, uracil and cytosine i.e. they mine of DNA is replaced by uracil. Various types of RNA are found in plant cells. These are (a) messenger RNA (mRNA) which carries the information contained in DNA (b) transfer RNA (t RNA) also known as soluble RNA which work as adaptor molecules for carrying amino acids to the site of Protein synthesis (c) ribosomal RNA (r RNA) which is associated with ribosome. All these three types are monogenetic RNA.

Short Notes on Anaeoploidy, Euploidy and Vacuole

ANAEOPLOIDY

It is change in number of chromosome which can be either due to loss of one or more chromosomes or due to addition or deletion of one or more chromosomes. It leads to variations in chromosome number and do not involve the whole of the Karyothype. The nuclei of the aneuploids contain chromosomes whose number is not true mullyple of the basic number (n). The aneuploidy arises due to non disjunction. The loss of one chromosome produces a Monosomic (2n—1) and the condition is termed as Monosomy. The gain of one chromosome produces Trisomic (2n—1) and the condition is known as Trisomy. In the same way the addition of two or more chromosomes is respectively known as Tetrasomy and Pentasomy, the individuals are known as Tetrasomic and Pentasomic. In some cases a pair of homologous chromosomes is lost (2n—2), such indivividuals are termed as Nullisomic and the condition is called Nullisomy.

EUPLOIDY:

It is a condition where one or more full sets of chromosomes are present in an organism. The euploids may e Monoploids, Diploids or Polyploids. Monoploids can be distinguished from haploids as they have a single basic set of chromosomes are in Barley 2n = x = 7 or in corn 2n = x = 10 while the Haploids have the half the somatic number of chromosomes found in normal individual i.e. each chromosome is represented once. In some cases as in male insects the haploids are produced due to parthenogenesis. In these insects the queen and drones are diploid females. The haploids also originate due to development of egg parthenogenetically in following plants such as Tomatoes and Cotton. The haploids also originate from pollen tube rather than from egg, synergids or antipodals of the embryo sac. Such haploids are known as Androgenic Haploids.

Significance: Haploids are characterised by a reduction in size of all vegetative and floral parts than a diploid. Haploids are used in production of homozygous diploids as haploids can be doubled by colchicines treatment. These homologous diploids are used for cultivation e.g. rice, wheat and tobacco.

VACUOLE:

Vacuoles are non protoplasmic liquid filled cavities in the cytoplasm and are surrounded by a membrane called the tonoplast. Tonoplast is selectively permeable; it allows certain substancesto enter in the vacuole. They are clear in plant cells. They are prominent in nature cells. They are filled with cell sap and act as store house, which often plays role in plant defence,

Which is necessary for plant cell enlargement. Plant vacuoles sometimes act as lysosome as they contain hydrolytic enzymes and after death of cells tonoplast lose its differential permeability and its enzyme causes lysis of the cell. Vacuole is filled with a fluid called cell sap which is water containing large number of soluble chemical substances such as inorganic salts, organic acids, soluble carbohydrates e.g. sugar, soluble proteins, and amino acids and in certain cells mucilage, anthocyamins, tannins, latex and alkaloids in varying proportions. The vacuole is this a tiny reservoir of the cell from which the cytoplasm draws water and other material according to its need.

Explain with examples that Mendel’s dominant factor fails to govern phenomenon of inheritance in many cases.

By incomplete dominance Mendel fails to govern the phenomenon of inheritance. In case of flower colour, Mendel found that red colour of flowers was dominant over white colour of flowers. The white coloured flowers reappeared in F₂.

Later a deviation to this rule was observed in plant species commonly called four o clock (Mirabilis jalapa). The plant produces flowers with red and white colours. When pure breeding red (R) flowered plants were crossed with pure breeding white (r) flowered plants, the F₁ plants were pink (Rr) flowered. Appearance of pink flowers, an intermediate shade between red and white, provided relief to those who believe in blending inheritance when F₂ hybrids were crossed (Rr × Rr), the F₂ generation showed a phenotypic ratio of 1 red : 2 pink : 1 white, instead of typical 3: 1 ratio. This ratio contradicted Mendel’s principles. Appearance of red and white factors (genes) in F₂indicates that these have not been altered or blended while present together in the pink flowered individuals. F₂ reds and whites are same as the parental red and white.

This was declared as case of incomplete dominance. In this case neither gene is dominant to the other. Each expresses itself in the presence of its allele to produce an intermediate effect.

Another important aspect of incomplete dominance in that phenotypic ratio (1: 2: 1) is the same as genotypic ratio (1 homozygous dominant: 2 heterozygous dominant: 1 homozygous recessive) resulting from cross when two monohybrids are crossed. When dominance is incomplete, a cross of two monohybrids (Rr × Rr) gives a phenotypic ratio of 1: 2: 1 which is identical to the genotypic ratio. The heterozygous individual shows incomplete dominance.

Incomplete dominance in four o clock.

CODOMINANCE

In case of blood types in human, both genes (A & B) produce an effect in a heterozygous individual. This is called codominance. The genes which govern A and B blood types are alleles. Each control the formation of a different red blood cell Protein or antigen. Antigen a in case of person having blood group A and antigen b in individuals with blood group B. Neither gene is dominant to the other. The heterozygous individuals with blood group AB contains both antigen a and b. Both proteins are detected in equal amounts in the red cells.

The case of incomplete dominance and codominance suggest that the dominance is not universal and absolute.

The gametes are always pure for a particular character (allele) and its examples

Mendel an Austrian monk performed a series of experiments on different varieties of Pea plant (Pisum sativum) in his garden at Brunn and published results in 1865 and 1869 in the journal of local Natural History Society. The science of heredity is called Mendelism. Mendel noticed in Pea seven pairs of contrasting characters, three of which are as follows

(1) Stem tall (6 to 7 ft :) or dwarf (3\4 to 1 1/2 ft :)

(2) Ripe seeds smooth or wrinkled

(3) Colour of cotyledons yellow or green.

He considered each pair of contrasting characters separately and as result of his experiments formulated two laws, the law of segregation and the law of independent assortment.

Law of segregation or Purity of gametes

This means that the genes or factors representing a pair of contrasting characters when brought in the cells offspring and later become separated in gametes so that each male or female gamete carries only one gene for the member of the pair but never both.

Mendel crossed a true breeding tall pea plant with a true breeding dwarf pea plant. He brought about the exchange of pollen between these pure varieties of pea plants and raised the seeds which were collected and grown. He found that in first hybrid generation (F₁ = first filial generation) all the plants were tall like one of the parents. Mendel called the characters which appeared in the hybrid dominant and that which was suppressed recessive. The tall hybrid plants of F₁ generation were then self fertilized and the resulting seeds were grown. It was discovered that in the second hybrid generation (F₂) so obtained the tall and dwarf plants were in the ratio of three to one. 

