Question

Describe the process of DNA replication with the help of a diagram.

Answer 1

Hint:DNA is a long polymer consisting of a long chain of monomers called deoxyribonucleotides. The DNA molecule is said to be a polynucleotide. Each nucleotide has three parts: a sugar, a nitrogen containing ring-structure called base and a phosphate group. The sugar present in DNA is a five carbon pentose called 2'-deoxyribose in which the-OH group on carbon 2 of ribose is replaced by hydrogen.

Complete answer:

>The mechanism by which a double-stranded DNA molecule is copied to produce two identical DNA molecules is called DNA replication.
>Replication is an important process because, whenever a cell divides, the two new daughter cells must contain the same genetic information which is in the form of DNA, as the parent cell.
>The DNA is copied by enzymes called DNA polymerases which act on single stranded DNA synthesizing a new strand complementary to the original strand. DNA synthesis always occurs in the direction 5' →3'.
>Replication fork is where the helix unwinds and new DNA is synthesized. At the replication fork a number of distinct events occur.
>Unwinding: The old strands that make up the parent DNA molecule are unwound and "unzipped" (i.e., the weak hydrogen bonds between the paired bases are broken) by the action of a special enzyme called helicase. Helicase forms a tight complex with DNA primase, producing a structure called primosome. After the separation of the strands, Single Stranded DNA Binding (SSB) protein attaches to the DNA and prevents the double helix from reforming.
>Leading and lagging strand: Synthesis of DNA by DNA polymerase occurs only in the 5' → 3' direction. The strands of the double helix of DNA run in the opposite direction to each other (one strand runs 5' 3' and the other 3'5'). The strand which is replicated in the same direction as the unwinding helix and so can be synthesized continuously is called leading strand. The other strand known as the lagging strand, is synthesized in the opposite direction and must be copied discontinuously resulting in “Okazaki fragments”.
>Priming: DNA polymerases enzymes require a short double stranded region to initiate or prime DNA synthesis; this is produced by an RNA polymerase, called primase, which is able to initiate synthesis of one single stranded DNA.
>The primase enzyme synthesizes a short RNA primer sequence on the DNA template creating a short double stranded region.
>Joining :The final step required to complete synthesis of the lagging strand is for the Okazaki fragments to be joined together by phosphodiester bonds which is carried out by a DNA ligase.

Note:Replication is an important process because, whenever a cell divides, the two new daughter cells must contain the same genetic information which is in the form of DNA, as the parent cell.
Speed of replication:
The DNA genome of complex eukaryotes is huge and the process of DNA replication should be incredibly fast. In them, chromosomes of 250 million pairs of bases can be replicated in several hours. The DNA replication speed in humans is about 50 nucleotides per second per replication fork (low speed compared to the speed of the bacterial DNA replication). But in humans the genome can be copied only in a few hours because many replication forks take place at the same time (multiple initiation sites).
The speed of replication of DNA in bacteria is much longer (about 1000 nucleotides per second) and that is a reason why the rate of errors is much higher during the process of bacterial replication.