# Explain how the enzyme telomerase maintains the integrity of chromosomes during DNA replication in eukaryotic cells.

> IB DP Biology Higher Level (2023 syllabus) — D1.1 DNA replication · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. 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.
2. 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.
3. 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.
4. 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

- [telomerase](https://www.gradenine.co.uk/glossary/telomerase)

## Related

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-enzyme-telomerase-maintains-4af76c1f) · Published by Druglandscape Ltd.