That is out of every 100 plants 75 were tall and 25 dwarf. He further found that the dwarf plants of F₂ generation produced only dwarf plants when self fertilized. In other words they bred true and were pure for the dwarf character (pure recessive). Similarly one third of tall plants of F₂generation produced only tall plants when self fertilized. That is they bred true and were pure for the tall character (pure dominants) when the tall hybrid of F₁ generation forms its gametes the two genes (T) and (t) segregate or separate and half of the male and female gametes (pollen grains and egg cells according to Mendel) receive the gene T and the other half the gene t. Thus each gamete carries T or t but not both. In other words a gametes pure with respect to one or the other number of the pair of contrasting characters.

The zygotes from which pure tall and hybrid tall arise are respectively known as homozygotes and heterozygotes.

The four o clock plant (Mirabilis jalapa) furnishes an excellent example.

This species consists of two varieties, one with white flowers and the other with red flowers. When a true breeding red flowered plant is crossed with a true breeding white flowered plant the F₁ hybrids have all pink flowers. When these hybrids are self fertilized they produce three types of plants, half bearing pink flowers (hybrids) one fourth bearing white flowers (pure recessive) and one fourth bearing red flowers (pure dominants).

Explain Law of independent assortment by Experiment of hybridization on Pea Plant

Mendel designed experiments in which two characters were examined simultaneously in order to know how two pairs of factors behave in relationship to one another when followed in the same cross. A cross involving two pairs of contrasting traits is called dihybrid cross.

Shape of Pea seeds can be either round or wrinkled. The round factor (R) is dominant to wrinkled (r). Similarly the colour of seeds may be yellow or green. The yellow factor (Y) is dominant to wrinkled (y). When plants homozygous for round seeds are crossed with those having wrinkled seeds, the F₂ generation showed the ratio of 3:1 between rounds and wrinkled. Similar F₂ratio was obtained when seeds with yellow factor were crossed with seeds with green factor.

Mendel crossed a plant with round yellow seeds (RRYY) to a plant with wrinkled green seeds (rryy). All F₁ dihybrids produced round yellow seeds (RrYy). This is a case of dominance and the results were expected. But whether genes for round and wrinkled and yellow and green would stay together when F₁dihybrids are allowed to self pollinate. The most suitable way to check it was to make a test cross between F₁ dihybrid and double recessive parents. A ration of 1: 1: 1: 1 was obtained when the cross was made.

Formation of four classes of gametes suggests that genes for seed shape and seed colour are behaving independently. In addition to old combination the F₁ dihybrids also produce new combinations Ry and rY. 


Thus the factors are free to form new combinations therefore it can be said that the factors assort independently knowing that dihybrid individuals form four different kinds of gametes in equal proportions, 16 possible combination are expected, when Mendel crossed F₁hybrids, he obtained, 315 round yellow (RY), 108 round green (Ry), 101 round wrinkled (rY) and 32 wrinkled green (ry). Statistical analysis shoed a ratio of 9: 3: 3: 1. This is called dihybrid ration when crosses were made for remaining five pairs of genes, they followed to same rule and produced same results i.e. the dihybrid and test cross ratio were established. These results led Mendel to formulate his second law of inheritance, the law of independent assortment. It states that: “The Members of one pair of factory (genes) segregate independently of members of other pairs of gamete formation as pair Rr segregated independent of Yy.

How Mendel succeeded in formulating in his laws of inheritance, while the previous workers failed

Mendel established principles that describe transmission of genes from parents to offspring. When Mendel began his studies, he knew nothing about chromosomes and role of meiosis in inheritance later investigation established parallel behaviour between chromosomes and Mendel’s unit of inheritance during meiosis. Mendel in 1856 performed his first hybridization experiment. Mendel was born in 1822 and died in 1884.

Mendel selected garden pea plant (Pisum sativum) because it showed several sharply contrasting characters that were without intermediate forms and were relatively unaffected by environmental factors. Also the flowers of garden pea are self pollinating in nature and so the crosses can be arranged according to the choice of breeder.

Mendel’s characters are:

(1) Length of Stem: tall or short

(2) Position of flowers: axial or terminal

(3) Colour of unripe pods: green or yellow

(4) Form of ripe pods inflated or constricted

(5) For of ripe seeds: round or wrinkled

(6) Colour of cotyledons: Yellow or green

(7) Colour of seed coat: Grey, brown or white.

The significance of Mendel’s work lies in his ability to formulate a scientific problem. The aim of his experiments was to study the numbers and kinds of offspring produced by hybrid individuals and to determine from the observation whether any statical relationships existed among these offsprings.

The pea plants that Mendel used in his experiments were grown from seeds obtained from plants that had ‘bred true’ when self fertilized for at least two generations before the experiment. Such plants are called true breeding. When self pollinated and self fertilized the offspring always resemble the parents for the given trait. Mendel performed corsses in which only one pair of contrasting alleles or alternative traits is being followed. Each such cross is known as monohybrid cross.

Using results of monohybrid crosses for various contrasting traits and to explain low unit factors could account for the results of monohybrid corsses.

The results Mendel’s eight years breeding experiments were read before the natural history. Society of Brunn in 1865 and in the following year these were published in the transactions of the society. But his work remained un noticed until 1990 when three distinguished botanists Hugo De Vries in Holland, Tschermak in Austria and Corrents in Germany discovered its significance. Since then Mendel’s work has formed the bosis of study of genetics. Mendel dies in 1884 before he could see his work accepted and appreciated. From the results of his experiments on carefully selected crossings, Mendel formulated certain laws to explain the interitance of characters as follows:

(1)        LAW OF UNIT CHARACTERS:

This means that all characters of the plant are units by themselves being independent of one another so far as their unheritance is concerned. There are certain factors or determiners (now called genes) of unit characters, which control the expressions of these characters during the development of the plants.

(2)        LAW OF DONUNANCE:

The characters are controlled by factors or genes. These occur in pairs (in chromosomes) and are responsible for tell ness and dwarfness separately. Tallness is dominant character while dwarfness is recessive character. The contrasting pairs of characters are allelomorphs. Thus tallness and dwarfness are allelomorphs.

Mendel gave two laws (1) Law of Segregation of gametes (2) Law of independent assortment.

(1)        FIRST LAW OF SEGREGATION

The factors for contrasting characters remain associated in pairs in the somatic cells of each plant throughout its whole life. Later when spores are formed as a result of reduction division, the factors located in homologous chromosomes become separated out, and each of the four gamete will have only one factor (tallness or dwarfness) of the pair but not both i.e. gamete becomes pure for a particular character. This law is also called law of purity of gametes.

Mendel made experiment on Pea plants and found the ratio 3:1 in this law of segregation.

(2)        SECOND LAW OF INDEPENDENT ASSORTMENT:

For dihybrid cross Mendel found that two pairs of contrasting characters are taken into idea. Mendel selected tall plant with red flower and a dwarf one with white flowers. Factors for tallness or dwarfness and red flowers or white are independently inherited and may be considered to be located in separate chromosome pairs when seeds were crossed the result became 9:3:3:1.

Mendel succeeded in formulating in his Laws of inheritance by doing experiments on plants in his garden and got good results & formed two laws. Later Mendel was considered as the father of Genetics. Later other scientists got base of Mendel’s work and enhanced in further work of Genetics.

