Questions: The End-Replication Problem and Telomerase

Question 1

Why does the end-replication problem specifically create a gap at the end of the lagging strand rather than the leading strand?

• A. The leading strand polymerase has lower proofreading fidelity near chromosome ends
• B. The lagging strand's last RNA primer cannot be replaced because there is no upstream Okazaki fragment to extend from when the primer is removed
• C. Helicase cannot unwind the very last segment of the chromosome, blocking leading strand synthesis
• D. The lagging strand runs in the 3'-to-5' direction, making it impossible for DNA polymerase to synthesize toward the end

Answer: (B)

On the lagging strand, each Okazaki fragment is primed by an RNA primer. Normally, when a primer is removed, the gap is filled by extending from the adjacent upstream fragment. But at the very end of the chromosome, the final Okazaki fragment's RNA primer has no upstream fragment — there is nothing to extend from when the primer is removed. This leaves a short single-stranded gap that cannot be filled by any known mechanism of conventional DNA replication. The leading strand has no analogous problem because it is synthesized continuously from a single primer toward the replication fork; its end is already fully replicated before the fork reaches the chromosome terminus.

Question 2

A researcher discovers cancer cells that are dividing indefinitely. Genetic analysis shows the cells' chromosomes are not shortening with each division. Which mechanism most likely explains this?

• A. The cancer cells have evolved a new form of DNA polymerase that can replicate without RNA primers
• B. The cells have reactivated telomerase or are using the alternative lengthening of telomeres (ALT) mechanism
• C. The cancer cells have lost their telomeres entirely, allowing chromosomes to be joined end-to-end for stable replication
• D. Mutations in the DNA damage checkpoint allow the cells to ignore telomere shortening signals

Answer: (B)

Maintaining telomere length is the key enabling condition for indefinite division (replicative immortality). Most adult somatic cells do not express telomerase; their telomeres shorten with each division until replicative senescence halts division. Cancer cells must solve this problem to divide indefinitely. The vast majority reactivate telomerase expression, restoring the ribonucleoprotein enzyme that adds TTAGGG repeats to the 3' overhang. A minority use the ALT (alternative lengthening of telomeres) pathway, a recombination-based mechanism. Loss of telomeres entirely would actually be catastrophic — exposed chromosome ends are recognized as double-strand breaks, triggering DNA repair and dangerous chromosome fusions.

Question 3

Progressive telomere shortening in somatic cells is harmful because it gradually erodes the coding DNA sequences at chromosome ends.

• T. True
• F. False

Answer: False

This is a common misconception about why telomere shortening matters. Telomeres are composed of repetitive, non-coding DNA sequences (TTAGGG in humans), repeated thousands of times. There are no protein-coding genes within telomeres. The reason telomeres exist is precisely to serve as expendable buffers: each cell division erodes a small number of these non-coding repeats rather than coding DNA. When telomeres become critically short — after many divisions — the cell triggers senescence or apoptosis, long before any coding sequence is at risk. Telomeres protect genes from the end-replication problem, they do not contain genes.

Question 4

In most adult somatic cells, telomerase actively prevents chromosomes from shortening with each cell division.

• T. True
• F. False

Answer: False

Telomerase is not active in most adult somatic cells — it is primarily expressed in germ cells, embryonic stem cells, and certain adult stem cell populations. Most somatic cells have silenced telomerase expression, which means their telomeres shorten with each replication cycle. This deliberate suppression is thought to be a tumor-suppressor mechanism: cells that have undergone too many divisions (and may have accumulated mutations) are halted by replicative senescence rather than allowed to continue dividing. Cancer cells escape this mechanism by reactivating telomerase or using ALT.

Question 5

Why does the end-replication problem lead to progressive chromosome shortening with each cell division, and why doesn't this immediately destroy critical genetic information?

Model answer: The end-replication problem arises because DNA polymerase requires an RNA primer and can only synthesize 5'-to-3'. On the lagging strand, the final RNA primer at the chromosome's end cannot be replaced — there is no upstream fragment to extend from — leaving a gap. Each round of replication therefore produces a slightly shorter chromosome. This progressive shortening does not immediately destroy critical information because telomeres are long tracts of non-coding repetitive sequence (TTAGGG in humans, thousands of repeats) that cap each chromosome end. These repeats serve as an expendable buffer: successive divisions consume telomeric repeats rather than coding sequences. Only when telomeres are critically shortened does the cell trigger senescence, long before any gene is at risk.

This answer captures both the mechanistic origin of the problem (primer requirement, no upstream fragment at the end) and the evolutionary solution (non-coding buffers that are expendable). Telomerase provides the additional layer: it uses an internal RNA template to re-extend the 3' overhang, restoring what replication eroded. The biological logic — non-coding buffers protect coding DNA; telomerase restores the buffers; most somatic cells lack telomerase as a tumor-suppressor strategy — connects the molecular mechanism to aging and cancer.