(a) Why does DNA replication occur in small replication forks and not in its entire length?

(b) Why is DNA replication continuous and discontinuous in a replication fork?

(c) State the importance of origin of replication in a replication fork.

OR

What is an operon? Explain the functioning of lac operon when in an open state.

(a) For long DNA molecules, as the two strands of DNA cannot be separated in its entire length (due to very high energy requirement), the replication occur within a small opening of the DNA helix, referred to as replication fork. And opening entire DNA at once will be more expensive process.

(b) - The DNA-dependent DNA polymerases catalyze polymerisation only in one direction, which is 5' to 3'.

- This creates some additional complications at the replicating fork.

- As a result, on 3’ to 5’ strand the replication is continuous. This is leading strand.

- While on the other 5’ to 3’ strand the replication is discontinuous. This is lagging strand.

- The discontinuously synthesised fragments are later joined by the enzyme DNA ligase.

(c) - The DNA polymerase cannot initiate replication on its own and also the replication doesn’t take place randomly at any place in DNA.

- In E. coli DNA there is an explicit region where the replication occurs. This region is known as origin of replication (Ori).

- This Ori has extremely preserved sequences of DNA amongst a variety of species.

- The replication of DNA begins in Ori as this region attract a few proteins which help out in opening and unwinding of DNA and will go ahead to initiation of replication.

OR

An Operon is a functioning unit of DNA containing a group of genes in the control of a single promoter and regulatory genes.

When lac operon is in open state, it functions as follows:

- The lac operon consists of one regulatory gene and three structural genes namely (z, y and a).

- The z gene codes for beta-galactosidase (β-gal), which is principally responsible for the hydrolysis of the disaccharide, lactose into its monomeric units, galactose and glucose.

- The y gene codes for permease, which boost permeability of the cell to β-galactosides.

- The ‘a’ gene encodes a transacetylase.

- Hence, all the three gene products are required for metabolism of lactose.

- When lactose is absent - When lactose is absent, i.e. gene produces repressor protein. It binds to the operator and as a result, prevents RNA polymerase to bind to operon. The operon is switched off.

- When lactose is present - Lactose work as inducer which binds to the repressor and forms inactive repressor. At this moment the RNA polymerase binds to the operator and transcribes lac mRNA. The Lac mRNA is polycistronic, i.e. produces all three enzymes β−galactosidase, permease and trans-acetylase. And the lac operon is switched on.