Define Monohybrid and Dihybrid cross. Explain inheritance of two pairs of contrasting characters showing independent assortment of tall-dwarf stems v/s yellow Green seeds in Pea plant

MONOHYBRID CROSS

If only one pair of contrasting character is being studied the hybrids are called Monohybrids.

DIHYBRID CROSS

When two different pairs of contrasting characters are studied simultaneously, the hybrids are called 

Dihybrids.

Mendel performed numerous experiments with garden pea keeping in view the simultaneous inheritance of two different pairs of contrasting characters and always found that one of the two contrasts out of each pair was dominant in F₁generation; and it was always the same contrast which was dominant in single trait inheritance. He always obtained a ratio of 9: 3: 3: 1 in F₂generation of which 9 were of dominant parental types 3: 3 were of two different recombination types which did not exist previously and one was showing both the recessive contrasts. Mendel realised of his breeding experiments, two different pairs of alleles is completely at random (i.e. depends on chance). This principle of inheritance was called the law of independent assortment”. In its simplest forms it can be stated that the alleles for two different pairs of contrasting characters are transmitted independent of one another during their inheritance to the next generation.

Tall dwarf and yellow green cotyledons

The gene for the length of Pea plant has two contrasts i.e. modified forms (called alleles) one of which produces tallness (T) and the other produces dwarfness (t), the two characters called as a pair of contrasting characters. Similarly, the gene for cotyledons colour has two modified forms (alleles) one of which produces yellow cotyledons (Y) and the other produces green cotyledons (y); the two characters called as a pair of contrasting characters. If a pure tall plant (TT) with yellow cotyledons (YY) is crossed with a pure dwarf plant (tt) with green cotyledons (yy), all the F₁ plants are tall and with yellow cotyledons. It indicates that tallness is dominant over dwarfness and yellow cotyledons are dominant over green. If F₁ tall and yellow plants are self corssed 9: 3: 3: 1 ratio is obtained in F₂ i.e. 9/16 are tall with yellow cotyledons, 3/16 are tall with green cotyledons, 3/16 dwarf with yellow cotyledons (3: 3 called recombinants, as these combination of characters did not exist before), 1/16 are dwarf with green cotyledons i.e. homozygous recessive for both traits.

During gamete formation T gene is free to go with y or Y. Similarly gene is independent of to go with either T or t. Thus the occurrence of a 9: 3: 3: 1 ration in F2 is truly evidence that the law of independent assortment is correct.

Describe Law of independence of Mendel or second law of Mendel

After the discovery of the mode of inheritance for a single pair of contrasting characters, Mendel proceeded to investigate the simultaneous inheritance of two different pairs of contrasting characters and the hybrids are called dihybrids. Mendel made experiments with garden Pea keeping view the simultaneous inheritance of two different pairs of contrasting characters out of each pair was dominant in F₁ generation and it was always the same contrast which was dominant in single trait inheritance. However he always obtained a 9:3:3:1 ration in F2 generation of which 9 were of dominant parental type, 3:3 were of two different recombinant types which did not exist previously and one was showing both successive contrasts.

From the results of his breeding experiments Mendel realized correctly that the assortment of two different pairs of alleles is completely at random (i.e. depends on chance) This prinaple of inheritance was called the “law of independent Assortment”. In its simplest form it can be stated that the alleles for two different pairs of contrasting characters are transmitted independent of one another during their inheritance to the next generation.

Mendel crossed a variety of pea having yellow round seeds (both dominant characters) with one having green wrinkled seeds (both recessive characters). The seeds obtained from these two pure forms were all yellow and round i.e. they showed (both dominant characters). When the hybrid plants (F₁) produced from these seeds were self fertilized they gave four types of seeds (1) rounded and yellow (2) wrinkled and yellow (3) rounded and green (4) wrinkled and green in the ratio of 9:3:3:1, i.e. in F₂ generation there were nine yellow round, green wrinkled. All the yellows taken together were to the greens as 3:1; all the rounds taken together were to the wrinkled as 3:1 but some of the yellows were now wrinkled and some of the greens were now round.

The results obtained from the above experiments may be explained as follows:

The dominant yellow colour of the seed may be represented by the gene Y and the recessive green colour by the gene Y. The dominant roundness of the seed may be represented by the gene R and the recessive wrinkleness by the gene r. The gametes of the yellow round parent will carry the gene YR and those of the green wrinkled Yr. The zygote formed by the union of two such gametes, will have YR Yr, and will give rise yellow round seed, showing both dominant characters. Now when the hybrid arising from this zygote forms its gametes the members of each pair of contrasting characters behave independently of those of the other pair that is a gamete bearing a gene for the roundness of the seed may also bear a gene for yellow or green cotyledons. Similarly a game bearing wrinkleness of seed may also bear a gene for green or yellow cotyledons. Thus there will be four kinds of male gametes and four kinds of female gametes and the four kinds in each case will be YR, Yr, yR, yr. On fertilization any one of the four female gametes will unite with any one of the four male gametes and hence there will be sixteen possible combinations.

Describe the brief life history of Mendel. State his cross with suitable example

Gregor Mendel (1822-84) was the first scientist who gave scientific study on genetics he entered a monastery in Brunn (Austria) where he carried on his scientific investigations on hybridization of plants. The results of his eight years breeding experiments were red before the Natural History Society of Brunn in 1865 and in the following years these were published in the transactions of that society but his work remained unnoticed until 1900 when three distinguished botanists Hygo De Vries in Holland, Tschermak in Austrua and Correns in Germany discovered its significance. Since then, Mendel’s work has formed the basis of the study of genetics. Mendel dies in 1884 before he could see his work accepted and appreciated.

Mendel’s experiment: Mendel selected for his work the common garden pea. In the pea he found a number of contrasting characters, flowers purple, red or while plants tall or dwarf and seed, yellow or green smooth or wrinkled. He concentrated his attention on only one pair of characters at a time and traced them carefully through many successive generations. In one series of experiments he selected tallness and dwarfness of plants.

The results he achieved in these experiments were the same in all cases. It did not matter whether he took the dwarf plant as the male and the tall plant as the female or vice versa.

MONOHYBRID CROSS

For monohybrid cross only one pair of contrasting characters is taken into consideration at a time. Mendel selected a pea plant, 2 meters in height and another 0.5 meter in height. He brought about artificial crossing between the two. The progeny that resulted from these crossing were all tall. This generation known as the first filial generation or F₁ generation was inbred. Seeds were collected and soon next year. They gave rise to a mixed generation of talls and dwarfs (but no intermediate) in the ratio of 3:1 i.e.  Three fourths talls and one fourth dwarfs. This generation is known as the second filial generation of F₂ generation. All dwarfs of F₂generation bred true producing dwarfs only in the third and subsequent generations. Seeds were collected separately from each F₂ tall plant and sown separately. It was seen that one third of the talls bred true to type, while the other two thirds again split up in the same ratio of 3:1. The F₂ratio is therefore 1:2:1 one fourth pure talls, half mixed talls and one fourth pure white.

