Question

To avoid the loss of terminal sequences different organisms use different strategies to replicate their DNA.

A. What features of the chromosome does E. coli have to circumvent this problem? Provide a detailed answer.

B. What strategy do prokaryotes use to replicate linear chromosomes to circumvent this problem? Provide a detailed answer.

C. What strategy do eukaryote use to replicate chromosomes to circumvent this problem? Provide a detailed answer.

Answer 1

C. Contrary to prokaryotic chromosome, the eukaryotic chromosome is linear and the chromosome at the ends is not completely replicated resulting in the shortening of the chromosome after each round of replication.At the ends of the chromosomes, especially in the lagging strand, the formation of okazaki fragments is hampered because the last RNA primer is positioned very far from the ends of the chromosome. Hence, single stranded overhangs are produced after each replication cycle.

To overcome this problem, the ends of the chromosome has thousands of special tandem repeat sequences of 5’ – TTAGGG-3’ known as Telomers or Telomer Caps. This telomere overhangs is susceptible for degradation by DNA repair enzymes which may recognize this overhangs as damaged DNA and hence the telomere is degraded slowly after repeated cell divisions. Telomerase are the enzymes which can prevent telomere shortening by extending the telomeric sequence of chromosomes. It is a reverse transcriptase enzyme which synthesizes DNA from RNA. The enzyme has RNA sequence which is complementary to the telomere sequences and adds nucleotides at the ends which compensates for the lost telomere nucleotides. The expression of telomerase is related to aging. As telomerase activity goes down as age progresses, the telomeres start shortening up leading to the shortening of chromosome and ultimate death of the cell.

A. Prokaryotes doesn't have linear chromosome but has circular chromosome just like E.coli has.

E.coli and other prokaryotes replicate their cicular chromosome by rolling circle mechanism and end sequences are not generally lost. However, termination is well defined in them. During replication catenane formation occurs where two circular strands are linked as chains and are termed as Catenanes.The decatenating is performed by DNA Gyrase which separtes the two strands.

There are specific terminatioin sequences present in the prokaryotes termed as "Ter sites" which is recognized and bound by Tus proteins. In E.coli, there are three ter sites namely Ter A, D and E. Thebinding of Tus protein to these Ter sites terminates the replication as the binding of helicase and ssb proteins is inhibited. This is how termination occurs in prokaryote. As such, there is no issue in replicating the ends of DNA. This problem mainly arises in eukaryotic replication.