Short Note on DNA Replication:
Replication is an essential property of genetic material. DNA replication is the process whereby DNA makes exact copies of itself. Approximately twenty or more enzymes and proteins are needed during replication. Replication occurs at the rate of between 50 nucleotides per second (in mammals) and 500 nucleotides per second (in bacteria). The hydrogen bonds between the complementary bases on the two strands of the parent DNA molecule break and the strands unwind, each strand acts as a template for the synthesis of a new strand complementary to itself. DNA polymerases move down the two single strands linking free nucleotides to their complementary bases on the templates. The process continues until all the nucleotides on the templates have joined with appropriate free nucleotides and two identical molecules of DNA are formed. Every time a cell divides it needs to make a duplicate copy of all its genes. It means, at each cell division the cell has to carry out replication of its entire DNA for transferring genetic information from parent cell to daughter cells.
Watson and Crick suggested a very simple mechanism of DNA replication on the basis of its double helical structure. During replication, the weak hydrogen bonds between the nitrogenous bases of the complementary nucleotides break so that the two polynucleotide strands of DNA separate and uncoil. The strands thus separated are complementary to each other. Each separated strand acts as a template for the synthesis or polymerization of other strands and two identical daughter DNA molecules are formed. Becuase of the specificity of base pairing, each nucleotide of the separated chains attracts its complementary nucleotide from the cell cytoplasm. Once the nucleotides are attached by the hydrogen bonds, their sugar radicals join through their phosphate components, completing the formation of a new polynucleotide chain.
Thus, each strand of the double helix DNA serves as a template or model on which its complementary strand is synthesized. This method of DNA replication is described as semiconservative because each daughter DNA molecule is a hybrid conserving one parental polynucleotide strand and the other one newly synthesized strand.
Basic Rule for DNA Replication:
The basic rules that are followed strictly during DNA replication can be summarized as follows:
(1) There is specificity of base pairing during DNA replication, i.e., adenine pairs with thymine and guanine with cytosine.
(2) Nucleotide monomers are added one by one to the 3′ end of the growing strand by the enzyme DNA polymerase.
(3) The sequence of nitrogenous bases in each daughter strand being formed is complementary to the base sequence in the template strand.
(4) C-3′ carbon of deoxyribose present on the 3′ end of the new polynucleotide chain of DNA has an OH group and is free to bind to another nucleotide. C-5′ carbon of deoxyribose on the 5′ end of the polynucleotide chain has a phosphate. Therefore, the new polynucleotide chain is always synthesized in 5′–>3′ direction.