Short note on Functions of Nucleus, Endoplasmic Reticulum, Plastids and Golgi Complex

FUNCTIONS OF NUCLEUS:

(1) Nucleus and Protoplasm of cell are responsible for various function of the cell. If they are separated, both of them die. Nucleolus is controlling centre of vital activities of the cell.

(2) Nucleus takes direct part in reproduction. Two reproductive nuclei called gametes (egg cell and male gamete) fuse together to give rise to oospore which grows into an embryo.

(3) Nucleus takes the initiative in cell division i.e. if is nucleus that divides first and is followed by the division of the cell. This is how the cell multiply in number and the plant body grows.

(4) Nucleus is regarded as the bearer of the hereditary characters i.e. it is through the media of two reproductive nuclei that the characteristics of parent plants are transmitted to the offspring. It is to benoted that it is the DNA of the nuclear reticulum that is the sole hereditary material of two reproductive nuclei.

ENDOPLASMIC RETICULUM:

It is a network of tube like structure distributed throughout the cytoplasm of the cell. Some of these tubes are connected with the nuclear membrane and some with cell membrane. They appear to be associated with enzyme formation, Protein synthesis, storage and transport of metabolic products. They may also contribute to the formation of the cell plate in nuclear division and of nuclear membrane around the newly formed nuclei.

ER is complex finely divided vascular system extending from nucleus throughout the cytoplasm to the margin of the cell. It may even extend to the neighbouring cells. Cavities of ER are surrounded by membraneas and are quite variable in shape and size. It exists in two forms rough or granular form with closely spaced granules ribosomes or cytoplasm side and smooth or a granular form.

PLASTIDS

Cytoplasm of platn cell consists of discoidal, oval or spherical bodies called Plastids. They are present in all plant except bacteria, fungi and blue green algae. Plastids are living. They are formed a fresh but arise from minute pre existing bodies called Protoplastids already present in embryonic cells. They multiply in number by division. On the basis of colour plastids are of three types i.e. Leucoplasts, chloroplasts and chromoplasts. One form of plastids can change in to another as Leucoplasts. Change into chloroplasts when the former are exposed to light for a prolonged period. Similarly chloroplasts change into leucoplasts in the continued absence of light. Similar changes take place in chromoplasts. In young tomato fruit the leucoplasts gradually change into chloroplasts which finally turn into chromoplasts as the fruit ripens. Leucoplasts are white and occur in storage cells of roots and underground stems. They convert sugar into starch. Chloroplasts are green plastids bearing green pigment chlorophyll. Chromoplasts are yellow, orange and red and are present in petals of flowers are fruits.

GOLGI COMPLEX

It was discovered by and named after Golgi (1898) from animal cells. Later Poger and Buvat (1957) discovered it from plant cells. It is found in all kinds of cells. It has a series of concentrically bent double membrane, which in reality are flattened membrane bound sacs which are refered to as cisterane, saccules or lamellar units. The individual vesicles are sometimes known as golgisomes or dictyosomes. The cisternae form a system of branched tubules. It is suggested that the golgi apparatus like nucleus envelope is also derived from Endoplasmic reticulum. Function of Golgi bodies is concerned with the secretion of Proteins and complex Polysaccharides. It is also supposed to be forming a part of cell pate and the secondary walls.

Describe Aneuploidy and Euploidy in relation to changes in the chromosome number

Change in chromosome number occur occasionally which produces a variety of effects. Aneuploidy in change in number of chromosomes which can be either due to loss of or more chromosomes or due to addition or deletion of one or more chromosomes. It leads to variation in chromosome number and do not involve the whole karyotype. The nuclei of Aneuploids contain chromosomes whose number is not true multiple of the basic number (n).

The Aneuploidy arises due to non disjunction. The loss of one chromosome produces a Monosomic (2n-1) and the condition is termed as Monosomy. The gain of one chromosome produces a Trisomic (2n+1) and the condition is known as Trisomy. In the same way the addition of two or more chromosomes is respectively known as Tetrasomy and Pentaasomy, the individuals are known as Tetrasomic and Pentasomic. In some cases a pair of homologous chromosomes is lost (2n-2); such individuals are termed as nullisomic and the condition is called Nullisomy.

Effects of Aneuploidy are

A.        MONOSOMY

The monosmoic lack a complete chromosome which creates genetic imbalance as the expression of only one allele at each locus of the chromosomes is inadequate. The expression of a genetic information during early development is very delicately regulated so that a sensitive balance of gene product is required to ensure normal development. Due to loss of one chromosome the equilibrium is not achieved therefore monosomy is not tolerated in diploids.

In many plants monosomy is observed such as maize, tobacco, the evening prim rose and oenothera. Such monosomic plants are usually less viable than their diploid derivatives. The monosomic can easily be produced in polyploids. A polyploid has several chromosomes of same type, therefore the loss becomes viable.

The number of possible monosomics in an organism will be equal to haploid chromosome number. In common wheat since 21 pairs of chromosomes are present, 21 possible monosomics are known. Monosomics were also isolated in cotton (2n=52) and in Tobacco (2n=48). In tomato which is a diploid (2n=24), rarely monosomics could be produced similarly monosomics have been produced in diploid maize.

Double monosomics (2n-1-1) i.e. loss of two chromosomes, but differ from nullisomics in respect that the chromosomes lost are non homologous. Also triple monosomics (2n-=-1-1-1) could be produced in polyploids like wheat.

B.         TRISOMY

The addition of extra chromosome produces somewhat more viable individuals in both animals and plants than does the loss of a chromosome.

As in monosomy the sex chromosome variation of trisomic type has less drastic effect on the phenotype that autosomal variation. Drosophilla females with three x chromosomes and a normal complement of autosomes (3x: 2A) may be fertile but less viable than normal (2x: 2A) females.

In some plants trisomic individuals are viable but their phenotype may become changed e.g. the diploid number of Jimson weed (Datura) in 24. Twelve different primary trisomic chromosomes are recognised in Jimson weed. Each kind of trisomy alters the phenotype. Secondarytrisomics, the chromosome is an isochromosome. Tertiary trisomics are also possible but rarely. The extra chromosome in tertiary trisomics is produced in the result of translocation.

Trisomy usually originates spontaneously due to production of n+1 type of gametes due to non disjunction of a bivalent. The trisomic are more often produced artifsicially by selfing triploids or by crossing triploid females with diploid males (3X × 2X). In plants and animals the trusomy may be delted during cytological observation of meiosis. As there copies of a chromosome are present, pairing of configurations are different. In some cases only two of these there homologues may synapse. At regions different members of trio may be paired and are known as Trivalents. In some cases one bivalent and one univalent may be present. The trivalent is usually arranged on the spindle so that during avaphase one member moves to one pole and two go to opposite pole. Trisomis are used to locate genes on specific chromosomes. If a particular gene is located on the chromosome involved in trigomy, segregation in the progemy of this trisomic will not follow a Mendelian pattern but the ratio will deviate from normal 3:1 and 1:1 test cross ratios.

