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Chapter 13: Problem 8

When the amino acid sequences of insulin isolated from different organisms were determined, differences were noted. For example, alanine was substituted for threonine, serine for glycine, and valine for isoleucine at corresponding positions in the protein. List the single-base changes that could occur in codons of the genetic code to produce these amino acid changes.

Short Answer

Expert verified
Question: List the possible single-base changes that could lead to the amino acid substitutions of alanine to threonine, serine to glycine, and valine to isoleucine. Answer: The possible single-base changes are: 1. Alanine to Threonine: - GCU -> ACU - GCC -> ACC - GCA -> ACA - GCG -> ACG 2. Serine to Glycine: - UCU -> UGU (Cysteine is a possible mutation) - UCC -> UGC (Cysteine is a possible mutation) - UCA -> GGA - UCG -> GGG - AGU -> GGU - AGC -> GGC 3. Valine to Isoleucine: - GUU -> AUU - GUC -> AUC - GUA -> AUA - GUG -> AUG (Methionine is a possible mutation)

Step by step solution

01

List the codons for the initial amino acids

First, we need to list the codons for the initial amino acids, alanine, serine, and valine. Alanine (Ala, A) is encoded by: GCU, GCC, GCA, GCG Serine (Ser, S) is encoded by: UCU, UCC, UCA, UCG, AGU, AGC Valine (Val, V) is encoded by: GUU, GUC, GUA, GUG
02

List the codons for the mutated amino acids

Next, we will list the codons for the mutated amino acids, threonine, glycine, and isoleucine. Threonine (Thr, T) is encoded by: ACU, ACC, ACA, ACG Glycine (Gly, G) is encoded by: GGU, GGC, GGA, GGG Isoleucine (Ile, I) is encoded by: AUU, AUC, AUA
03

Find the single-base changes that could lead to the amino acid changes

Now, we need to find the single-base changes in the codons that can lead to the given amino acid changes. 1. Alanine to Threonine: Change one base in the codon for alanine to convert it to a codon for threonine. - GCU -> ACU - GCC -> ACC - GCA -> ACA - GCG -> ACG 2. Serine to Glycine: Change one base in the codon for serine to convert it to a codon for glycine. - UCU -> UGU (UGU encodes for Cysteine, but this change is still a possible mutation) - UCC -> UGC (Likewise, UGC encodes for Cysteine) - UCA -> GGA - UCG -> GGG - AGU -> GGU - AGC -> GGC 3. Valine to Isoleucine: Change one base in the codon for valine to convert it to a codon for isoleucine. - GUU -> AUU - GUC -> AUC - GUA -> AUA - GUG -> AUG (AUG encodes for Methionine, but this change is still a possible mutation) So, these are the possible single-base changes that could lead to the given amino acid substitutions.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Amino Acid Sequence
The sequence of amino acids in a protein dictates its structure and function. This sequence is determined by the corresponding sequence of nucleotides in the DNA, which are transcribed into messenger RNA (mRNA) and then translated into amino acids during protein synthesis. Each amino acid is specified by a set of three nucleotides called a codon.

For instance, insulin, a hormone critical for regulating blood sugar levels, consists of a specific amino acid sequence. If a mutation occurs in the DNA or mRNA sequence, it can result in an amino acid substitution in the protein. A single amino acid change can significantly affect a protein's properties, potentially altering its ability to function correctly. This can be illustrated through the exercise example, where variations in the amino acid sequences of insulin from different organisms were observed, impacting its activity.
Codon Substitution
Codon substitution refers to the change of a single nucleotide in a codon, which can alter the amino acid for which that codon codes. Such mutations can be synonymous, causing no change in the amino acid sequence, or nonsynonymous, leading to an altered amino acid sequence. The latter was highlighted in the exercise, where single-base changes could result in alanine being replaced by threonine, serine by glycine, and valine by isoleucine.

It is essential to note that due to the redundancy of the genetic code, some amino acids are encoded by multiple codons, and not all substitutions will change the encoded amino acid. However, the exercise focuses on those specific changes where a single nucleotide alteration leads to a different amino acid, which illustrates the precision required in the genetic code for proper protein synthesis.
Protein Synthesis
Protein synthesis is a fundamental biological process involving two main stages: transcription and translation. During transcription, the genetic information in DNA is copied into mRNA. In translation, this mRNA is used as a template to assemble amino acids into a polypeptide chain, which then folds to form a functional protein. Each codon within the mRNA corresponds to a specific amino acid.

In the given exercise, understanding protein synthesis helps explain how mutations in DNA or mRNA lead to variations in amino acid sequences and, consequently, in protein function. It is the specific order of codons in mRNA that dictates the sequence of amino acids in a protein and therefore any change in the codon sequence due to mutation can have downstream effects, potentially resulting in the malfunction of the protein or an entirely altered activity.

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Most popular questions from this chapter

Recent observations indicate that alternative splicing is a common way for eukaryotes to expand their repertoire of gene functions. Studies indicate that approximately 50 percent of human genes exhibit alternative splicing and approximately 15 percent of disease-causing mutations involve aberrant alternative splicing. Different tissues show remarkably different frequencies of alternative splicing, with the brain accounting for approximately 18 percent of such events [Xu et al. (2002). Nucl. Acids Res. 30:3754-3766]. (a) Define alternative splicing and speculate on the evolutionary strategy alternative splicing offers to organisms. (b) Why might some tissues engage in more alternative splicing than others?

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis. (a) Can you find any support for the doublet code notion in the existing coding dictionary? (b) The amino acids Ala, Val, Gly, Asp, and Glu are all early members of biosynthetic pathways and are more evolutionarily conserved than other amino acids. They therefore probably represent "early" amino acids. Of what significance is this information in terms of the evolution of the genetic code? Also, which base, of the first two within a coding triplet, would likely have been the more significant in originally specifying these amino acids? (c) As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be latearriving amino acids. In addition, they are considered to have been absent in the abiotic Earth. All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?

Write a paragraph describing the abbreviated chemical reactions that summarize RNA polymerase-directed transcription.

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.

A glycine residue is in position 210 of the tryptophan synthetase enzyme of wild-type \(E\). coli. If the codon specifying glycine is GGA, how many single- base substitutions will result in an amino acid substitution at position \(210 ?\) What are they? How many will result if the wild-type codon is GGU?
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