Transcript Chapter 14, Mutation and DNA repair

Chapter 14
Molecular
Mechanisms of
Mutation and
DNA Repair
Jones and Bartlett Publishers © 2005
Definitions
• Mutation = an inherited change in the
genetic material of an organism.
• Mutagens vs. teratogens
• Germline vs. somatic mutation
• Spontaneous vs. induced mutations
Types of Mutations
• Most common – base-pair substitutions
– Transition mutations – Pu to Pu, Py to Py.
• GA & A  G
• TC&CT
– Transversion mutations – Pu to Py or Py to Pu.
• A  T, A  C, G  T, or G  C.
• T  A, T  G, C  A, or C  G.
Types of Mutations
• Neutral mutations result when amino acid
substitutions do not change protein
function.
• Similar amino acids can be substituted for
each other – isoleucine for leucine.
• Missense mutations result when the amino
acid substitution changes protein function.
– Temperature sensitivity often is a missense
mutation.
Types of Mutations
• Nonsense mutations – change in codon to
UGA, UAA, or UAG.
• This results in premature stopping of
protein synthesis.
• They can be lethal or severe in phenotype.
Types of Mutations
• The 2nd most common type of mutations are
insertion/deletions of base pairs.
• This can cause frameshifts.
• Deletions or insertions of one or two bases
usually results in dramatic differences in
protein production.
Types of Mutations
• Why are these mutations important?
– They can affect mRNA and protein production,
eventually determining the phenotype.
• Silent mutations – produce no change in
amino acid sequence (due to degeneracy of
the genetic code.) (aka synonymous
mutations).
– CUU codes for leucine, but so does CUC, CUA,
CUG, UUA, and UUG.
A single base pair mutation in the b-globin gene
changes one amino acid in the coded protein
A base pair addition/deletion (frameshift) mutation
results in multiple amino acid changes
downstream from the point of mutation
Types of DNA Replication Errors
• Tautomeric shifts lead to mismatched
bases.
• DNA slippage – runs of the same base, or
repeated sequences.
• Depurination and deamination
The inheritance of the fragile-X syndrome
(caused by expansion of triplet repeats CGG)
With each passing generation, the number of triplets increases. Once
a threshold number of repeats is reached, the disease phenotype
becomes visible. In this pedigree, such a value is reached in
generation 3.
Triplet repeat mutations are created by
template slippage during DNA replication
followed by mismatch repair
Part of a messenger DNA containing a G quartet
Insertion of a transposon creates short direct repeats
in the target DNA flanking the inserted transposon
Definition of direct and inverted
repeat DNA sequences
Structure of a retrotransposon (retrovirus
genomes also have direct repeats at the ends)
Recombination between 2 transposons
in the same DNA molecule has different consequence
depending upon their relative orientation
Recombination between transposons in 2 different DNA
molecules can lead to duplication or deletion mutations
The technique of replica plating proves
that mutations arise spontaneously rather
than being induced by a selective agent
A method for detecting recessive lethals
on the X-chromosome in Drosophila
5-methylcytosines are hot
spots of mutation-1
The mechanism of
methylation of cytosine
in the
5-position
5-methylcytosines are hot
spots of mutation-2
Deamination of
cytosine leads to
uracil
while deamination
of 5-methylcytosine leads to
thymine
Uracil is not a
normal
component of
DNA and can be
recognized and
removed.
Thymine is a
normal
component of
DNA and is not
recognized as a
source of potential
mutation.
Excision repair of uracil in DNA
Mechanism of mutagenesis by the tautomerization
of the thymine analog 5-Bromouracil
The keto and enol
forms of DNA
bases
are called
tautomers. Both
thymine and
5-bromouracil can
assume these 2
alternative states.
Base analogs like 5-bromouracil can
induce mutations
Structure of 2 alkylating mutagens
Structure of a frameshift mutagen
It resembles
a base pair
Ultraviolet light causes joining (crosslinking)
of adjacent pyrimidine bases
Induction of mutations by radiation
is linearly related to exposure dose
Annual exposure of human beings in the
United States to various forms of ionizing radiation
People exposed to increased radiation from the
Chernobyl accident have a doubling of mutation rate
A low spontaneous mutation rate is achieved
by 3 successive accuracy-enhancing steps
Mechanism of post-replication mismatch correction
based on the methylation of the parental DNA strand
Steps in the excision repair of an
apurinic or apyrimidinic (AP) site
Repair of a bubble created in DNA
by the addition of a bulky agent
There is a mistake in this
diagram. The damaged
segment is displaced by a
DNA helicase (not a DNA
polymerase). During the
displacement of a strand by
a DNA polymerase, DNA
synthesis is concerted with
strand displacement and no
gap is formed.
Repair of a damage site by recombinational repair
Recombinational
repair is used
when excision
repair fails
Restoration of the wild type phenotype by a second
mutation compensating for the first mutation
A mutation that
rescues another
mutation is called
a suppressor
mutation
The Ames test for the detection of a
chemical mutagen
Most chemicals that
act as mutagens in
bacteria cause
cancer in animals
Chapter 15
• Cell cycle regulation via checkpoints
• Transcriptional activation via p53 protein.
• Sporadic (99%) vs. familial (1%) cancers
– Contact inhibition
– Cell senescence (associated with telomerase
activity; higher in cancer cells)
– Cancers are clonal.
Oncogene- assoc. with tumor progression