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 5th 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.
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