Explain how the enzyme telomerase maintains the integrity of chromosomes during DNA replication in eukaryotic cells.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In eukaryotic cells, the ends of linear chromosomes present a unique challenge during DNA replication. Without a specific mechanism, chromosome ends shorten with each round of replication, ultimately leading to the loss of coding sequences.
Model answer (4 marks)
1. DNA polymerase cannot copy the very end of the lagging strand because there is no free 3'-OH for the final RNA primer to be extended from, leaving an unreplicated 3' overhang and causing shortening of the lagging strand end after primer removal.
2. Telomeres are repetitive, non-coding sequences (e.g. TTAGGG in humans) that act as a buffer; their loss does not immediately erode coding sequences.
3. Telomerase is a reverse transcriptase that carries an RNA template complementary to the telomere repeat; it uses this template to extend the 3' end of the parental (template) strand.
4. The extension by telomerase creates a new 3' overhang to which primase can bind, allowing a primer to be synthesised and DNA polymerase to extend the complementary strand, thereby restoring telomere length and preventing net loss of chromosomal material.
2. Telomeres are repetitive, non-coding sequences (e.g. TTAGGG in humans) that act as a buffer; their loss does not immediately erode coding sequences.
3. Telomerase is a reverse transcriptase that carries an RNA template complementary to the telomere repeat; it uses this template to extend the 3' end of the parental (template) strand.
4. The extension by telomerase creates a new 3' overhang to which primase can bind, allowing a primer to be synthesised and DNA polymerase to extend the complementary strand, thereby restoring telomere length and preventing net loss of chromosomal material.
Examiner tips
- Use the exact terminology (telomerase, reverse transcriptase, RNA template, telomere repeat).
- Show the sequence of events: loss of 3' overhang → telomerase extension → new primer site → DNA polymerase action.
- Keep each point concise and directly linked to the marks.
Common mistakes
- Confusing the role of telomerase with DNA polymerase; telomerase extends the template strand, not the nascent strand.
- Omitting the fact that telomerase carries its own RNA template.
Mark scheme (4 marks)
- DNA polymerase cannot replicate the very end of the lagging strand because there is no free 3'-OH for the final primer to be extended from, leaving an unreplicated 3' overhang / causing shortening of the lagging strand end after primer removal.
- Telomeres are repetitive, non-coding nucleotide sequences (e.g. TTAGGG in humans) located at the ends of eukaryotic chromosomes that act as a buffer, so shortening does not immediately erode coding sequences.
- Telomerase is a reverse transcriptase that carries its own RNA template complementary to the telomere repeat sequence, which it uses to extend the 3' end of the parental (template) strand.
- Extension of the template strand by telomerase provides a new region to which primase can bind and synthesise a primer, allowing DNA polymerase to then synthesise the complementary strand, thereby restoring telomere length and preventing net loss of chromosomal material.
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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