Download Some mutations affect a single gene, while others affect an entire

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8.7
6E
Mutations
Key Concept Mutations are changes in DNA that
may or may not affect phenotype.
Some mutations affect a single gene, while
others affect an entire chromosome.
In biology a mutation means a change in an organism’s DNA. A mutation can happen during replication and affect a single gene. A mutation
can also happen during meiosis and affect a whole chromosome.
Gene Mutations
DNA
A mutation is a change in an organism’s DNA.
Normal
Point m
DNA
G A T C T C A G G C T A
DNA
mRNA
C U A G A G U C C G A U
mRNA
protein
Leu
Glu
mutated
G A T C T A A G G C T A
G A T C T T C A G G C T A base
mRNA
mRNA
C U A G A U U C C G A U
C U A G A A G U C C G A U
protein
protein
Leu
Leu
Asp
Glu
Ser
Val
added
A base
T
Asp
Arg
Frameshift mutation (deletion)
G A T C T C A G C T A A
added base
What is the difference between a gene mutation and a
chromosomal mutation?
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Asp
DNA
DNA
Errors in meiosis can result in changes in large parts of a chromosome.
mRNA
mRNA
T
C U A G A A G U C C G A U
C U A G A G
Recall that crossing over is a normal process in which chromosomes
exchange pieces. Errors in crossing over or in other parts of meiosis can
protein
protein
result in chromosomes
with
two copies
of the same gene. PiecesLeu
of Glu
Val
Arg
Leu
Glu
non-homologous chromosomes might even be exchanged.
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Ser
Point
mutation
Frameshift
mutation (insertion)
DNA
G A T C T T C A G G C T A
Chromosomal Mutations
Types of Mutations
U C G A U U
Ser
Ile
deleted
base
G
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There are different types of gene mutations.
• A point mutation is when an incorrect
nucleotide is put into a DNA molecule
during replication. If the error is not fixed
by DNA polymerase, the DNA is perma­
nently changed. For example, the figure to
the right shows a CTC codon that is changed
to a CTA codon. As a result, the wrong
amino acid is added—aspartic acid instead
of glutamic acid.
• A frameshift mutation is the addition
or removal of a nucleotide in the DNA
Normal
sequence. This results in a change in the
A T C Recall
DNA
G importance
G C T A
T C Athe
readingGframe.
of the
reading frame from Section 8.5. Think back
to the sentence
RAT.” If
mRNA
C U A G “THE
C C ATE
A G UCAT
G A THE
U
the letter E is removed, or deleted, from the
first “THE,” the reading frame is shifted. The
protein
Leu
Glu TET
SerHERAsp
result is “THC
ATA
AT…” The
reading frame is also shifted if a nucleotide is
Frameshift mutation (insertion)
added, or inserted.
protein
Frames
DNA
mRNA
protein
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Mutations may or may not affect phenotype.
Whether a mutation affects an organism depends on many different
things.
• Type of mutation A point mutation only affects one codon. A
frameshift mutation usually has a bigger effect because it changes the
whole reading frame and can affect many codons.
U G C
• Impact on the amino acid sequence A change in one codon can
still have a big effect. For example, if a codon for an amino acid is
changed into a stop codon, transcription would end at the wrong
U G C cysteine
place. A point mutation may also have no effect. Recall that more
than one codon can code for the same amino acid. For example,
CGU, CGC, CGA, and CGG all code for arginine. A point mutation that changes the last nucleotide of this codon would have no
U G A stop
effect on the resulting amino acid.
• Impact on the resulting protein Some changes might not affect
U G A
the resulting protein’s shape or function. Other changes might
This point mutation changed
prevent the protein from functioning. For example, a mutation
could change the active site of an enzyme and prevent the enzyme a codon for cysteine into a
stop codon.
from binding to its substrate.
• Type of cell Recall that mutations that occur in germ cells can be
passed on to offspring. Mutations in body cells cannot be passed on
to offspring.
Give one example of a mutation that would not affect an
organism’s phenotype.
Mutations can be caused by several factors.
Mutations happen. But cells have tools to repair them. For example,
DNA polymerase has a “proofreading” function to fix errors. However,
mutations can happen faster than the body’s repair system can work.
Some mutations are the result of errors that happen normally in the cell.
Other mutations are caused by things in the environment.
• Replication errors DNA polymerase proofreads replication, but a
small number of errors are not fixed. Over time, there are more and
more errors. Eventually these mutations affect how the cell works.
There is evidence that a build-up of mutations is a major cause
of aging.
Interactive Reader
147
• Mutagens Some conditions and substances in the environment
can cause DNA mutations—such as UV light and some chemicals.
Things in the environment that can change DNA are called
mutagens.
If mutations cause changes that affect the control over cell division,
cancer may result.
What are two examples of mutagens?
8.7 Vocabulary Check
mutation
point mutation
frameshift mutation
mutagen
Mark It Up
Go back and highlight
each sentence that
has a vocabulary
word in bold.
2. What is the difference between a point mutation and a frameshift
mutation?
8.7 The Big Picture
3. Is a mutation that happens during meiosis likely to affect a single gene
or an entire chromosome? Explain.
4. Give an example of one mutation that will affect an organism’s phenotype and one mutation that will not affect phenotype.
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1. What is the difference between a mutation and a mutagen?