Question

a. Explain how eukaryotic chromosomes become shortened after each round of replication.

b. Why does this not happen in prokaryotes?

c. How does the cell attempt to remedy this problem

Answer 1

a. Eukaryotic centrosomes shorten during each replication because of the presence of telomeres on their ends. These telomeres are repeat sequences of TTAGG with a 3’ G rich overhang which is single stranded. Telomeres protect the chromosomes from degradation. They prevent inter-chromosomal fusion.

DNA replication occurs via leading and lagging strand synthesis. The lagging strand synthesis requires primers as the direction of replication is in 5’ to 3’ direction. The 3’ G rich overhang is easily extended in leading strand synthesis. A G rich specific nuclease will then remove the synthesized 5’-3’ sequence produced. The processing of the 5’ strand forms the G tail, which is followed by 5’ resectioning to form the 3’ overhang. However, in lagging strand synthesis, removal of RNA primers forms the Okazaki fragments. This creates a 3’ gap where the 3’ end primer is removed. This is known as end replication problem. Although telomerase tries to extend this G-rich sequence, it may be incomplete. Thus, there is shortening of the telomere repeats in subsequent generations. DNA repair processes also cannot solve this problem.

b. Prokaryotes have a single circular chromosome containing DNA while eukaryotes have several linear chromosomes. Due to the circular chromosome, there are no telomeres present as the ends are not free. Thus, telomeres are not required to prevent inter-chromosomal fusion. Due to absence of telomeres, prokaryotes are able to continue and form complete chromosomes after replication. Hence, prokaryotes will not shorten its chromosome during subsequent generations.

c. The problem of chromosome shortening is solved to a certain extent by the Telomerase enzyme. This enzyme has a reverse transcriptase TERT and a RNA component TERC, which is the template used for extension of telomeres on the lagging strand. The TERC sequence of 451 bp will bind to the complementary sequence at end of chromosomes. The TERT reverse transcriptase will then use this TERC sequence to synthesize telomere DNA. TERT is a reverse transcriptase that can uses RNA instead of DNA as a template. Hence, it is a RNA dependent DNA polymerase. The 3’ overhang is replicated by telomerase when it is recruited by shelterin to the ends of chromosome. Telomerase also can associate with other proteins such as TEP1, hGAR1, C1/C2, hstau, etc which have functions in nuclear localization, assembly and stability of the telomerase. There is further relocation of the telomerase as it adds bases to the TERC and repeats the steps. This is followed by completion of the sequence by actions of DNA polymerase.