C.        Tetrasomy:

They have particular chromosome, there are at least tetrasomics available in wheat. E. R sears was able to synthesize a complete set of compensating Nullisomic Tetra somic (2n + 2 + 2) where addition of a pair or homologous chromosomes would compensate for the loss of another pair of homologous. Sun non homologous chromosomes are able to compensate for each other are called Homoeologous chromosomes.

2) Euplody is a condition where one or more full sets of chromosomes are present in an organism. Euploids may be Monoploids, Diploids of Polyploids.

Monoploids can be distinguished from haploids as they have a single basic set of chromosomes as in Barly 2n = x = 7 or in corn 2n = x = 10 while the Haploids have half the somatic number of chromosomes found in normal individual i.e. each chromosome is represented once. In some cases as in the male insects the haploid are produced due to parthenogenesis. In these insects the queen and drones are diploid females. The haploids also originate due t development of egg parthenogenetically in flowering plants such as Tomatoes and Cotton. The haploids also originate from pollen tube rather than form egg, synergids or antipodals of the embryo sac. Such haploids are known as Androgenic Haploids.

Haploids can be produced artificially by any one of the following methods.

(1)        X-ray Treatment                                   (2)        Delayed Pollination

(3)        Temperature shock                               (4)        Colchicine treatment

(5)        Distant hybridization                              (6)        Anther or pollen culture

SIGNIFICANCE OF EUPLOIDY:

Haploids are characterized by reduction in size of all vegetative and floral parts than a diploid. The haploids are used in production of homozygous diploid as haploids can be doubled by colchicines treatment. These homozygous diploids are used for cultivation e.g. rice, wheat and Tobacco.

What are lipids

Bloor in 1943 proposed the term lipid for those naturally occurring compounds which are insoluble in water but soluble in organic solvent. These are also the compounds containing carbon, hydrogen and oxygen like carbohydrate but contain much lesser ration of oxygen than carbohydrates.

Lipids are important diverse group of biological molecules widely distributed among plants and animals.

(i) Acyl glycerol (fats and oil)

(ii) Waxes

(iii) Phospholipids

(iv) Terpenoids

(i)         ACYL GLYCEROL

These are found in plants and provide energy for different metabolic activity and are very rich in chemical energy. When compared an equal amount of acyl glycerol contains over twice the energy content than carbohydrate. It consists of glycerol molecule linked to three fatty acids. This condensed molecule is also called a Tetracyl glycerol (Trigly  ceride). They are two types

(a) Saturated acyl glycerol: They contain saturated fatty acids i.e. they do not contain any double bond between carbon atom. They solid at ordinary temperature, mostly found in animals e.g. stearin.

(b) Unsaturated acyl glycerol: They are liquid at ordinary temperature. They are found in plant also called oil e.g. Linolin found in cotton seed contains Linoleic acid.

(ii)        WAXES

Waxes are simple lipids with one molecule of fatty acids forming bond with one molecule of long chain alcohol e.g. Bee’s wax.

Waxes are found as protective coating on stems, stalks, leaves, petals, fruit waxes are water repellent and non reactive.

(iii)       PHOSPHOLIPIDS

Phospholipids is most important class of lipids from biological point of view. Phospholipid is similar to Triacyl glycerol or an oil except that one fatty acid is replaced by phosphate group. Phospholipid molecule consists of two end which are called hydrophilic (water loving) end (heat) and hydrophobic (water fearing) end (tail).

Phospholipids are present in all living cells frequently associated with membranes and are related to vital functions such as regulations of cell permeability and transport processes. Proteins of cell membrane depend on their phospholipids component.

(iv)       TERPENOIDS

Terpenoid is a large and important class of lipids built up of isoprenoid (C₅H₈) units. Steroids, carotenoids and terpens are important classes of it. They help in oxidation reduction processes as terpens, some are components of essential oils of plants e.g. Menthal, camphor, mint etc. Plant pigments like carotene, xanthophylls are also form of terpenoids.

(a) Terpenses: They are volatile in nature produce special fragrance. Some of them are used in perfumes e.g. Myrcene from oil of bay Geraniol from rose, limonene from lemon oil and Menthol from peppermint oil.

(b) Carotenoids: They consist of fatty acids like carbon chain which are conjugated by double bonds and carrying 6 membrane carbon ring at each end. These compounds are pigments producing red, orange, yellow, cream and brown colours in plants.

Another group of pigmented compounds are Tetra phyrrol which are present as an important part of familiar chlorophyll and cytochromes pigments.

What is cell cycle

All cells come from cells; with these words Rudolf virchow captured the crucial importance of cellular reproduction for both are descended from pre existing cells, cellular reproduction is absolutely essential for continue existence of life on arch. Cell reproduces by division process, form daughter cells. Each daughter cell also inherits about half of the parents’ cell cytoplasm, including full complement of organelles. Each round of growth and cell division is called a cell cycle.

It is a natural question what makes the cell divide or why the reproducing cells stop dividing. If we take a unicellular organisms for example Amoeba it is seen that it feeds, grows and attains certain size, afterwards. It divides; on the other hand if amoeba is starved it shrinks and stops dividing. Therefore it appears as if the cell division is a way to keep a constant ration between the amount of cytoplasm and nucleus. It means that the division of cell is regulated by the amount of nucleoplasm. This can be explained by the fact that the nucleus governs the activities of cell and can efficiently control only over certain amount of cytoplasm. Hence the growth and development of every living organism depends on the growth and multiplication of its cell.

In unicellular organisms cell division is means of reproduction and by this process, two or more new individuals arise from the mother cells. In multicellular organisms new individuals develop from single cell, the zygote; it is multiplication of this cell and its descendants that determine the development and growth of the individual.

In general energy cell has two periods in the life cycle. Interphase (non division) and division (which produce two daughter cells). Many cells undergo a continuous alteration between divisions and non-division.

The enents occurring from the completion of one division until the beginning of next division constitute cell cycle. This cycle is repeated at each cell generation but the length of the cycle varies considerably in different types of cells. Division may take place by Mitosis or Meiosis. The cell cycle can be considered as the complex series of phenomena by which cellular material is divided between daughter cells.

Interphase

The non dividing initial phase of the cycle as the interval between two divisions is called interphase. During interphase cell caries out work, grows and prepares itself for next division and duplicates its DNA for it.

Interphase is divided into three sub stages G₁ (gap one) S (synthesis) and G₂(gap two) phase.

G₁ is phase of where no DNA synthesis occurs. In this phase synthesis and organisation of substrate (RNA) and formation of enzymes required to DNA synthesis occur. At a point late in G₁, a cell follows one of two paths, either it withdraws form the cycle and enters a resting phase or G₀stage or enters into S phase to complete the cycle. Cells that enter G₀remain viable and metabolically active but do not divide.

S phase is synthesis stage because during S-phase DNA synthesis takes place and DNA content of nucleus gets doubled. This phase of constant duration in similar cells of species. In G₂ sub-stage ribosomes are spindle formation.

