5.05 Telomeres

Question 1

What are telomeres, and why are they essential for chromosome integrity? (5 marks)

Mark Scheme:

1. Telomeres are protective DNA sequences located at the ends of chromosomes. (1 mark)
2. They prevent the loss of essential genetic information during DNA replication. (1 mark)
3. DNA polymerase cannot fully replicate the ends of linear DNA, leaving a small section unreplicated. (1 mark)
4. Telomeres add non-coding DNA at chromosome ends, ensuring functional DNA is preserved. (1 mark)
5. Telomeres function like “plastic tips on shoelaces,” capping chromosome ends and maintaining stability. (1 mark)

Question 2

Explain the replication challenge at chromosome ends and how telomeres address it. (5 marks)

Mark Scheme:

1. DNA polymerase cannot replicate the very ends of linear DNA strands, leaving a small section unreplicated. (1 mark)
2. Without telomeres, essential genetic material would be lost with each round of replication. (1 mark)
3. Telomeres consist of repeated short DNA sequences, which are non-coding and sacrificial. (1 mark)
4. These sequences allow replication enzymes to complete duplication of functional DNA. (1 mark)
5. The enzyme telomerase replenishes telomeres in certain cells, ensuring chromosome integrity. (1 mark)

Question 3

Describe the role of telomerase in maintaining telomere length and its importance in cell division. (6 marks)

Mark Scheme:

1. Telomerase is an enzyme that adds extra bases to telomeres after DNA replication. (1 mark)
2. It replenishes the telomeres, preventing them from becoming too short. (1 mark)
3. This is crucial for actively dividing cells, such as stem cells, which need to divide indefinitely. (1 mark)
4. Telomerase ensures that essential genetic material is not lost, enabling continued cell division. (1 mark)
5. Differentiated cells, which lack telomerase, experience telomere shortening, leading to limited division potential. (1 mark)
6. Telomerase activity is often high in cancer cells, enabling uncontrolled cell division. (1 mark)

Question 4

What is the relationship between telomeres and ageing? (5 marks)

Mark Scheme:

1. Telomeres shorten with each cell division in non-dividing or differentiated cells that lack telomerase activity. (1 mark)
2. Over time, telomeres become critically short, exposing essential DNA and leading to cell death. (1 mark)
3. This telomere shortening is thought to contribute to the ageing process and natural cell death. (1 mark)
4. Anti-ageing research explores ways to prevent telomere loss, potentially slowing or reversing ageing. (1 mark)
5. Telomere length is often used as a biological marker for cellular ageing and lifespan. (1 mark)

Question 5

Compare the roles of telomeres in actively dividing and differentiated cells. (6 marks)

Mark Scheme:

1. In actively dividing cells (e.g., stem cells), telomerase replenishes telomeres, allowing indefinite division. (1 mark)
2. These cells maintain telomere length, ensuring essential DNA is not lost. (1 mark)
3. In differentiated cells, telomerase is usually inactive, leading to gradual telomere shortening with each division. (1 mark)
4. This shortening limits the number of divisions, contributing to cell death and ageing. (1 mark)
5. Actively dividing cells rely on telomeres for long-term division potential, while differentiated cells rely on telomeres to protect DNA for their limited lifespan. (1 mark)
6. Telomere dynamics reflect the differing needs for proliferation and stability in these cell types. (1 mark)

Question 6

Why is telomerase activity high in cancer cells, and what are the implications? (5 marks)

Mark Scheme:

1. Cancer cells exhibit high telomerase activity, enabling them to replenish telomeres after each division. (1 mark)
2. This prevents telomere shortening, allowing cancer cells to divide indefinitely. (1 mark)
3. High telomerase activity is a key factor in the uncontrolled proliferation of cancer cells. (1 mark)
4. Targeting telomerase activity is a potential strategy for cancer therapy, aiming to limit cell division. (1 mark)
5. However, reducing telomerase activity must be balanced to avoid adverse effects on normal stem cells that also rely on telomerase. (1 mark)

Question 7

Explain how telomeres act as “plastic tips on shoelaces” for chromosomes. (4 marks)

Mark Scheme:

1. Telomeres cap the ends of chromosomes, protecting them from degradation and fusion with other chromosomes. (1 mark)
2. Like plastic tips on shoelaces, telomeres prevent the fraying of DNA ends. (1 mark)
3. This protection ensures the stability of genetic material during replication and cell division. (1 mark)
4. Telomeres allow functional DNA to be preserved, maintaining chromosome integrity over multiple cell divisions. (1 mark)

Question 8

Discuss the clinical significance of telomeres and telomerase in ageing and disease. (6 marks)

Mark Scheme:

1. Telomere shortening is associated with ageing, as cells lose their ability to divide and function. (1 mark)
2. This contributes to age-related conditions, such as weakened immune function and tissue degeneration. (1 mark)
3. Cancer cells often reactivate telomerase, enabling them to divide indefinitely and evade normal cellular limits. (1 mark)
4. Telomerase inhibitors are being developed as potential cancer treatments to limit tumor growth. (1 mark)
5. In contrast, telomerase activation in normal cells could potentially slow ageing, but this poses risks of promoting cancer. (1 mark)
6. Understanding telomere dynamics offers opportunities for therapies targeting ageing and disease processes. (1 mark)

Question 9

What happens to a cell if telomeres become critically short? (4 marks)

Mark Scheme:

1. If telomeres become critically short, essential DNA sequences at the chromosome ends are exposed. (1 mark)
2. This leads to genomic instability, as chromosomes may degrade or fuse with others. (1 mark)
3. Cells typically enter senescence (a state of permanent growth arrest) or undergo apoptosis (programmed cell death). (1 mark)
4. This mechanism prevents the propagation of damaged cells but contributes to ageing and tissue decline. (1 mark)

Question 10

How do telomeres balance cell longevity and genomic stability? (5 marks)

Mark Scheme:

• Telomeres protect chromosome ends, preventing degradation and maintaining genomic stability. (1 mark)
• By shortening with each division, they limit the lifespan of differentiated cells, reducing the risk of accumulating mutations. (1 mark)
• In stem cells, telomerase maintains telomere length, allowing these cells to divide indefinitely for tissue repair and regeneration. (1 mark)
• This balance ensures that proliferative capacity is maintained without compromising genetic integrity. (1 mark)
• Telomere dysfunction or excessive shortening disrupts this balance, contributing to ageing or cancer. (1 mark)