50 DNA Replication Exercise B: DNA Replication (con\'t) Completion stage of repl

Question

50 DNA Replication Exercise B: DNA Replication (con't) Completion stage of replication: Telomerase In prokaryotes, once ligase is finished connecting the Okazaki fragments, the DNA has been successfully replicated. This is due to the circular shape of their DNA However, eukaryotic DNA is linear. In the lagging strand this poses a problem. At the end of the chromosomes, . Thymine .. DNA Polymerase " known as the telomeres, there is nowhere for the DNA-UracilDNA Polymerase ll Polymerase ll, to connect due to the absence of an RNA -Deoxyribose , primase primer (Fig. 15). If these D-nucleotides at the telomeres -Ribose a, Ligase were left unsynthesized, the DNA strands would eventually become shorter with every replication. It is thought that aging is a product of malfunctioning ta - Cytosine -SS8P Helicase Adenine , Guanine p. DNA Polymerase I Telomerase . Phosphate telomerase Fig. 15. Completion phase of replication unsynthesized D-nucleotides of the lagging strand 5. Why are there unsynthesized nucleotides at the telomere of the lagging strand? 6. Why is it problematic for the D-nucleotides on the lagging strand of the telomeres to be left unsynthesized? Telomerase is an enzyme that attaches to the unsynthesized end of the lagging strand and catalyzes the synthesis of DNA from its own RNA template, akin to reverse engineering. At one end of the telomerase a few R-nucleotides combine with the end of the unreplicated strand (Fig. 16a). Telomerase then adds D-nucleotides to the end of the lagging strand (Fig. 16b). 2 In Fig. 16, record the a) Telomerase: Step 1 complementary base Lagping strand Epairs of the extending DNA of the lagging strand. Lagging strand Fie. 16. Completion phase of replication: telomerase extends the unreplicated strand. www.thebiologyprimer.com

Explanation / Answer

5.

The enzyme DNA polymerase (DNA dependent DNA polymerase) can add nucleotides to the 3’ OH end of the existing strand, but cannot start polymerization de novo. Notice that it can only polymerize an existing strand, it needs a primer to start addition of nucleotides.

During replication of DNA, small RNA fragments called primers initiate this polymerization. Once replication is completed, the RNA fragments in the DNA-RNA hybrids are degraded. However, the 3’OH ends serve as templates to fill the gap by addition of nucleotides. However, at the ends, when RNA primers are removed, there is no 3’ OH available to fill the gaps. The polymerases cannot add nucleotides to the 5” phosphate ends.

6.

If the D-nucleotides on the lagging strand of the telomeres are left unsynthesized, with each round of replication, the telomere will fall short of that length of unduplicated strand. Eventually, the DNA strands will become shorter with every replication.

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