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

17. Mutation, Repair, and Recombination

Types of Mutations

Genetics

17. Mutation, Repair, and Recombination

Types of Mutations

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Problem 11a

Textbook Question

Two different mutations are identified in a haploid strain of yeast. The first prevents the synthesis of adenine by a nonsense mutation of the ade-1 gene. In this mutation, a base-pair substitution changes a tryptophan codon (UGG) to a stop codon (UGA). The second affects one of several duplicate tRNA genes. This base-pair substitution mutation changes the anticodon sequence of a tRNAᵀʳᵖ from

3′−ACC−5′ to 3′−ACU−5′

Do you consider the first mutation to be a forward mutation or a reversion? Why?

Verified step by step guidance

Understand the definitions: A forward mutation is a change in the DNA sequence that creates a new mutant allele, while a reversion (or reverse mutation) restores the original function of a gene, either by correcting the original mutation or by compensating for its effects.

Analyze the first mutation: The nonsense mutation in the ade-1 gene changes a tryptophan codon (UGG) to a stop codon (UGA). This prevents the synthesis of adenine by prematurely terminating translation, which is a forward mutation because it introduces a new mutant allele.

Consider whether the mutation restores function: Since the nonsense mutation disrupts the function of the ade-1 gene and does not restore the original function, it cannot be classified as a reversion.

Evaluate the context of the question: The first mutation is a base-pair substitution that introduces a stop codon, which is consistent with the definition of a forward mutation. It does not reverse or compensate for any prior mutation.

Conclude: The first mutation is a forward mutation because it introduces a new change that disrupts the normal function of the ade-1 gene, rather than restoring its original function or compensating for a previous mutation.

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Key Concepts

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

Forward Mutation

A forward mutation refers to a change in the DNA sequence that alters a gene's function, typically resulting in a loss of function. In the context of the ade-1 gene mutation, the nonsense mutation introduces a premature stop codon, leading to the inability to synthesize adenine. This type of mutation is considered forward because it creates a new phenotype that differs from the wild type.

Reversion Mutation

A reversion mutation, or back mutation, occurs when a mutated gene returns to its original sequence, restoring the original function. This can happen through various mechanisms, such as a second mutation that corrects the first. In the case of the ade-1 gene, if a subsequent mutation restored the ability to synthesize adenine, it would be classified as a reversion.

Nonsense Mutation

A nonsense mutation is a specific type of point mutation where a single nucleotide change results in a premature stop codon in the mRNA sequence. This leads to the production of a truncated protein that is often nonfunctional. In the example provided, the change from a tryptophan codon to a stop codon in the ade-1 gene exemplifies a nonsense mutation, which significantly impacts the gene's function.

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Textbook Question

Two different mutations are identified in a haploid strain of yeast. The first prevents the synthesis of adenine by a nonsense mutation of the ade-1 gene. In this mutation, a base-pair substitution changes a tryptophan codon (UGG) to a stop codon (UGA). The second affects one of several duplicate tRNA genes. This base-pair substitution mutation changes the anticodon sequence of a tRNAᵀʳᵖ from

3′−ACC−5′ to 3′−ACU−5′

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Textbook Question

Two different mutations are identified in a haploid strain of yeast. The first prevents the synthesis of adenine by a nonsense mutation of the ade-1 gene. In this mutation, a base-pair substitution changes a tryptophan codon (UGG) to a stop codon (UGA). The second affects one of several duplicate tRNA genes. This base-pair substitution mutation changes the anticodon sequence of a tRNAᵀʳᵖ from

3′−ACC−5′ to 3′−ACU−5′

Do you consider the second mutation to be a forward mutation or a reversion? Why?

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