What is DNA replication?

Definition:

DNA replication is a fundamental process occurring in all living organisms to copy DNA. In this process, each strand of the parent double-stranded DNA molecule acts as a template for the synthesis of the complementary daughter strand. It proceeds from 5’ to 3’ direction.

DNA replication is the process by which DNA makes a copy of itself during cell division.  

DNA replication 

1. The first step in DNA replication is to ‘unzip’ the double helix structure of the DNA? molecule.
2. This is carried out by an enzyme? called helicase which breaks the hydrogen bonds? holding the complementary? bases? of DNA together (A with T, C with G).
3. The separation of the two single strands of DNA creates a ‘Y’ shape called a replication ‘fork’. The two separated strands will act as templates for making the new strands of DNA.
4. One of the strands is oriented in the 3’ to 5’ direction (towards the replication fork), this is the leading strand?. The other strand is oriented in the 5’ to 3’ direction (away from the replication fork), this is the lagging strand?. As a result of their different orientations, the two strands are replicated differently: 

           
                     

              Leading Strand:

4. A short piece of RNA ?called a primer? (produced by an enzyme called primase) comes along and binds to the end of the leading strand. The primer acts as the starting point for DNA synthesis.
5. DNA polymerase? binds to the leading strand and then ‘walks’ along it, adding new complementary? nucleotide? bases (A, C, G and T) to the strand of DNA in the 5’ to 3’ direction.
6. This sort of replication is called continuous.

       Lagging strand:
                                
                            

4. Numerous RNA primers are made by the primase enzyme and bind at various points along the lagging strand.
5. Chunks of DNA, called Okazaki fragments, are then added to the lagging strand also in the 5’ to 3’ direction.
6. This type of replication is called discontinuous as the Okazaki fragments will need to be joined up later.                             

         Once all of the bases are matched up (A with T, C with G), an enzyme called exonuclease strips away the primer(s). The gaps where the primer(s) were are then filled by yet more complementary nucleotides.

The new strand is proofread to make sure there are no mistakes in the new DNA sequence.

Finally, an enzyme called DNA ligase? seals up the sequence of DNA into two continuous double strands.

The result of DNA replication is two DNA molecules consisting of one new and one old chain of nucleotides. This is why DNA replication is described as semi-conservative, half of the chain is part of the original DNA molecule, half is brand new.

Following replication the new DNA automatically winds up into a double helix. 

Mechanism of DNA replication:

                                            

                 Three different mechanisms were proposed for the replication of DNA.

Conservative model: According to this model Both parental strands stay together after DNA replication and a new DNA double helix with two daughter strands is created.

Semiconservative model: According to this model, two double-stranded DNA molecules are produced and each double-stranded DNA contains one parental and one daughter strand following replication

Dispersive model: According to this model, the parental and daughter DNA are interspersed in both