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Summary Chapter 19 - Gene Mutations and DNA Repair
Chapter 19 of Genetics by Brooker.
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Chapter 19 - Gene mutation and DNA repair19.1 - Effects of mutations on gene structure and function
Gene mutations are molecular changes in the DNA sequence of a gene. A point
mutation is a change in a single base pair within the DNA. Several mutations can be a point
mutation:
- Base substitution: one base is substituted for another
- Transition: a change of a pyrimidine to another pyrimidine
- Transversion: a purine and a pyrimidine are interchanged
Gene mutations can alter the coding sequence within a gene:
- Silent mutations are those that do not alter the amino acid sequence of the
polypeptide even though the base sequence has changed.
- Missense mutations are base substitutions for which an amino acid change does
result.
- Nonsense mutations involve a change from a normal codon to a stop codon.
- Frameshift mutations involve the addition or deletion of a number of nucleotides that
is not divisible by 3.
Expect for silent mutations, new mutations are more likely to produce polypeptides that have
reduced rather than enhanced function. When a missense mutation has no detectable effect
on protein function, it is referred to as a neutral mutation.
Gene mutations can occur outside of a coding sequence and influence gene
expression. Promoter mutations that increase transcription are termed up promoter
mutations. A down promoter mutation causes the promoter to become less like the
consensus sequence, decreasing its affinity for transcription factors and decreasing the
transcription rate.
Gene mutations are also given names that describe how they affect the wild-type
genotype and phenotype:
- A wild type is relatively prevalent genotype. Many or most genes exist as multiple
alleles, so a population may have two or more wild types alleles.
- A mutation may change a wild type genotype by altering the DNA sequence of a
gene. When such a mutation is rare in a population, the result is generally referred to
as a mutant allele.
- A reverse mutation, more commonly called a reversion, changes a mutant allele back
to a wild type.
- A deleterious mutation decreases the chances of survival and reproduction
- A lethal mutation results in the death of a cell or organism
- A beneficial mutation enhances the survival or reproductive success of an organism.
Some mutations results in conditional mutants, in which the phenotype is affected only under
a defined set of conditions.
Suppressor mutations reverse the phenotypic effects of another mutation.
Suppressors or suppressor mutations are second site mutations, which may restore another
mutation:
- Intragenic suppressor: the second mutation site is within the same gene
, as the first → produces a change in protein structure that compensates for
an abnormality in protein structure caused by the first mutation.
- Intergenic suppressor: second mutation can occur in a different gene from
the first mutation → involve change in the expression of one gene that
compensates for a loss-of-function mutation affecting another gene.
Changes in chromosome structure can affect the expression of a gene. In some
cases, a chromosomal rearrangement may affect a gene because a chromosomal
breakpoint - a region where two chromosomes pieces break and rejoin with other
chromosome pieces- occurs within a gene. A breakpoint in the middle of a gene is very likely
to inhibit gene function because it separates the gene into two pieces. In other cases, a
gene may be left intact, but its expression may be altered when it is moved to a new
location. When this occurs, the change in gene location is to have a position effect.
Mutations can occur in germline or somatic cells. The term germ line refers to cells that
give rise to the gametes such as eggs and sperm. A germ-line mutation can occur directly in
a sperm or egg cell, or it can occur in a precursor cell that produces gametes. If a mutant
gamete participates in fertilization, all cells of the resulting offspring will contain the mutation.
The somatic cells are all cells of the body excluding the germ-line cells. A somatic mutation
has occurred within a single embryonic cell. An individual that has somatic regions that differ
genotypically from each other is called a genetic mosaic.
19.2 Random nature of mutations
The results of the Lederberg's supported the random mutation hypothesis, now known as
the random mutation theory. According to this theory, mutation is a random process - it can
occur in any gene and does not require the exposure of an organism to an environmental
condition that causes specific types of mutations to happen.
19.3 Spontaneous mutations
Geneticists categorize the causes of mutations in one of two way:
- Spontaneous mutations: changes in DNA structure that result from natural biological
or chemical processes.
- Induced mutations: caused by environmental agents
Spontaneous mutations can arise by depurination, deamination and tautomeric shifts.
- Depurination: involves the removal of a purine from the DNA. A mutation may result
during subsequent rounds of DNA replication. Because a complementary base is not
present to specify the incoming base for the new strand, any of the four bases are
added to the new strand in the region that is opposite the apurinic site.
- Deamination: involves the removal of an amino group from the cytosine base. This
produces a uracil. A mutation may result because uracil hydrogen bonds with
adenine during DNA replication. Therefore, if a DNA template strand has uracil
instead of cytosine, a newly made strand will incorporate adenine into the daughter
strand instead of guanine.
- Tautomeric shift: the tautomers are bases, which exist in keto and enol or amino and
imino forms. These forms can interconvert by a chemical reaction that involves the
migration of hydrogen atoms and a switch of a single bond and an adjacent double
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