Question

9. The process of DNA replication creates a particular problem for replicating the ends of linear chromosomes.
a) Describe why the DNA replication machinery has difficulty replicating DNA ends.

b) Telomerase has been identified as an enzyme that can reverse the outcome of end replication.
Describe the near-universal mammalian chromosome telomere sequence and how telomerase solves the problem of end replication.

Answer 1

Answer a) The eukaryotic chromosomes are linear or rod-shaped and they have ends. These linear ends face difficulty in replicating DNA at the ends because the DNA at the very end of the chromosome cannot be fully copied in each round of replication, resulting in a slow, gradual shortening of the chromosome. During replication of ends, one of the two new strands at a replication fork is made continuously and the other strand is produced in many small pieces called Okazaki fragments. There is a short stretch of DNA remains that does not get covered by an Okazaki fragments, there's no way to get the fragment started because the primer would fall beyond the chromosome end. The part of the DNA at the end of a eukaryotic chromosome goes uncopied in each round of replication, leaving a single-stranded overhang. Over multiple rounds of cell division, the chromosome will get shorter and shorter as this process repeats.

Answer b) The Near-universal mammalian chromosome telomere consist of hundreds or thousands of repeats of the same short DNA sequence 5'-TTAGGG-3' in humans and other mammals. Telomerase is an RNA-dependent DNA polymerase that can make DNA using RNA as a template.

The telomerase has a special complementary RNA that contains a complementary sequence to telomeric end that extends the overhanging strand of the telomere DNA by using complementary RNA as a template. When the overhang is long enough, a matching strand can be made by the normal DNA replication machinery (that is, using an RNA primer and DNA polymerase), producing double-stranded DNA and complete the synthesis of telomeric ends.