A-Level Biology May/June 2025 Q5(a): Outline the role of telomeres in eukaryotic chromosomes.

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  Telomeres are regions of repetitive, non-coding DNA located at the ends of eukaryotic chromosomes.

  1. Their primary role is to act as a protective buffer, preventing the loss of important genes and genetic information from the ends of the chromosome during successive rounds of DNA replication. This is necessary because DNA polymerase cannot fully replicate the 3' end of the lagging strand, leading to chromosome shortening with each cell division.
  2. By protecting the coding regions, telomeres allow cells to undergo many repeated cycles of mitosis and cell division without damaging essential genetic material.
  3. Furthermore, telomeres prevent the ends of chromosomes from being recognised as damaged DNA by the cell's repair mechanisms, which stops them from fusing with other chromosomes and thus maintains chromosomal stability.

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  How the marks are awarded

  • B1 — The answer correctly states that telomeres prevent the loss of genes or genetic information from the ends of chromosomes.
  • B1 — The answer links the function of telomeres to allowing cells to carry out many repeated mitoses or cell divisions.
  • B1 — The answer explains that telomeres prevent the fusion of chromosome ends or prevent them from being recognised as damaged.

  Common mistakes

  • Confusing telomeres with centromeres, stating they hold sister chromatids together.
  • Describing the role of the enzyme telomerase (which rebuilds telomeres) instead of the role of the telomere structure itself.
  • Vaguely stating that telomeres 'get shorter' without explaining the consequence, which is the prevention of gene loss.
  • Stating that telomeres contain genes, when they are in fact non-coding sequences.

  Examiner tip: For 'outline the role' questions, always state the direct function of the structure and then explain the biological importance or consequence of that function for the cell or organism.

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