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Mutations
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Terms and concepts
• clone
– population of cells that are genetically
identical
• genome
– all genes present in a cell or virus
• haploid – one set of genes
• diploid – two sets of genes
• genotype
– specific set of genes an organism possesses
• phenotype
– set of observable characteristics
2
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Mutation
• Change in the genetic material
• Mutations may be neutral, beneficial, or harmful
• Mutagen: Agent that causes mutations
• Spontaneous mutations: Occur in the absence of a
mutagen
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Mutation
• Mutations
• Transformation
• Transduction
• Conjugation
• Jumping Genes
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Mutation
• Base substitution
(point mutation,
Neutral mutation)
• Missense mutation
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•Change in one base
•Result in possible
change in amino acid
Figure 8.17a, b
Mutation
• Nonsense mutation
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• Results in a nonsense
codon
Figure 8.17a, c
Mutation
• Frameshift mutation
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•Insertion or deletion of one
or more nucleotide pairs
Figure 8.17a, d
Mutation
• Ionizing radiation (X rays and gamma rays) causes
the formation of ions that can react with
nucleotides and the deoxyribose-phosphate
backbone.
• Nucleotide excision repairs mutations
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Mutation
• UV radiation
causes thymine
dimers
• Light-repair
enzymes
separates thymine
dimers through
photoreactivation
• SOS repair for
extremely
damaged DNA
(this process is
error proned).
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Figure 8.20
The Frequency of Mutation
• Spontaneous mutation rate = 1 in 109 replicated
base pairs or 1 in 106 replicated genes
• Mutagens increase to 10–5 or 10–3 per replicated
gene
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Selection
• Positive (direct) selection detects mutant cells
because they grow or appear different.
• Negative (indirect) selection detects mutant cells
because they do not grow.
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Induced Mutations
• caused by chemical or physical agents that damage
or alter the chemistry of DNA, or that interfere with
DNA repair mechanisms
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Base analogs
• similar to
nitrogenous
bases
• incorporated
into DNA during
replication
• have different
base-pairing
characteristics
Figure 11.27a
17
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Specific mispairings
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• occur when
mutagen
changes base’s
structure and
pairing
characteristics
– e.g., alkylating
agents
Figure 11.28
18
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DNA-damaging agents
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• severely
damage DNA so
that it can’t
serve as
template for
replication
• repair
mechanisms
allow survival,
but also cause
mutations
19
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thymine
dimer
Figure 11.29
Caused by uv light
Replica Plating
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Figure 8.21
The Ames Test for Chemical Carcinogens
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Figure 8.22
Genetic Transfer and Recombination
• Vertical gene transfer •Occurs during
reproduction, between
generations of cells
• Horizontal gene
transfer
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•Transfer of genes
between cells of the same
generation
Genetic Recombination
• Exchange of
genes between
two DNA
molecules
• Crossing over
occurs when
two
chromosomes
break and
rejoin
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Figure 8.23
Recombination
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Figure 8.25
Transformation
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Figure 8.24
Transformation
DNA binding
protein
nuclease – nicks and degrades one
strand
competence-specific
protein
Figure 13.17
Streptococcus pneumoniae
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Transduction
Phage protein coat
Bacterial
chromosome
Recombinant
1 A phage infects the
donor bacterial
cell.
2 Phage DNA and proteins
are made, and the
bacterial chromosome is
broken down into pieces.
Bacterial
DNA
Donor
bacterial
DNA
Phage
DNA
3 Occasionally during phage assembly,
pieces of bacterial DNA are packaged
in a phage capsid. Then the donor cell
lyses and releases phage particles
containing bacterial DNA.
Recipient cell
4 A phage carrying bacterial
DNA infects a new host cell,
the recipient cell.
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Recipient
bacterial
DNA
Recombinant cell
5 Recombinant can occur,
producing a recombinant
cell with a genotype
different from both the
donor and recipient cells.
Figure 8.28
Transduction
Figure 13.20
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Specialized Transduction
• also called restricted transduction
• carried out only by temperate phages that
have established lysogeny
• only specific portion of bacterial genome is
transferred
• occurs when prophage is incorrectly excised
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Specialized
transduction
Lysogeny
Figure 13.20
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Conjugation
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Figure 8.27a
Conjugation
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Figure 8.27b
Conjugation
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Figure 8.27c
Plasmids
• Conjugative plasmid
Carries genes for sex
pili and transfer of the
plasmid
• Dissimilation plasmids
Encode enzymes for
catabolism of unusual
compounds
• R factors
Encode antibiotic
resistance
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Plasmids
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Figure 8.29
Transposons
• Segments of DNA
that can move from
one region of DNA
to another
• Contain insertion
sequences for
cutting and resealing
DNA (transposase)
• Complex
transposons carry
other genes
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Figure 8.30a, b