Chapter 13 Mutation and DNA repair

Chapter 13 – Altering the Genetic Material: Mutation and
DNA Repair
Chapter Outline
1. Consequences of mutations
2. Causes of mutations
3. DNA repair
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13.1 Consequences of Mutations
Section 13.1 Learning
Outcomes
1. Describe different types of
point mutations
2. Outline how mutations in a
protein-coding gene may
affect the amino acid
sequence of a polypeptide
3. Explain how mutations
outside the coding sequence
can affect gene expression
4. Distinguish between
mutations in somatic cells
and germ-line cells
5. Classify mutations when
provided with examples

13.1 Consequences of Mutations
• A mutation is a heritable change in the genetic material
• Mutations are essential to the continuity of life; they are the
source of variation for natural selection
• New mutations are more likely to be harmful than beneficial
• DNA repair systems reverse most DNA damage before a
permanent mutation can occur
• Cancer is a disease caused
by gene mutations; cigarette
smoke contains chemicals
that mutate DNA and may
lead to lung cancer

13.1 Consequences of Mutations
Gene Mutations May Alter the DNA Sequence of a Gene
• A point mutation affects only a single base pair within the DNA
• A base substitution involves a change where one base is
replaced by another
• Ex: T (in top strand) was replaced by G, and corresponding
A (in bottom strand) was replaced with C
• A single base pair can be added or deleted and cause a point
mutation
• Ex: A single base pair (A-T) has been added to the sequence

13.1 Consequences of Mutations
Gene Mutations May affect the Amino Acid
Sequence of a Polypeptide
• A point mutation within
the coding region may
be classified as a silent
mutation, missense
mutation, nonsense
mutation, or frameshift
mutation based on the
impact on the
polypeptide

13.1 Consequences of Mutations
Mutations in the Coding Sequence of a Gene Are
Known to Cause Many Human Diseases
• People are affected by thousands of genetic diseases
• Some genetic disorders are
caused by a mutation in a
single gene while others
involve interactions among
multiple genes
• Mutations that are present in
germ cells can be passed from
parents to offspring
• Mutations can also occur in
the cells of a person’s body;
these mutations can cause
cancer but are not passed
from parents to offspring

13.1 Consequences of Mutations
Gene Mutations That Occur Outside of Coding
Sequences Can Influence Gene Expression
• Mutations within noncoding sequences can affect gene expression

13.1 Consequences of Mutations
Mutations Can Occur in Germ-Line or Somatic Cells
• The time and location of a
mutation determines its
severity and heritability
• Early in development
versus adult cell
• Germ-line cell versus
somatic cell
• Germ-line cells give rise to
gametes (sperm and egg
cells) and somatic cells are
all other body cells
• Only mutations in germ-
line cells can be passed
from parents to
offspring

13.2 Causes of Mutations
Section 13.2 Learning
Outcomes
1. Analyze the results of the
Lederbergs’ experiment,
and explain how they were
consistent with the random
theory of mutation
2. Discuss the difference
between spontaneous and
induced mutations, and
provide examples of each
3. Describe the Ames test for
determining if a substance
is a mutagen, and analyze
data that are generated
from the test

13.2 Causes of Mutations
The Lederbergs Used Replica Plating to Show That
Mutations Are Random Events
• The experiments of Joshua and Esther Lederberg addressed
questions about the cause of mutations
• They documented the presence of E. coli cells that were
resistant to T1 bacteriophage (a virus that infects bacteria)

13.2 Causes of Mutations
The Lederbergs Used Replica Plating to Show That
Mutations Are Random Events
• Data indicated that the mutations happened before exposure to
the virus, consistent with these mutations occurring randomly

13.2 Causes of Mutations
Mutations May Be Spontaneous or Induced
• In addition to spontaneous mutations that occur at a background
rate, various chemical and physical agents can induce mutations
• Mutations can be categorized as spontaneous or induced
• Spontaneous mutations result
from abnormalities in biological
processes (ex: mistake during
DNA replication)
• Background mutation rate
is ~1 mutation per 1 million
genes
• Induced mutations are caused
by environmental agents,
called mutagens, that alter the
structure of DNA
• Mutagens can be classified
as chemical or physical

13.2 Causes of Mutations
Mutagens Alter DNA Structure in Different Ways
• Chemical mutagens can cause covalent modifications, act as base
analogs, or cause distortion of the double helix
• Nitrous acid causes a covalent modification, removing –NH2 and
replacing it with =O, which changes C to U
• Base analogs have structures
that are similar to DNA bases,
but don’t pair correctly and
cause errors in DNA replication
• Mutagens that distort the helix
structure lead to additions and
deletions during replication

13.2 Causes of Mutations
Mutagens Alter DNA Structure in Different Ways
• Physical agents such as X-rays and UV light can damage DNA
structure
• Ionizing radiation (ex: X-rays) has high energy and penetrates
deeply to create free radicals
• Can cause deletions or breaks in one or both DNA strands
• Nonionizing radiation (ex: UV light)
has less energy and can only
penetrate the surface
• UV rays can cause thymine dimers
to form; if not repaired, these
dimers can cause gaps or
incorporation of incorrect bases

13.2 Causes of Mutations
Testing Methods Determine if an Antigen is a Mutagen
• The Ames test investigates whether a substance is a mutagen
• The Ames test uses
Salmonella
typhimurium that
cannot synthesize
histidine due to a
point mutation
• The Ames test
monitors the rate
at which a second
mutation (that
restores capacity
for histidine
synthesis) occurs

13.3 DNA Repair
Section 13.3 Learning
Outcomes
1. List the general features of
DNA repair systems
2. Describe the steps of
nucleotide excision repair
(NER)

13.3 DNA Repair
• All living organisms require the ability to repair damage to DNA in
order to minimize mutation
• Cells contain
several systems
that detect and
repair damage

13.3 DNA Repair
• Nucleotide excision repair (NER) is the most common DNA repair
system; it is found in all eukaryotes and prokaryotes
• The region encompassing several
nucleotides in the damaged strand
is removed
• The intact
strand is used
as a template
for resynthesis
of a normal
complementary
strand

Chapter 13 Summary
13.1 Consequences of mutations
• Gene mutations may alter the DNA sequence of a gene
• Mutations may affect the amino acid sequence of a polypeptide
• Mutations in the coding sequence of a gene are known to cause
many human diseases
• Gene mutations that occur outside of coding sequences can
influence gene expression
• Mutations can occur in germ-line or somatic cells
13.2 Causes of mutations
• Mutations may be spontaneous or induced
• Mutagens alter DNA structure in different ways
• Testing methods determine if an agent is a mutagen
13.3 DNA repair
• DNA repair systems consist of proteins that sense DNA damage
and repair it before a mutation occurs