Ch. 8 - Molecular Biology of Transcription and RNA Processing

Sanders - Genetic Analysis: An Integrated Approach 3rd Edition

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Sanders 3rd Edition

Ch. 8 - Molecular Biology of Transcription and RNA Processing

Problem 22a

Chapter 8, Problem 22a

The human β-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are

Speculate about the way in which this base substitution causes mutation of β-globin protein.

Verified step by step guidance

Step 1: Understand the context of the problem. The β-globin gene is part of the hemoglobin protein, and mutations in this gene can lead to disorders such as β-thalassemia. The problem involves a single nucleotide substitution at the end of intron 2, which may affect RNA splicing.

Step 2: Analyze the sequences provided. The wild-type sequence at the end of intron 2 is 5′-CCUCCCACAG-3′, while the mutant sequence is 5′-CCUCCCACUG-3′. The substitution changes the last nucleotide of the intron from 'A' to 'U'.

Step 3: Recall the role of splice sites in RNA splicing. Splice sites are specific sequences at the boundaries of introns and exons that are recognized by the spliceosome. The 3′ splice site typically includes a conserved sequence ending in 'AG'.

Step 4: Consider the impact of the mutation. The substitution from 'A' to 'U' disrupts the conserved 'AG' sequence at the 3′ splice site. This may prevent the spliceosome from recognizing the site, leading to improper splicing of the pre-mRNA.

Step 5: Speculate on the consequences for the β-globin protein. Improper splicing could result in the retention of intron 2 in the mature mRNA or the use of a cryptic splice site. This would likely produce an aberrant β-globin protein with altered structure and function, potentially causing a disease phenotype such as β-thalassemia.

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Key Concepts

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

Base Pair Substitution

A base pair substitution is a type of mutation where one nucleotide in the DNA sequence is replaced by another. This can lead to changes in the corresponding mRNA and potentially alter the amino acid sequence of the resulting protein. Depending on the nature of the substitution, it can be classified as silent, missense, or nonsense, each having different implications for protein function.

Intron and Exon Structure

Introns and exons are components of eukaryotic genes, where exons are the coding sequences that are expressed in the final mRNA, while introns are non-coding sequences that are removed during RNA splicing. The presence of introns can influence gene expression and the final protein product, as mutations in introns can affect splicing efficiency and the inclusion of exons in the mature mRNA.

Protein Structure and Function

The structure of a protein is determined by the sequence of amino acids, which is in turn dictated by the mRNA sequence. Changes in the mRNA due to mutations can lead to alterations in the amino acid sequence, potentially affecting the protein's folding, stability, and function. In the case of β-globin, mutations can lead to disorders such as sickle cell disease if the protein's function in oxygen transport is compromised.

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Textbook Question

Wild-type E. coli grows best at 37°C but can grow efficiently up to 42°C. An E. coli strain has a mutation of the sigma subunit that results in an RNA polymerase holoenzyme that is stable and transcribes at wild-type levels at 37°C. The mutant holoenzyme is progressively destabilized as the temperature is raised, and it completely denatures and ceases to carry out transcription at 42°C. Relative to wild-type growth, characterize the ability of the mutant strain to carry out transcription at What term best characterizes the type of mutation exhibited by the mutant bacterial strain? (Hint: The term was used in Chapter 4 to describe the Himalayan allele of the mammalian C gene.)

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Textbook Question

A mutant strain of Salmonella bacteria carries a mutation of the rho protein that has full activity at 37°C but is completely inactivated when the mutant strain is grown at 40°C. Speculate about the kind of differences you would expect to see if you compared a broad spectrum of mRNAs from the mutant strain grown at 37°C and the same spectrum of mRNAs from the strain when grown at 40°C.

The human β-globin wild-type allele and a certain mutant allele are identical in sequence except for a single base-pair substitution that changes one nucleotide at the end of intron 2. The wild-type and mutant sequences of the affected portion of pre-mRNA are

This is one example of how DNA sequence change occurring somewhere other than in an exon can produce mutation. List other kinds of DNA sequence changes occurring outside exons that can produce mutation. In each case, characterize the kind of change you would expect to see in mutant mRNA or mutant protein.

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Textbook Question

Microbiologists describe the processes of transcription and translation as 'coupled' in bacteria. This term indicates that a bacterial mRNA can be undergoing transcription at the same moment it is also undergoing translation.

How is coupling of transcription and translation possible in bacteria?

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Textbook Question

A full-length eukaryotic gene is inserted into a bacterial chromosome. The gene contains a complete promoter sequence and a functional polyadenylation sequence, and it has wild-type nucleotides throughout the transcribed region. However, the gene fails to produce a functional protein. List at least three possible reasons why this eukaryotic gene is not expressed in bacteria.

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