Question

I am confused with the end replication problem. So, at the end DNA polyermase III is unable to bind to the end because there is a missing of a 3' hydroxyl group. Given that, why can't a DNA primase simply add a primer at the end in which DNA polymerase III binds to it and finishes the replication and then a DNA polymerase I binds to the primer and replaces it with DNA and therefore solving the problem?

Can someone explain why my train of thought is wrong?

Answer 1

Ans-For lagging-strand DNA replication, short RNA primers are made by RNA primase. These are then extended by DNA polymerase to form Okazaki fragments. When these RNA primers are removed, there is no way to synthesize lagging-strand sequence that is complementary to the small region at the end of the chromosome (which is at least as large as an RNA primer). So, with continuing cell division, sequence is lost from the ends of linear chromosomes.

During chromosome replication, the enzymes that duplicate DNA cannot continue their duplication all the way to the end of a chromosome, so in each duplication the end of the chromosome is shortened (this is because the synthesis of Okazaki fragments requires RNA primers attaching ahead on the lagging strand). The telomeres are disposable buffers at the ends of chromosomes which are truncated during cell division; their presence protects the genes before them on the chromosome from being truncated instead. The telomeres themselves are protected by a complex of shelterin proteins, as well as by the RNA that telomeric DNA encodes