Chromosomes become double stranded with two chromatids each of various periods of cell cycle G₁ period is variable, it lasts, for 3 to 4 hours. S and G₂periods are constant. After completion of interphase cell division (mitosis = M) is a dynamic period of vigorous and continual activity. It is also divided into 4 phases with specific events.

Describe in detail physiochemical nature of plasma membrane and cytoplasm

Plant cell is a unit or independent tiny or independent tiny or microscopic mass of protoplasm enclosing in it a denser spherical or oval body called the nucleus and bounded by a distinct wall the cell wall. Protoplasm and nucleus are living, while the cell wall is non living; the latter has been formed by the protoplasm to maintain its shape and firmness and to afford necessary protection. Cells vary widely in shape and size. They may be spherical, oval, polygonal, cubical or narrow and elongated.

PLASMA MEMBRANE

All cells are enclosed in plasma membrane that serves as their outer boundary, separating the cytoplasm from the external environment. This is called plasma membrane. It allows the cell to take up and retain certain substances while excluding others. All membrances have the same basic molecular organisation. They have double layer of phospholipids interspersed with proteins.

The phospholipids molecules in the plasma membrane are arranged in two parallel layers. Their non polar hydrophobic ends face each other, where as their polar hydrophilic ends are associated with carbohydrates, protein etc.

Plasma membrane also contains several types of lipids like cholesterol. In certain animal cells cholesterol may constitute upto 50 percent of the lipid molecules in plasma membrane. It is absent plasma membrane of most plant and bacteria cells.

In 1972 singer and Nicholson proposed a working model of plasma membrane known as fluid mosaic model. It is the lipid bilayer is retained as the core of the membrane. These lipid molecules are present in a fluid state. Capable of rotating and moving laterally within the membrane. The structure and arrangement of membrane proteins in the fluid mosaic model are like ice bergs in the sea. The proteins occur as a mosaic of discontinuous particles that penetrate deeply into and even completely through the lipid sheet. The components of plasma membrane are mobile and capable of coming together to engage in various types of transient or semi permanent interaction.

A class of proteins that are directly incorporated within the lipid bilayer are intrinsic proteins some of these proteins are believed to provide a channel through which water soluble substances such as ions can pass back and forth between the extra cellular and intra cellular compartment. Extrinsic proteins are a class of proteins located entirely outside the lipid bilayer on extra cellular or cytoplasmic surface. It exhibit loose association with membrane surface. Those proteins which possess lipid or carbohydrate side chains are arranged as mosaics within the cell membrane.

Plasma membrane functions as protection of cell cytoplasm, and to regulate the flow of solutions and material in and out of the cell.

CYTOPLASM

Protoplasm outside nucleus is called cytoplasm. Cytoplasm appears as a semi fluid colloid that fills the cell. The cytoplasm exhibits active streaming movements around the inner surface of the cell. This movement is known as cyclosis. Cytoplasm is composed of several types of organelles occupying half the volume of cell and a fluid of matrix, the cytosol (cell division) in which the organelles reside. The cytosol is a watery solution of slats, sugar, amino acids, proteins, fatty acids, nucleotides and other material. Giving shape and organization to the cytoplasm is a network of protein fibres, the cytoskeleton many organelles and individual molecules of the cytoplasm are thought to be attached to the cytoskeleton. 

Observations under electron microscope reveal that the cytoplasm is not a simple colloid since it contains many different kinds of minute organelles and also a mesh of tiny filaments the micro fibrils that form a sort of skeleton giving rigidity to cell and helping unicellular organisms in movement. A variety of them are membrane bound. These organelles are grouped on the basis of membrane. Membrane bound organelles are endoplasmic reticulum, Mitochondria, Golgi apparatus lysogomes, plastids and microbodies. Non membrane organlles are Ribosome, Centriole and vacuole.

Describe inversions and translocations in the structure of chromosomes

INVERSIONS:

The inversion is a type of chromosomal aberration in which a segment chromosome is turned around 180° and inserted into the chromosome. An inversion does not involve a loss of genetic information but simply rearranges the linear gene sequence. An inversion involves two breaks along the length of the chromosome prior to the reinsertion of the inverted segment.

In those cases where the centromere is not the part of the rearranged chromosome segment, the inversion is said to be paracentric on other other hadn if the centrosome is a part o the inverted segment, the inversion is known as Pericentric.

The organism with one inverted chromosome and one non-inverted homologue present, are called inversion heterozygotes.

Pairing between such chromosome is not possible until they form an inversion loop. If crossing over does not occur within the inverted segment of the inversion heterozygote, the homologues will segregate normally. When crossing over occurs within the inversion loop, abnormal chromatids are produced. A single cross over produces two parental chromatids and two recombinant chromotids. In case of a paracentric inversion one recombinant is Dicentric i.e. having two centrosome and one recombinant is a centric i.e. lacking a centromere. Both contain duplications and deletions of chromosome segments as well. During avaphase, an acentric chromatid mones roudomly to one pole or the other or may be lost, while a decentric chromatid is pulled in two directions. 

This polarized movements produces Dicentric Briodges. A deventric chewmated break at some point so that part of the chromatid goes into newxt gamete and other part into another gamete during the reduction division. In this way gametes which contain other recombinant chromatid are deficient in genetic material when such a gamete participates in fertilization the zygote most often develops abnormally.

During pericentric inversion each tetrad yield two parental chromatids containing complete set of genes and the recompinant chromatids have duplications and deletions as they are diresctly involved inc crossing over. No acentric or dicentric chromatids are produced. Gametes receiving these chromatids also produce inviable embryos.

The inversion results in new positioning of genes relative to other genes. If the expression of a gene is altered as a result of its relocation, a change in phemotype may result. Such a change is called position effect. In Drosophhilla females, heterozygous for sex linked recesive mutation with eye (W⁺/W/, X chromosome bearing the wild tupe allele (w) amy be inverted and the while locus moves to a point adjacent to centro metric heterochromatin. If the inversion is not present the heterozygous female ahs wild type red eye because the white allele is recessive. Females with X chromosome inversion have eyes that are mottled or variegated having red and white patches. Relation of W⁺allele next to a heterochromatin area seems to cause a loss of complete dominance over the recessive allele. Other genes located on X chromosome will also behave in the same manner of shifted to some other place.

Genetic consequences of Inversion:

The process of inversion maintains a set of alleles at a series of adjacent loco provided they are contained within inversion. Because the recovery of cross over products is suppressed in inversion heterozygotes, a particular gene sequence is preserved intact in the viable gametes. If this gene order provides a survival advantage to organism having it, the inversion is beneficial from evolutionary point of view. For example if the set of alleles AB, DeF is more adaptive than the sets AbcDEF or abcdEF, the favourable set will not be disturbed by the crossing over if it is maintained within a heterozygous inversion.

