Question

Explain how telomeres are maintained. Include chromosomal structures and enzymes.

Answer 1

Telomeres are the ends of the linear chromosomes which contain a repetitive sequences for example in vertebrates TTAGGG. They protect the ends of the chromosome from deterioration/shortening which would eventually lead to loss of genetic information. Why is there loss of information? This is because of okazaki fragments which need their RNA primer to join ahead of the fragment synthesized. Thus at the end a primer without any 3' template cannot be made and a portion of 5' strand is not synthesized. This will shorten the chromosome length with each replication cycle. To prevent this, we have an enyzme called telomerase which replicates the ends of linear chromosomes. At the 3' overhang, there are species specfic tandem repeats (TTAGGG in vertebrates) to which the telomerase binds. Telomerase is a template independent enzyme. In its core it has a RNA fragments complimentary to the repeat at the 3' end. The telomerase makes complimentary base pairing and extend the 3' overhang. With the help of extended 3' overhang a primer for 5' is established by primase and okazaki fragments are synthesized by DNA polymerase in 5' to 3' direction. In this way telomeres are extended and the structural intergrity of chromosomes at its ends is maintained.