Q53 — Telomerase — Mechanism of Telomere Extension

Theoretical 2 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license

At the ends of chromosomal DNA molecules of eukaryotes, there are special nucleotide sequences called telomeres, which shorten during each division of the somatic cell. Telomeres do not contain genes, but consist of multiple repeats of a short nucleotide sequence, such as TTAGGG. When DNA replication is completed (Fig. 1a), a single-strand protrusion (TTAGGG) occurs at the 3’ end of the 5’-3’ (lagging) strand of DNA. The enzyme telomerase containing the CAAUCC sequence in associated RNA allows for a complete duplicate of the DNA molecule along the entire length of the chromosome (Fig. 1b-e).

Figure 1. The mechanism of action of telomerase: (a) replication complete, 3’ overhang present; (b) telomerase binds via RNA:DNA base-pairing; (c) telomerase extends the 3’ overhang using its RNA as template; (d) telomerase translocates; (e) DNA polymerase fills in the complementary strand. Figure 1. The mechanism of action of telomerase. 5’-3’ is the “maternal” DNA strand, and 3’-5’ is the “daughter” strand.

Using the information and figures, determine whether the following statements are true or false.

A. The protein component of telomerase is sufficient to elongate telomeres.

B. The mother strand of DNA is sequentially elongated on the 3' end, using the telomerase RNA as a template.

C. Loss-of-function mutations in the telomerase protein are likely to cause diseases such as progeria or bone-marrow failure.

D. Telomere length has a threshold value in cells and in HeLa cells telomerase is repressed to maintain this threshold value.

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Question reproduced from IBO 2022, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)