Chapter 8
Section 8.7: Mutations
Objectives
• SWBAT distinguish between different types
of mutations.
• SWBAT explain why mutations may or may
not affect phenotype.
• SWBAT list factors that cause mutations.
Vocabulary
•
•
•
•
Mutation
Point mutation
Frameshift mutation
Mutagen
Mutations
• Mutation is a change in an organism’s DNA.
• There are many different types of mutations.
– Mutations that can affect a single gene occur
during replication.
– Mutations that affect a group of genes or an
entire chromosome happen during meiosis.
Mutations
• Base substitution: the replacement of one
base by another.
• The insertion or deletion of single bases. Such
mutations involve a “frame shift” in the
process of translation.
• Inversion of a section of DNA.
• Duplication or deletion of a section of DNA
Inversion Mutation
Mutations
• Most mutations that alter fitness, in an
evolutionary sense, will lower it.
• However, from time to time, a mutation will
be beneficial and its fitness will allow it to
increase in a population.
Types of Gene Mutations
• Point Mutation – a mutation in which one
nucleotide is substituted for another.
– An incorrect nucleotide is put in the place of the
correct one.
– Usually this type of error is caught by DNA
Polymerase’s proofreading but sometimes it will
slip through.
– A point mutation can sometimes, though not
always, be very significant (ex. sickle cell
anemia).
Types of Gene Mutations
• Frameshift Mutation – involves the insertion or
deletion of a nucleotide in the DNA sequence.
– This usually has greater effects because a frameshift
mutation shifts the entire sequence of base pairs
which follow it.
– An example of how it works:
• You start with the sequence THE CAT ATE THE RAT.
• If you remove the first E, then everything shifts to the left
and you read THC ATA TET HER AT. The meaning of the
sequence has been altered.
Any time the
reading frame
is shifted by
either a
deletion or
insertion, a
whole
nucleotide
sequence
loses its
meaning.
mutated
base
Which type of mutation do the two diagrams represent? Which
type of mutation is going to have the bigger impact?
Chromosomal Mutations
• During meiosis, homologous chromosomes
exchange DNA segments via crossing over.
– On occasion, homologous chromosomes do not
align properly and segments of different sizes are
swapped.
– This is called gene duplication because one
chromosome may have two copies of a gene or
genes while the other may have no copies of the
gene or genes.
Chromosomal Mutations
Chromosomal Mutations
• Translocation – a piece of one chromosome
moves to a non-homologous chromosome.
Mutations Effects
• Chromosomal mutations affect lots of genes
and tend to have a big effect on an organism.
• A mutation may break up a gene causing the
gene not to work, or it could make a new
hybrid gene with a new function (which might
turn out to be adaptive – or not).
• Translocated genes may also come under the
control of new promoters making the genes
more or less active than usual.
Mutations Effects
• Gene mutations can do a number of things
such as:
– Change amino acid sequences
– Change how proteins are folded – thus changing
their function
– Alter the active site of an enzyme so the enzyme
ceases to be a catalyst
– A gene substitution could also insert a premature
stop codon into a forming polypeptide chain.
Mutations Effects
• Some gene mutations do not affect an
organism’s phenotype.
– Recall that many codons code for the same amino
acid – thus some substitutions have no effect.
– For example, is AAG changes to AAA, the resulting
protein still has the correct amino acid, lysine.
– A mutation that does not affect the resulting
protein is called a silent mutation.
Mutations and Natural Selection
• Mutations occur in both body and germ cells
but they can be passed on to offspring only in
germ cells.
– Mutations occurring in germ cells are the
underlying source of genetic variation, which is
the basis for natural selection!
– Mutations in the germ line affect offspringphenotype and are often very harmful.
– Typically the phenotypes that result are less
adaptive, though not always.
Mutations and Natural Selection
– Natural selection typically removes the mutant
alleles (mutant genes) from the population.
– Occasionally, a mutation results in a more
adaptive phenotype, which are favored by natural
selection and increase in a population.
Factors Causing Mutation
• Mutagens – agents in the environment that
can change DNA.
– They speed up the rate of replication errors and
can even break DNA strands.
– Some are natural (ex. ultraviolet rays from the
sun) while others are human caused (toxic
pollution, agricultural herbicides and pesticides,
industrial chemicals, radioactivity, etc.).
– Mutagens are used to treat cancer (a positive)
Silent Spring