Telomere Replication

Summary

The replication of linear chromosomes presents a unique challenge at their ends, where the lagging strand cannot be fully replicated. DNA polymerases can only elongate from a free three-prime hydroxy group, and without one at the chromosome's end, the strand cannot be completed. This leads to chromosome shortening with each cycle. Telomerase, an enzyme that adds repetitive sequences to chromosome ends, solves this issue. It extends the parental strand, allowing DNA polymerase to complete the lagging strand and fully replicate the chromosome, preserving genetic information at the ends of chromosomes.

Takeaways

• 😀 The ends of linear chromosomes face unique problems during DNA replication.
• 😀 DNA polymerases can only elongate DNA from a free 3' hydroxyl group.
• 😀 The lagging strand is synthesized in segments, requiring RNA primers at intervals.
• 😀 The leading strand is synthesized continuously in the 5' to 3' direction.
• 😀 The lagging strand cannot fully replicate the chromosome ends due to the lack of a 3' hydroxyl group at the very end.
• 😀 Even with an RNA primer at the end of the chromosome, the lagging strand cannot be completed.
• 😀 The issue of chromosome shortening during replication is known as the 'end replication problem.'
• 😀 Telomerase solves the end replication problem by adding repetitive sequences to the chromosome ends.
• 😀 Telomerase uses an RNA template to extend the parental strand in the 5' to 3' direction.
• 😀 The lagging strand is completed by DNA polymerase alpha, which includes a DNA primase subunit.

Q & A

What problem do linear chromosomes face during DNA replication?

Linear chromosomes face the problem of replicating their ends during DNA replication because DNA polymerases can only elongate from a free three prime hydroxy group, which is not available at the chromosome ends.

Why does the lagging strand stop short during DNA replication?

The lagging strand stops short because even if an RNA primer is placed at the very end of the chromosome, the strand still cannot be completed. The final RNA primer would need to be removed, leaving no three prime hydroxy group to prime DNA synthesis at the chromosome ends.

How does the leading strand differ from the lagging strand during DNA replication?

The leading strand is synthesized continuously in the 5' to 3' direction all the way to the end of the template, while the lagging strand is synthesized in small fragments and stops short at the end of the chromosome.

What is the role of RNA primers in DNA replication?

RNA primers provide three prime hydroxy groups at regular intervals along the lagging strand template, enabling DNA polymerase to initiate the synthesis of the new strand.

What is the 'end replication problem'?

The end replication problem refers to the inability to fully replicate the ends of linear chromosomes during each cell division, leading to progressive shortening of the chromosomes over time.

How does the enzyme telomerase solve the end replication problem?

Telomerase solves the end replication problem by recognizing the repeat sequences at the chromosome ends (telomeres), elongating the parental strand in the 5' to 3' direction, and adding additional repeats to extend the chromosome ends.

What is the role of telomerase in the replication of chromosome ends?

Telomerase uses an RNA template to add repetitive DNA sequences to the ends of chromosomes, extending them and compensating for the loss that occurs during DNA replication.

What is the function of DNA polymerase alpha in completing the lagging strand?

DNA polymerase alpha, which carries DNA primase as one of its subunits, completes the lagging strand after telomerase elongates the parental strand. It synthesizes the DNA to fill in the gaps created during lagging strand synthesis.

Why is there no three prime hydroxy group available at the ends of chromosomes?

At the very ends of chromosomes, there is no three prime hydroxy group because the RNA primers used to initiate DNA synthesis cannot be replaced at the very tip of the chromosome, resulting in an incomplete replication of the final segment of DNA.

What happens to chromosomes if the end replication problem is not solved?

If the end replication problem is not solved, chromosomes would progressively shorten with each replication cycle, potentially leading to the loss of essential genetic information and cellular dysfunction.