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.
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