TRANSLOCATIONS:

The transfer of a section of one chromosome to a non homolofue is called tranlocation. In Drosophilla, translocation was first recognised gentetically by the unusual behaviour of second chromosome gene known as pale, which had the phemotypic effect of diluting certain eye colours. Although pale was lethal in homozygous conditions. Bridges found that its lethality and phenotypic effect could be suppressed by the presence of another gene present on third chromosome which is also lethal when homozygous. The lethality of the latter, in turn, was suppressed by the presence of the former, linkave analysis soon sowed that the pale effect was caused by deficiency for a small section of genes on the tip of second chromosome which had linked to third chromosome between genes ebony and rough. In other words deficiency and translocation has transferred a gene from chromosome 2^nd to 3^rd.

At present a variety of translocation are known among which following are most common.

(1)        Simple translocation: They are produced by single break in chromosome and transfer of a broken piece of this chromosome directly into the end of another.

(2)        Shifts or Intercalary Translocation: They are more common and are produced by involving three breaks so that two break section of one chromosome is inserted within the break produced in non homologous chromosome.

(3)        Reciprocal Translocation: These are interchanges which occur when single breaks into two non homologous chromosomes produce an exchange of chromosome sections between them.

GENETICS CONSEQUENCES OF TRANSLOCATIONS:

(1)        Process of translocation provides of one the proof that genes are present on chromosomes.

(2)        Translocation has also helped us to understand the position effect i.e. when a chromosome rearrange ment involves no change in the amount of genetic material but only in the order of genes, the term position effect is used to describe any associated phenotypic alteration.

(3)        Translocation sometimes is the cause of sterility.

Describe deficiency and Duplications in the structure of chromosomes

Deletion or deficiency represents the loss of segments of a chromosome which may include the loss of a single gene or part of a gene. Deletions become the first chromosome aberrations when in 1917 bridges demonstrated that a sex linked recessive gene (EO) for cosine eye colour came t expression in a Drosoplulla presumed to be heterozygous condition. He explained this unusual expression of EO gene by saying that a section of the homologue containing the dominant allele (+) of EO was missing due to deltion. Deficiencies may be classified of terminal type is produced when a single break occur near the end of a chromosome. Intercalary type of deletion on the other hand results by two breaks which occur some where in the length of the chromosome.

In either of the types of deficiencies the deleted chromosome is left without genes carried away in the delted portion unless the deleted part rejoins to the same or to another chromosome without centromer the deleted segment cannot move towards the pole of the spindle during cell division but lags the dividing cell and is excluded from the chromosome group when the nuclear membrane forms around the chromosomes of the daughter cell.

GENETIC CONSEQUENCES OF DELETION:

(1)        Recessive genes in heterozygous conditions due to deletion of its dominant allele express itself. Thus the expression of recessive allele in deleted heterozygous condition is called pseudo dominance. The wings with cut margins or notch is a sex linked recessive trait in Drosophilla. Notch express itself as a dominant in heterozygous deletion females. Another sex linked recessive trait Eosin eye colour express itself as pseudo dominant allele in deletion heterozygous condition.

(2)        Duplications:

The presence of a section of chromosome in excess of the normal amount in known as Duplication. The repeated section of the chromosomal material may be present in one pair of homologous chromosomes or may have been transposed to a non homologue or on occasion may even exist independently with its own centromere. Following are possible types.

(1)        TENDEM:

When any segment of chromosome exists twice in a normal order i.e. sequence de-de.

(2)        PEVERSE TENDEM:

In which the duplicated sequence is in reverse order i.e. sequence deed.

(3)        DISPLACED HOMOBRACHIAL TENDEM:

In this case the duplicated segment place itself in a new position at the same chromosome arm as shown by the sequence of ab-be.

(4)        DISPLACED HETEROBRACHIAL:

In this case the duplicated segment align itself on different arm of the same chromosome.

(5)        TRANSPOSITION TANDOM:

In this case the duplicate segment is shifted to different atm.

CONSEQUENCES DUPLICATION:

Duplication has proved a useful mechanism by which genetists can study dosage effect. It has also helped us to study the phemotypic effect of alleles.

Describe ultra structure and morphology of chromosomes present in the nucleus

Nucleus is filled with a Prein rich substance called Nucleoplasm or Karyolymph. In the nucleoplasm are numerous fine stands in the form of network called chromatin network or nuclear reticulum which is composed of nucleic acids, Deoxyribo nucleic acid (DNA) and Protein. During cell division chromatin changes its shape to form chromosomes. Chromosomes contain hereditary units called genes that carry the hereditary information from generation to generation. Chromosomes vary in number from species to species e.g. 20 in corn. Chromosomes are elongated and each is composed of two parts the arm and centromere. Before cell division each chromosome consists of two threads called chromoneres. Chromosome has different shapes as under:

(i)         Meta centric

Chromosome with equal arms resembling the letter V.

(ii)        SUB META CENTRIC

Chromosome with unequal arms and resembling the letter J.

(iii)       ACROCENTRIC OR SUBTELOCENTRIC

Rod like chromosomes with one arm very small and the other very long. The centromere is subterminal.

(iv)       TELOCENTRIC

Location of centromere at the end of chromosome.

ULTRA STRUCTURE OF CHROMOSOMES:

Eukaryotic chromosomes are composed of chromatin, a complex of DNA and Protein. Most eukaryotic chromosomes are about 60% Protein and 40% DNA. A significant amount of RNA is also associated with chromosomes because they are the sites of RNA synthesis. DNA of the chromosome exists as one very long double stranded fiber, a duplex which extends unbroken through the entire length of the chromosome. If the strand of DNA from a single chromosome were laid out in a straight line, it would be more than 7 feet (2 meter) long. This is much too long to fit into a cell. In the cell however DNA is coiled, thus fitting into much smaller space.

It we gently disrupt a eukaryotic nucleus and examine DNA with an electron microscope, we find that it resembles a string of beads. Every 200 ucleotides, DNA duplex is coiled about a complex of histones which are small very basic polupeptides rich in amino acids arginine and lysine. Eight of these histones form the core of an assembly called a nucleosome. Because so many of their amino acids are basic, histones are very positively charged. DNA duplex which is negatively charged, is strongly attracted to the histones and wrap tightly around the histone core of each nucleosome. The core thus acts as a ‘form’ that promotes and guides the nucleosomes wraps up into higher order coils called supercoils.

Highly condensed portion of the chromatin are called hetero chromatin. Some remain condensed permanently, so that their DNA is never expressed. The remainder of the chromosome called enchromatin is not condensed except during cell division, when the movement of the chromosomes is facilitated by the compact packaging that occurs at that stage. At all other times enchromatin is an open configuration and its genes can be activated. The genes are carried in the chromosomes. Chromosomes can be separately identified visually, but the genes re very small units and have not molecules borne in the chromosomes of the cell nuclei. It is observed that the chromosomes and genes behave so in inheritance that the genes cannot be considered outside chromosome. At the time of meiosis the separation of homologous pairs of chromosomes can be seen and it takes place as also required for the segregation of gene pairs. Chromosomes are also the kinds of organelles that segregate regularly when eukaryotic cells divide. In early twentieth century it was that the chromosomes were the vehicle for the information of heredity.

