Nucleic Acids

Introduction

     Nucleic acids are giant macromolecules having a complex structure and very high molecular weight. They are polymers of nucleotides and hence, described as polynucleotides.

Deoxyribose Nucleic Acid (DNA)

     With the exception of a few viruses, DNA forms the genetic material in all living organisms. In the prokaryotic cells DNA occurs in the cytoplasm. In the eukaryotic cells it is confined largely to the nucleus and forms the main component of chromosomes. A small amount of DNA is found in the cytoplasm in the organelles like mitochondria and plastids. It is called extra- nuclear DNA.

Experiment to prove that DNA is Genetic Material Griffith Effect

     The fact that DNA is genetic material came from the experiments using bacteria and viruses. The first series of experiments were performed by a British bacteriologist F. Griffith in 1928, using the bacterium Diplococcus pneumoniae which causes pneumonia in mammals.

Blender Experiment by Hershey and Chase

     Hershey and Chase devised an experiment in the year 1952 with two different preparations of T₂ phage. T₂ phage contains both DNA and proteins. It is possible to examine which of the two components has the information to programme for the production of more viral particles.

Replication of DNA

     The most significant feature of DNA molecule is its capacity to make its own identical copies. This property is called replication. It occurs regularly during the synthetic phase of the cell cycle, every time a cell divides.

Mechanism of DNA Replication

     It is a complex process and involves the participation of over a dozen enzymes and other protein factors. The intertwined DNA starts separating from a particular point and this is known as the origin of replication.

Leading Strand

     On the other parent strand, the enzyme forms short stretches of DNA once again in the 5'-3' direction starting from an RNA primer. These short DNA fragments are referred to as Okazaki fragments and because this strand is synthesised and put together in small pieces it is referred to as lagging strand. These short pieces are joined together by the enzyme DNA ligase.

Ribose Nucleic Acid (RNA)

     RNA occurs mostly in the cytoplasm in the eukaryotic cells. A small amount occurs in the nucleus of the cell, as a constituent of nucleolus. RNA is a single polynucleotide chain composed of nucleotides of adenine, guanine, cytosine and uracil. Thymine nucleotides are absent.

Summary

     Nucleic Acids are giant macromolecules having a complex structure and very high molecular weight. There are two types of nucleic acids in a cell, namely De-oxy ribose nucleic acid and Ribose nucleic acid.