This is a comprehensive and detailed practice material that contains questions and answers for Chapter 17 of Mastering Biology.
*Essential Study Material!!
12/10/2020 Chapter 17
Chapter 17
Due: 11:59pm on Monday, December 14, 2020
You will receive no credit for items you complete after the assignment is due. Grading Policy
Point Mutations
DNA polymerase is very accurate and rarely makes a mistake in DNA replication. Occasionally, however, an error in replication, known as a point mutation, is
introduced. There are two general categories of point mutations—frameshift mutations (also called base-pair insertions or base-pair deletions) and base
substitution mutations (shown in the diagram).
Part A - Base substitution mutations and frameshift mutations
If a segment of DNA were replicated without any errors, the replicated strand would have the following sequence of nucleotides:
5' - ACTACGTGA - 3'
Sort the following replicated DNA sequences by the type of point mutation each contains (frameshift, base substitution, or neither), as compared to the correct
sequence shown above.
Sort the items into the appropriate bins.
Hint 1. How to approach the problem
In order to determine the type of point mutation contained in a DNA sequence, it is helpful to write the mutated sequences below the correct sequence
so you can see the differences. The table below displays examples.
Correct Sequence ATGCAT
Substitution 1 ATCCAT
Substitution 2 ATGCTT
Frameshift 1 ATGGCAT
Frameshift 2 ATCAT
Frameshift mutations will change the number of nucleotides in a sequence; base substitution mutations will change the composition of the sequence
but not the number of nucleotides.
Hint 2. How many bases are in each mutated segment?
The correct DNA sequence contains nine bases. Which of the mutated sequences also contain(s) nine bases?
Select all that apply.
ANSWER:
Correct
A base substitution mutation can occur if the DNA polymerase inserts the wrong nucleotide base as it synthesizes a new strand of DNA. A frameshift
mutation can occur if the DNA polymerase leaves out a nucleotide or adds an extra nucleotide to the sequence. Certain forms of cancer occur
because of mutations in DNA sequences that are located in so-called mutational hotspots. These hotspots are locations in the DNA sequence where
mutations occur more often than in other places.
Part B - Types of base substitution mutations
When a base substitution mutation occurs, one nucleotide in a replicating DNA sequence is substituted for another, which results in the production of a mutant
strand of DNA. The result of the mutation depends on how the substituted nucleotide base alters the string of amino acids coded by the mutant DNA.
The three types of base substitution mutations are nonsense mutations, missense mutations, and silent mutations. Each type is defined by how it affects
protein synthesis.
U C A G
UUU UCU UAU UGU U
Phe Tyr Cys
UUC UCC UAC UGC C
U Ser
UUA UCA UAA Stop UGA Stop A
Leu
UUG UCG UAG Stop UGG Trp G
CUU CCU CAU CGU U
His
CUC CCC CAC CGC C
C Leu Pro Arg
CUA CCA CAA CGA A
Gln
CUG CCG CAG CGG G
AUU ACU AAU AGU U
Asn Ser
AUC Ile ACC AAC AGC C
A Thr
AUA ACA AAA AGA A
Met or
Lys Arg
AUG start ACG AAG AGG G
GUU GCU GAU GGU U
Asp
GUC GCC GAC GGC C
G Val Ala Gly
GUA GCA GAA GGA A
Glu
GUG GCG GAG GGG G
Label the four mutated DNA segments shown below according to the type of point mutation each represents. Use the codon table above to determine how
each mutation would affect the amino acid coding for each segment.
Drag the labels to their appropriate locations to identify the type of point mutation shown.
,12/10/2020 Chapter 17
Hint 1. How to approach the problem
All base substitution mutations alter one base within the DNA sequence, causing a new base pair to form as the DNA replicates. This change in turn
alters the mRNA sequence when the DNA is transcribed. The effect of this nucleotide substitution on the translation of the mRNA to a protein will
depend on what amino acid is now coded for by the altered base sequence. There are three possible outcomes that can result:
nonsense mutation: the new base changes an amino acid codon to a stop codon, ending protein synthesis and resulting in a shorter and
usually nonfunctional protein.
missense mutation: the new base leads to the insertion of a different amino acid than had been coded for, possibly altering the resulting
protein’s structure.
silent mutation: the new base does not change the amino acid sequence coded for by the original DNA strand.
You will have to refer to the table of amino acid codons to predict which of the three possible outcomes will occur for each mutant DNA segment.
Hint 2. How do the three types of base substitution mutations differ?
Drag the terms on the left to the appropriate blanks on the right to complete the sentences.
ANSWER:
Reset Help
1. A nonsense mutation leads to early termination of protein synthesis.
2. Protein synthesis ends when a stop codon is produced.
3. A change in any triplet of bases in the mRNA sequence may or may not affect the coding of a(n)
amino acid and change the structure of the synthesized protein.
4. A silent mutation results in the insertion of the same amino acid as was coded for by the original
nucleotide sequence.
5. Base substitution mutations alter a(n) nucleotide in the original DNA sequence.
6. A missense mutation can dramatically alter protein structure because it results in the insertion of
a different amino acid than was coded for by the original nucleotide sequence.
Correct
Point mutations in DNA sequences can profoundly affect protein synthesis, or they can have no effect at all. Point mutations can be beneficial to an
organism but are more commonly neutral or harmful.
Part C - Severity of point mutations
Generally speaking, which of the following mutations would most severely affect the protein coded for by a gene?
Hint 1. How to approach the problem
You know that each type of genetic mutation will affect the sequence of the gene, which will in turn affect the protein it codes for to varying extents. You
need to consider how base substitution mutations typically affect proteins compared to how frameshift mutations typically affect proteins.
Hint 2. Which mutation will affect an amino acid sequence the least?
Of the mutations listed below, which one will likely change the amino acid sequence of the protein coded by the gene the least?
ANSWER:
a base substitution at the beginning of the gene
a base substitution at the end of the gene
a frameshift deletion at the beginning of the gene
a frameshift deletion at the end of the gene
ANSWER:
a base substitution at the beginning of the gene
a base substitution at the end of the gene
a frameshift deletion at the beginning of the gene
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