Structure of DNA

Structure of DNA

DNA is the hereditary material as it contains the genetic information. It is the most important constituent of a chromosome. The most widely accepted model of DNA is the double helical structure of James Watson and Francis Crick. They proposed the three-dimensional model of DNA on the basis of X-ray diffraction studies of DNA obtained by Rosalind Franklin and Maurice Wilkins. In appreciation of their discoveries on the molecular structure of nucleic acids Watson, Crick and Wilkins were awarded Nobel prize for Medicine in 1962.

Chemical Composition of DNA molecule

DNA is a large molecule consisting of millions of nucleotides. Hence, it is also called a polynucleotide. Each nucleotide consists of three components.

1. A sugar molecules – Deoxyribose sugar.

2. A nitrogenous base.

There are two types of nitrogenous bases in DNA. They are

(a) Purines (Adenine and Guanine)

(b) Pyrimidines (Cytosine and Thymine)

3. A phosphate group

Nucleoside and Nucleotide

Nucleoside = Nitrogen base + Sugar

Nucleotide = Nucleoside + Phosphate

The nucleotides are formed according to the purines and pyrimidines present in them.

1. Watson and Crick model of DNA

1. DNA molecule consists of two polynucleotide chains.

2. These chains form a double helix structure with two strands which run anti-parallel to one another.

3. Nitrogenous bases in the centre are linked to sugar-phosphate units which form the backbone of the DNA.

4. Pairing between the nitrogenous bases is very specific and is always between purine and pyrimidine linked by hydrogen bonds.

·                 Adenine (A) links Thymine (T) with two hydrogen bonds (A = T)

·                 Cytosine (C) links Guanine (G) with three hydrogen bonds( C ≡ G)

This is called complementary base pairing.

5. Hydrogen bonds between the nitrogenous bases make the DNA molecule stable.

6. Each turn of the double helix is 34 A° (3.4 nm). There are ten base pairs in a complete turn.

7. The nucleotides in a helix are joined together by phosphodiester bonds.

2. DNA Replication

DNA replication is one of the basic process that occurs within a cell. DNA molecule produces exact copies of its own structure during replication process. The two strands of a DNA molecule have complementary base pairs, the nucleotides of each strand provide the information needed to produce its new strand. The two resulting daughter cells contain exactly the same genetic information as the parent cell. DNA replication involves the following steps

Origin of replication

The specific points on the DNA, where the replication begins, is the site of origin of replication. The two strands open and separate at this point forming the replication fork.

Unwinding of DNA molecule

The enzyme called helicase, bind to the origin of replication site. Helicase separates the two strands of the DNA. The enzyme called topoisomerase separates the double helix above the replication fork and removes the twists formed during the unwinding process. Each of the separated DNA strands function as a template.

Formation of RNA primer

An RNA primer is a short segment of RNA nucleotides. The primer is synthesized by the DNA template close to the origin of replication site.

Synthesis of new complementary strand from the parent strand

After the formation of RNA primer, nucleotides are added with the help of an enzyme DNA polymerase and a new complementary strand of DNA is formed from each of the parent strand. The synthesis is unidirectional.

In one strand, the daughter strand is synthesized as a continuous strand which is called leading strand. In the other strand, short segments of DNA are synthesized. This strand is called lagging strand. The short segments of DNA are called Okazaki fragments. The fragments are joined together by the enzyme, DNA ligase.

The replication stops when the replication fork of the two sides meet at a site called terminus, which is situated opposite to origin of replication site

3. Significance of DNA

·                 It is responsible for the transmission of hereditary information from one generation to next generation

·                 It contains information required for the formation of proteins.

·                 It controls the developmental process and life activities of an organism.