Table of contents

1. Introduction to Genetics51m
2. Mendel's Laws of Inheritance3h 37m
3. Extensions to Mendelian Inheritance2h 41m
4. Genetic Mapping and Linkage2h 28m
5. Genetics of Bacteria and Viruses1h 21m
6. Chromosomal Variation1h 48m
7. DNA and Chromosome Structure56m
8. DNA Replication1h 10m
9. Mitosis and Meiosis1h 34m
10. Transcription1h 0m
11. Translation58m
12. Gene Regulation in Prokaryotes1h 19m
13. Gene Regulation in Eukaryotes44m
14. Genetic Control of Development44m
15. Genomes and Genomics1h 50m
16. Transposable Elements47m
17. Mutation, Repair, and Recombination1h 6m
18. Molecular Genetic Tools19m
- Genetic Cloning
  9m
- Methods for Analyzing DNA
  9m
19. Cancer Genetics29m
- Overview of Cancer
  21m
- Cancer Mutations
  7m
20. Quantitative Genetics1h 26m
21. Population Genetics50m
- Hardy Weinberg
  19m
- Allelic Frequency Changes
  31m
22. Evolutionary Genetics29m

8. DNA Replication

Telomeres and Telomerase

Genetics

8. DNA Replication

Telomeres and Telomerase

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2:16 minutes

Problem 11a

Textbook Question

There is a problem completing the replication of linear chromosomes at their ends. Describe the problem and identify why telomeres shorten in each replication cycle.

Verified step by step guidance

Understand the structure of linear chromosomes: Linear chromosomes have ends called telomeres, which are repetitive DNA sequences that protect the chromosome from degradation and prevent the loss of essential genetic information during replication.

Review the mechanism of DNA replication: DNA polymerase synthesizes new DNA strands in the 5' to 3' direction. However, it requires a primer to initiate synthesis and cannot fully replicate the very ends of the lagging strand due to the removal of the RNA primer.

Explain the end-replication problem: During replication, the RNA primer at the very end of the lagging strand is removed, but DNA polymerase cannot fill in the gap because there is no upstream DNA to provide a starting point. This results in the progressive shortening of the chromosome ends with each replication cycle.

Describe the role of telomeres: Telomeres act as a buffer zone, absorbing the loss of DNA sequences during replication. However, as cells divide repeatedly, telomeres become progressively shorter, eventually leading to cellular senescence or apoptosis when they reach a critically short length.

Introduce the enzyme telomerase: In certain cell types, such as germ cells and stem cells, the enzyme telomerase can extend telomeres by adding repetitive sequences to the ends of chromosomes, counteracting the shortening process and maintaining chromosome integrity.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Telomeres

Telomeres are repetitive nucleotide sequences located at the ends of linear chromosomes. They protect the chromosome from deterioration or fusion with neighboring chromosomes. Each time a cell divides, a portion of the telomere is not replicated, leading to gradual shortening over successive replication cycles.

DNA Replication

DNA replication is the process by which a cell duplicates its DNA before cell division. It involves unwinding the double helix and synthesizing new strands complementary to the original ones. However, due to the nature of DNA polymerase, which cannot fully replicate the ends of linear chromosomes, telomeres are progressively shortened with each replication.

Hayflick Limit

The Hayflick limit refers to the number of times a normal somatic human cell can divide before cell division stops, which is typically around 40-60 divisions. This limit is largely due to telomere shortening, as critically short telomeres trigger cellular senescence or apoptosis, preventing further cell division and contributing to aging.

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Related Practice

Multiple Choice

Which cell type contains the most telomerase

Telomeres are composed of what type of DNA molecule?

In the discussion, we focused on how DNA is organized at the chromosomal level. Along the way, we found many opportunities to consider the methods and reasoning by which much of this information was acquired. From the explanations given in the chapter, what answers would you propose to the following fundamental questions:

How do we know that satellite DNA consists of repetitive sequences and has been derived from regions of the centromere?

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Textbook Question

There is a problem completing the replication of linear chromosomes at their ends. What is the function of telomerase, and how does it operate to synthesize telomeres?

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Textbook Question

In 1994, telomerase activity was discovered in human cancer cell lines. Although telomerase is not active in most human adult cells, all cells do contain the genes for telomerase proteins and telomerase RNA. Since inappropriate activation of telomerase may contribute to cancer, why do you think the genes coding for this enzyme have been maintained in the human genome throughout evolution? Are there any types of human body cells where telomerase activation would be advantageous or even necessary? Explain.

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Textbook Question

Telomeres are found at the ends of eukaryotic chromosomes. Why is telomerase usually active in germ-line cells but not in somatic cells?

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Textbook Question

Telomeres are found at the ends of eukaryotic chromosomes. What is the functional role of telomeres?

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Textbook Question

Telomeres are found at the ends of eukaryotic chromosomes. How does telomerase assemble telomeres?