Describe Mitotic cell division or Mitosis or Somatic cell division in plants and its importance

Cell division leading to the development of the vegetative body (soma) of the plant is known as somatic cell division. It includes the division of the nucleus called mitosis (mitose=thread) or karyokinesis (karyon=nut or nucleus) (Kinesis=movement) or indirect nuclear division and the division of cytoplasm is called cytokinesis (kytos=cell). It occurs in growing regions as in the root tip and stem tip.

In Mitosis there are four stages as under:

(1)        PROPHASE

The first sign of the prophase is the appearance of a number of separate slender, crooked threads called chromosomes. The chromosomes are spirally coiled. The individual chromosomes are always double with two threads called chromatids. Chromosomes are composed of nucleoproteins. As prophase proceeds the chromosomes relex their coils and thicken somewhat. Their double nature becomes more apparent. As prophase advances a chromosomal substance accumulates in a sheath or matrix round each chromosome and the chromatids become closely coiled in it. Each chromatid divides longitudinally into two. Thus at this stage each chromosome consists of four threads.

In well fixed chromosomes some unstained gaps or constrictions are seen; those are the attachment regions called centrosomes. The nucleoli lose their staining power and disappear completely. The nucleus then rapidly passes into the next stage the metaphase.

(2)        METAPHASE

The nuclear membrane and nucleolus disappear and a spindle like body called nuclear spindle is formed. Spindle may be of nuclear origin or more probably of cytoplasmic origin. Commonly in root tips it appears as two opposite polar caps outside the nuclear membrane. The membrane then disappears and the spindle extends into the nuclear area. The chromosomes move to the equatorial plane on the spindle and there clearly apart from one another. At this stage the chromatids come even more closely together. From the centrosome of each pair of chromatids fibre like extensions called tractile fibres are formed towards the opposite poles through the nuclear spindle. The number of chromosomes is normally constant for a particular species of plants and this number is also normally even, expressed as 2n (or 2x) or diploid. Chromosome numbers cover a wide range but 24 seem to be a common figure.

(3)        ANAPHASE

In the end of metaphase the centrosomes of each pair of chromatids appear to repel each other. They diverge and move ahead towards the opposite poles along the course of tractile fibres. The chromatids soon become separated from each other. Anaphase covers the shortest period in mitosis.

(4)        TELOPHASE

At each pole the chromatids (daughter chromosomes) form a close group. The nuclear spindle disappears and so does the matrix. A nuclear membrane is formed round each group of chromatids. The chromatids duplicate themselves into full chromosomes which recognize themselves within the nuclear membrane. Nucleoli reappear at definite points on certain chromosomes. The nuclear sap reappears and each nucleus increases in size. It passes into metabolic stage or prepares for next division.

Importance: In this complicated process of nuclear division the constituents of chromosomes are equally appropriated to two daughter nuclei and thus they become qualitatively and quantitatively similar to the mother nucleus. Chromosomes are the bearers of hereditary characters and because of even distribution of chromosomal substances particularly DNA, two daughter nuclei possess all characteristics and quantities of the mother nucleus. 

What are Nucleic acids

Nucleic acids are present in all organisms from virus to man. Nucleic acids are the long chain of poly nucleotide in which mononucleotides are linked with each other. There are two kinds of nucleic acids, Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA). DNA is found mainly in the chromatin of the cell nucleus whereas most of the RNA (90%) is present in the cytoplasm and a little (10%) in the nucleolus.

Nucleic acid is a polymer of nucleotide. Nucleotide is a molecule which consists of following three parts.

(i) Pentose sugar (5 carbon)

(ii) Phosphoric acid (H₃PO₄)

(iii) A nitrogenous base.

Pentose sugar found in nucleotide is either Ribose (C₅H₁₀O₅) or Deoxyribose (C₅H₁₀O₄). Ribose is found in RNA nucleotides while Deoxyribose sugar is found in DNA nucleotides. Both of them are distinguished primarily of the basis of this Pentose sugar. This sugar behaves as basic skeleton.

Phosphoric acid is common in all nucleotides. It is attaclid with 5 carbons of Pentose sugar in each nucleotide.

There are two basic types of nitrogenous bases i.e. Pirine and Pyrinidine. Purine includes two nitrogenous bases named Adenine (A) and Gunine (G) while pyrimidine includes three nitrogenous bases cytosine (C), thyamine (T) and uracil (U)> The nucleotides differ on the basis of their nitrogenous bases. Formation of nucleotide takes place in two steps. At first step nitrogenous base combines with pentose sugar at its first carbon to form a nucleoside. At the second step the phosphoric acid combines with 5^th carbon of pentose sugar to form a nucleotide.

Nucleic acids are store house and transmitters of genetic information. They make it possible for cells to function on specific patterns and give rise to new cells that either function similarly or develop new functions according to the plans enclosed in the nucleic acid. Genetic information is encoded in a nucleic acid molecule in particular and simple fashion. DNA and RNA are basically similar structure because both of these are polynucleotide chains but the nucleotides of both are different.

DNA AS HEREDITARY MATERIAL

Transformation of one type of bacteria into another type and infection of bacteria by bacteriophage provides first evidence that DNA is the hereditary material.

Griffin discovered that living bacteria can acquire genetic material form dead bacteria and transform live bacteria from non virulent to virulent. A bacteriophage consists solely of DNA and protein. When it infects a bacterium, the phage injects its DNA only into the bacterium; the phage injects its DNA only into the bacterium where it directs the synthesis of more phages. In all nucleotides of DNA, phosphate and deoxyribose sugar are always common but the nitrogenous bases are different. In other words we can say that each DNA has specific sequence of nitrogenous bases.

Watson and Crick in 1953 gave the model of DNA. DNA has double helices with two polynucleotides running in opposite directions and connected with each other by hydrogen bonds. In fact two polynucleotides are coiled about the same axis and could be separated only by uncoiling. The bases are set at right angles to the long axis. In two nucleotides running opposite each other adenine always pairs with thyamine and cytosine with guanine. Thus the number of A and T or C and G is always constant in a DNA molecule.

There are about 10 base pairs in each turn.

Duplication of DNA: DNA has the power of self duplication. Two strands separate from one another. Each of these then synthesize its complimentary strand from the pool of nucleotides. Thus in each of the newly formed DNA molecule there will be one parental and the other newly formed strand.

RNA (RIBONUCLEIC ACID)

It consists of sugars, bases and phosphoric acid. Sugar here is ribose as against deoxyribose in DNA. RNA occurs as a single stranded molecule though in some cases a double stranded RNA may also be present. In RNA the bases are adnine, guanine, uracil and cytosine i.e. thyamine of DNA is replaced by uracil. Various types of RNA are found in plant cells. These are messenger of RNA (mRNA) which carries the information; another is transfer RNA (tRNA) also known as soluble and third is ribosomal RNA (rRNA) which is associated with ribosome. All these three types are non genetic RNA. Sometimes there is present genetic RNA also. It has the power of self duplication.