17. Mutation, Repair, and Recombination

Induced Mutations

17. Mutation, Repair, and Recombination

Induced Mutations

3:24 minutes

Problem 34e

Textbook Question

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.
A double-stranded break that occurs during G₁ and prevents completion of DNA replication

Verified step by step guidance

Step 1: Identify the type of DNA damage described in the problem. A double-stranded break (DSB) is a severe form of DNA damage where both strands of the DNA helix are broken. This type of damage can prevent DNA replication and cell division if not repaired.

Step 2: Recognize the cell cycle phase mentioned in the problem. The damage occurs during the G₁ phase, which is the first gap phase before DNA replication begins in the S phase. This is important because the repair pathway used depends on the availability of a sister chromatid, which is only present after DNA replication.

Step 3: Determine the DNA repair pathway suitable for the G₁ phase. Since the cell is in G₁ and no sister chromatid is available, the Non-Homologous End Joining (NHEJ) pathway is typically used to repair double-stranded breaks during this phase. NHEJ directly ligates the broken DNA ends without requiring a homologous template.

Step 4: Explain the reasoning behind the choice of NHEJ. NHEJ is active throughout the cell cycle but is particularly important in G₁ because it does not rely on homologous recombination, which requires a sister chromatid. While NHEJ is error-prone and may lead to small insertions or deletions, it is the primary mechanism for repairing DSBs in G₁.

Step 5: Conclude by emphasizing the importance of repairing double-stranded breaks promptly. If left unrepaired, DSBs can lead to genomic instability, cell cycle arrest, or apoptosis. NHEJ ensures the cell can proceed to the next phase of the cycle despite the absence of a homologous template.

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Key Concepts

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

Double-Stranded Breaks (DSBs)

Double-stranded breaks are severe forms of DNA damage where both strands of the DNA helix are broken. This type of damage can lead to genomic instability if not repaired properly, as it can result in the loss of genetic information or chromosomal rearrangements. DSBs can occur due to various factors, including ionizing radiation, chemical agents, or during normal cellular processes like DNA replication.

Homologous Recombination (HR)

Homologous recombination is a critical DNA repair pathway that repairs double-stranded breaks using a homologous sequence as a template. This process is most active during the S and G₂ phases of the cell cycle when sister chromatids are available. HR is a precise repair mechanism that ensures the accurate restoration of the original DNA sequence, thus maintaining genomic integrity.

Non-Homologous End Joining (NHEJ)

Non-homologous end joining is a DNA repair pathway that directly ligates the broken ends of DNA without the need for a homologous template. This pathway is predominant in the G₁ phase of the cell cycle, making it crucial for repairing double-stranded breaks that occur before DNA replication. While NHEJ is faster and more efficient, it can lead to insertions or deletions at the repair site, potentially causing mutations.

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Textbook Question

A fragment of a wild-type polypeptide is sequenced for seven amino acids. The same polypeptide region is sequenced in four mutants.

Use the available information to characterize each mutant.

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Textbook Question

Experiments by Charles Yanofsky in the 1950s and 1960s helped characterize the nature of tryptophan synthesis in E. coli. In one of Yanofsky's experiments, he identified glycine (Gly) as the wild-type amino acid in position 211 of tryptophan synthetase, the product of the trpA gene. He identified two independent missense mutants with defective tryptophan synthetase at these positions that resulted from base-pair substitutions. One mutant encoded arginine (Arg) and another encoded glutamic acid (Glu). At position 235, wild-type tryptophan synthetase contains serine (Ser) but a base-pair substitution mutant encodes leucine (Leu). At position 243, the wild-type polypeptide contains glutamine and a base-pair substitution mutant encodes a stop codon. Identify the most likely wild-type codons for positions 211, 235, and 243. Justify your answer in each case.

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Textbook Question

Alkaptonuria is a human autosomal recessive disorder caused by mutation of the HAO gene that encodes the enzyme homogentisic acid oxidase. A map of the HAO gene region reveals four BamHI restriction sites (B1 to B4) in the wild-type allele and three BamHI restriction sites in the mutant allele. BamHI utilizes the restriction sequence 5′-GGATCC-3′. The BamHI restriction sequence identified as B3 is altered to 5′-GGAACC-3′ in the mutant allele. The mutation results in a Ser-to-Thr missense mutation. Restriction maps of the two alleles are shown below, and the binding sites of two molecular probes (probe A and probe B) are identified.

DNA samples taken from a mother (M), father (F), and two children (C1 and C2) are analyzed by Southern blotting of BamHI-digested DNA. The gel electrophoresis results are illustrated.

Using A to represent the wild-type allele and a for the mutant allele, identify the genotype of each family member. Identify any family member who is alkaptonuric.

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Textbook Question

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.

A cytosine that has been deaminated to uracil

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Textbook Question

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.

A double-strand break that occurs just after replication in an actively dividing cell

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Textbook Question

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.

A thymine dimer induced as a result of UV exposure

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Textbook Question

Common baker's yeast (Saccharomyces cerevisiae) is normally grown at 37°C, but it will grow actively at temperatures down to approximately 25°C. A haploid culture of wild-type yeast is mutagenized with EMS. Cells from the mutagenized culture are spread on a complete-medium plate and grown at 25°C. Six colonies (1 to 6) are selected from the original complete-medium plate and transferred to two fresh complete-medium plates. The new complete plates (shown) are grown at 25°C and 37°C. Four replica plates are made onto minimal medium or minimal plus adenine from the 25°C complete-medium plate. The new plates are grown at either 25°C or 37°C and the growth results are shown.

Classify the nature of the mutations in colonies 1, 2, and 5.

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Textbook Question

Common baker's yeast (Saccharomyces cerevisiae) is normally grown at 37°C, but it will grow actively at temperatures down to approximately 25°C. A haploid culture of wild-type yeast is mutagenized with EMS. Cells from the mutagenized culture are spread on a complete-medium plate and grown at 25°C. Six colonies (1 to 6) are selected from the original complete-medium plate and transferred to two fresh complete-medium plates. The new complete plates (shown) are grown at 25°C and 37°C. Four replica plates are made onto minimal medium or minimal plus adenine from the 25°C complete-medium plate. The new plates are grown at either 25°C or 37°C and the growth results are shown.

Which colonies are prototrophic and which are auxotrophic? What growth information is used to make these determinations?

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Show more practices

Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.A double-stranded break that occurs during G1 and prevents completion of DNA replication

Key Concepts

Double-Stranded Breaks (DSBs)

Homologous Recombination (HR)

Non-Homologous End Joining (NHEJ)

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Using your knowledge of DNA repair pathways, choose the pathway that would be used to repair the following types of DNA damage. Explain your reasoning.
A double-stranded break that occurs during G₁ and prevents completion of DNA replication