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1)why does DNA replication occur in small replication forks not in its entire length? 2)why is DNA replication continuous and discontinuous in a replication fork? 3)explain the importance of "origin of replication" in a replication fork.

1) DNA replication occurs in small replication forks not in its entire length in one time as DNA is a very large molecule and only that part of DNA opens up which is being replicated. Opening of whole DNA molecule would be energetically more expensive process. 

2) The main enzyme involved in DNA replication is the DNA-dependent DNA polymerase. This enzyme catalyses the polymerisation of deoxynucleotides along the 5′ → 3′ direction, and hence, replication is continuous along the 3′ → 5′ strand (leading strand) and discontinuous along the template, i.e., the 5′ → 3′ direction (lagging strand).

 Okazaki fragments are short DNA segments on the lagging strand, formed in the 5’ – 3’ direction, starting from RNA primers. A separate RNA primer is needed for the synthesis of each Okazaki fragment. These discontinuously synthesized fragments are later joined by the enzyme DNA ligase.

 3) Ori stands for Origin of replication. This site has the highly conserved sequence of DNA among various species. The replication of DNA starts here because this site attracts some proteins which help in the opening and unwinding of DNA and this leads to the initiation of replication. 

 

@Monisha, good job!!!

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For long DNA molecules , since tghe two strands of DNA cannot be separated in its entire length due to very high energy requirement for this , the replication occur within small openings of the DNA helix at the ORI , which are reffered as replication fork.

Synthesis of new strands takes place on both the templates simultaneously i.e., bidirectional ; but on any template , the DNA-DEPENDENT DNA POLYMERASES catalyse polymerisation only in one direction ,i.e.,3' - 5' of the template. this creates certain additional complications .So, on of the templates strands i.e., with polarity 3' - 5' , replication is continuous , while on the other i.e., 5' -3' it is discontinuous. The discontinuously synthesised fragments are later joined by the enzyme DNA ligase.`

Replication does not initiate randomly at any place in DNA .there are specific region in E.coli DNA where the replication opriginates .hence without these replicaqtion doesnt occur at required sites.

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For long DNA molecules , since tghe two strands of DNA cannot be separated in its entire length due to very high energy requirement for this , the replication occur within small openings of the DNA helix at the ORI , which are reffered as replication fork.

Synthesis of new strands takes place on both the templates simultaneously i.e., bidirectional ; but on any template , the DNA-DEPENDENT DNA POLYMERASES catalyse polymerisation only in one direction ,i.e.,3' - 5' of the template. this creates certain additional complications .So, on of the templates strands i.e., with polarity 3' - 5' , replication is continuous , while on the other i.e., 5' -3' it is discontinuous. The discontinuously synthesised fragments are later joined by the enzyme DNA ligase.`

Replication does not initiate randomly at any place in DNA .there are specific region in E.coli DNA where the replication opriginates .hence without these replicaqtion doesnt occur at required sites.

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