Replication of DNA

Essential property of genes is self duplication or replication. DNA possesses this property. Two strands of DNA molecule are complementary to each other. First step is relication is breaking of hydrogen bonds that bind together two strands of DNA. Two strands thus separate from each other and unwind. Each strand then directs the synthesis of new strand complementary to itself out of free deoxyribo nucleotides present as tri-phosphates in the environment of DNA. In this process each strand acts as a model or template to which free nucleotides are linked by hydrogen bonds with their complementary deoxyribo-nucleotides in the strand under the catalytic influence of the enzyme DNA polymerase: Thus an adenine base in free nucleotide would pair with thymine in DNA strand, a cytosine base in free nucleotide with guanine base in DNA strand and so on. This is followed by the formation of sugar phosphate bonds between successive nucleotides which arrive on the template. The result would be that each single strand of original DNA molecule would form double stranded DNA molecule in which new strand is complementary to and specified by the old strand. Further, new strand that is added to an old DNA strand is equivalent to other strand of the parental DNA molecule. Two daughter DNA molecules are thus formed which are identical to parental molecule. It should be noted that every daughter molecule is half old, half new. Each daughter molecule contains one newly synthesized strand and one strand belongs to old parental molecule which is entirely conserved. For this reason the replication of DNA is describe as Semi-conservative. It may noted that the breaking of parental DNA molecule and the unwinding of two strands occur in small segment of DNA molecule at a time. The synthesis of new strands starts at one end of old molecule and proceeds step by step to other end. This property of replication of DNA is called autocatalysis. There are three main hypothesis to explain the replication of DNA.

(A) Dispersive hypothesis (B) Conservative (C) Semi-conservative

(A)       Dispersive hypothesis: During the early stages of development of biochemical history of DNA some bio-chemists proposed that replication of DNA follows a dispersive mechanism. According to these both of the strands of daughter DNA have some parts new and some parts from parental DNA. Due to lack of experimental evidence dispersive mechanism of DNA replication was rejected very early.

(B)       Conservative replication: Some of the biochemists have also suggested conservative mechanism for DNA replication. According to this view both parental strands are retained by the parent DNA molecule and both the strands of daughter DNA are synthesized new under the guidance of parental strands. This was also rejected due to lack of evidence.

(C)       Semi-conservative replication: In E. Coli cells DNA is replicated in semi conservative manner. Each daughter DNA receives one strand from its parent and synthesize its second strand from its cell’s raw material. After replication both daughter DNA molecules appear as haploid of old (parental and new strands. This view is accepted by almost every biochemist as the only mechanism of DNA replication.

DNA replication mechanism: Replication DNA is possible only when the parent molecule uncoils and both of its strands get apart. For unwinding and separation of the strands of the parent molecule, at the time of replication, three types of enzymes are involved. These are (1) Gyrase or Helicase enzymes uncoil replicating DNA and force it to become straight and uncoiled. (2) DNA binding proteins (B – protein), or melting enzymes react with both strands of the parent DNA and by breaking hydrogen bonds gap both strands apart. (3) Helix destabilizing proteins react with single stranded DNA and do not allow them to recoil during replication. By the activity of enzymes Gyrase, DNA B – proteins and helix destabilizing proteins, origin site (single stranded region) is created at which replication starts bi-directionally.

Replication is catalyzed by numbers of enzymes called DNA polymerases and ligases. DNA polymerases are enzymes which are responsible for replication. These enzymes of DNA polymerases are known to exist. They are (1) DNA polymerase I (2) DNA polymerase II (3) DNA polymerase III