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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1:30 minutes

Problem 11c

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′

Assuming there are no other mutations in the genome, will this double-mutant yeast strain be able to grow on minimal medium? If growth will occur, characterize the nature of growth relative to wild type.

Verified step by step guidance

Understand 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, which is essential for yeast growth on minimal medium. Without adenine, the yeast cannot grow unless the mutation is suppressed or adenine is supplemented.

Analyze the second mutation: The second mutation alters the anticodon of a tRNAᵀʳᵖ from 3′−ACC−5′ to 3′−ACU−5′. This change allows the tRNA to recognize the stop codon UGA and insert tryptophan during translation, potentially suppressing the nonsense mutation in the ade-1 gene.

Determine the effect of the second mutation: The modified tRNAᵀʳᵖ can suppress the nonsense mutation by inserting tryptophan at the UGA stop codon. This suppression may restore the production of a functional Ade-1 protein, enabling adenine synthesis.

Evaluate the growth of the double-mutant strain: If the suppression is effective, the double-mutant yeast strain will be able to synthesize adenine and grow on minimal medium. However, the growth may not be as robust as the wild type due to potential inefficiencies in suppression or other cellular effects of the modified tRNA.

Characterize the nature of growth: If growth occurs, it is likely to be slower or less efficient than the wild type because the suppression mechanism is not perfect, and the modified tRNA may interfere with normal translation processes. This would result in a phenotype that is partially rescued relative to the wild type.

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Key Concepts

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

Nonsense Mutation

A nonsense mutation is a type of point mutation where a single nucleotide change results in a premature stop codon in the coding sequence of a gene. This leads to the truncation of the protein, often rendering it nonfunctional. In the context of the ade-1 gene in yeast, the mutation prevents the synthesis of adenine, which is essential for growth on minimal medium.

tRNA and Anticodon

Transfer RNA (tRNA) is a type of RNA molecule that helps decode a messenger RNA (mRNA) sequence into a protein. Each tRNA has an anticodon that pairs with a corresponding codon on the mRNA. The mutation described changes the anticodon of a tRNAᵀʳᵖ, which may affect the translation of specific amino acids, potentially impacting protein synthesis and cellular function.

Minimal Medium Growth

Minimal medium is a growth medium that contains only the essential nutrients required for the growth of an organism, such as carbon sources, salts, and a nitrogen source. For yeast to grow on minimal medium, it must be able to synthesize all necessary metabolites, including adenine. The presence of the nonsense mutation in the ade-1 gene suggests that the double-mutant strain may not be able to grow on minimal medium unless the tRNA mutation compensates for the loss of adenine synthesis.

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Describe the purpose of the Ames test. How are his⁻ bacteria used in the Ames test? What mutational event is identified using his⁻ bacteria?

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In studies of the amino acid sequence of wild-type and mutant forms of tryptophan synthetase in E. coli, the following changes have been observed:

Determine a set of triplet codes in which only a single-nucleotide change produces each amino acid change.

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In numerous population studies of spontaneous mutation, two observations are made consistently: (1) Most mutations are recessive, and (2) forward mutation is more frequent than reversion. What do you think are the likely explanations for these two observations?

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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 first mutation to be a forward mutation or a reversion? Why?

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Textbook Question

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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Why are frameshift mutations likely to be more detrimental than point mutations, in which a single pyrimidine or purine has been substituted?

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What is the phenotypic effect of inserting a Ds element into the maize C gene? How do Ds and Ac produce maize kernels that are mostly yellow with purple spots?

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If you were to look up Gaucher disease on the OMIM website, you would see that there are three major types, designated Type I (OMIM 230800), Type II (OMIM 230900), and Type III (OMIM 231000). All three types are mutations of the gene for acid-β-glucosidase, encoded on chromosome 1. Different mutations of this gene produce the three types of Gaucher disease that differ somewhat in their symptoms and disease severity.

Thinking about the production or function of the acid-β-glucosidase enzyme, why do you suppose different mutations of this gene produce differences in symptoms and disease severity?

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