Principles of Biochemistry (2+1)

7.2.1.1 The Replication process

DNA is replicated by enzymes known as DNA-directed DNA polymerase or DNA polymerase.

DNA polymerase: These enzymes use single stranded DNA as templates on which to catalyse the synthesis of the complementary strand from the appropriate deoxynucleoside triphosphates.

All the DNA polymerases can add a nucleotide only to the free 3' -OH group of a base-paired polynucleotide so that DNA chains are extended only in the5’ to 3’ direction.

Because the two strands of parent DNA run in opposite directions (one in 5’ to 3’ and the other 3’ to 5’), only one strand can grow continuously in the 5’-to-3’ direction (Lagging strand).

The other strand must be formed in short segments, called Okazaki fragments (after their discoverer, Reiji Okazaki), as the DNA unwinds (Lagging strand).   

The breaks or gaps in this daughter strand are called nicks.

To complete the formation of this strand, the Okazaki fragments are connected by action of the enzyme DNA ligase.

DNA unwinding during replication process

The process of DNA unwinding does not have to begin at an end of the DNA molecule. It may occur at any location within the molecule. Unwinding usually occurs at several interior locations simultaneously and that DNA replication is bidirectional for those locations; that is, it proceeds in both directions from the unwinding sites.

The result of this multiple-site replication process is formation of “bubbles” of newly synthesized DNA. The bubbles grow larger and eventually coalesce, giving rise to two complete daughter DNAs. Multiple-site replication enables large DNA molecules to be replicated rapidly.

1. DNA is replicated by enzymes known as DNA-directed DNA polymerases or simply DNA polymerase. These enzymes use single-stranded DNA (ssDNA) as templates on which to catalyze the synthesis of the complementary strand from the appropriate deoxynucleoside triphosphates. The reaction occurs through the nucleophilic attack of the growing DNA chain's 3'-OH group on the alpha-phosphoryl of an incoming nucleoside triphosphate.

2. One strand of the performed DNA Serves as the Primer. DNA polymerase adds nucleotides sequentially to the 3' end of one strand, the primer. Therefore, the synthesis of the new DNA strand is in the 5' --> 3' direction.The DNA polymerase cannot start a new DNA strand by itself; it can only add to a preexisting strand and even then can add new nucleotide residues only if a template strand is available.

3. The Other Strand of the Preformed DNA Serves as a Template

Nucleotides are added to the primer strand in accordance with the base sequence of the template strand, following Watson-Crick base pairing.

Wherever a thymine residue occurs in the template strand, an adenine residue is added to the daughter strand , and vice versa.

Similarly, wherever a guanine residue is present in the template strand, a cytosine residue is inserted in the daughter strand and vice versa.

The product of DNA Polymerase is thus a base-paired duplex.