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MUTATIONS
And their effect
Mutations
 This is any change in the DNA sequence
that is not immediately and properly
repaired, or any inherited change in the
sequence of DNA.
 Mutations that occur in the body cells cause
cell death or cancer, and are not passed on
to the next generation.
 Mutations are usually recessive and are
inherited in a Mendelian way.
Mutations
 The majority of mutations are
disadvantageous.
 Mutations may result from mistakes in DNA
replication, or may be caused by mutagenic
agents.
 Mutations occur spontaneously. (A normal
spontaneous mutation rate for a single gene
is 1 mutation in 106 – 108 replications, this
may be increased by mutagenic agents.)
Mutations
 There are 2 main types of mutations:
– Gene Mutations – a change in a nucleotide
coding.
– Chromosome Mutations – these can effect:
 The number or arrangement of genes on the
chromosomes.
 The number of chromosomes.
 The number of sets of chromosomes.
DNA Repair
 There are several complex DNA repair
systems in the body. Some mend specific
problems, e.g. the damage caused by UV
radiation.
 In rare cases, a human is born without the
enzyme system that repairs this damage,
these people have to live out of the sun.
DNA Repair
 Usually enzymes can tell when there is a
wrong base pairing, e.g. A with G.
 The repair enzymes go to work, cutting out a
chunk from one side of the DNA containing
the mistake. Then new enzymes, using the
base pairing code, make a new side from
the opposite strand of DNA.
 The new strand is put into place by another
enzyme system.
Mutagenic Agents
Ionising Radiation
This sometimes causes simple base changes by
substituting one base for another, (this may or
may not have a bad effect.)
It can also cause deletions of large portions of
genetic material.
Mutagenic agents cont
Ultra-violet light
This can cause 2 adjacent thymine bases to bond
to each other to form a thymine dimer. An
unrepaired dimer inactivates the strand of DNA, so
it can not replicate or make mRNA and thus
protein.
Mutagenic agents cont
X-rays
The effect of x-rays is cumulative. An overdose of xrays causes more harmful mutations.
e.g. Muller’s experiments with Drosophila
ref pg 136 txtbk
Mutagenic agents cont
Chemicals
Mustard gas, formaldehyde, nitrous acids,
epoxides, acridine dyes and alkylating
agents can all cause mutations.
Nitrous acid, found in cigarette smoke
changes cytosine to uracil. It can also
substitute wrong, but similar chemicals for
the bases adenine and guanine so when a
substituted base DNA tries to replicate all
sorts of mistakes are made.
Mutagenic agents cont
There is mounting evidence of a correlation
between mutagens and carcinogens.
Many cancers are caused by a mutation in
the body, thus disrupting metabolism and
growth causing disease.
Gene Mutations
Substitution
A single base is changed. This can alter a codon
which may or may not have an effect.
 GAA to GAG would have no effect as it codes for
the same amino acid.
 GAA to GUA changes glutamic acid to valine
causing sickle-cell anaemia.
 A change to the ‘stop’ codon would stop
production of a polypeptide at the wrong place.
 A change in the ‘stop’ codon would cause the
polypeptide to go on and on and on.
Substitution
Substitution
Gene Mutations
Inversion
The positions of 2 nucleotides are inverted. If
this happens within a codon it will only affect
1 amino acid.
Gene Mutations
Insertion and Deletion
Adding or removing a nucleotide causes a
shift in the reading frame, this is known as a
“frame shift mutation”
This results in the wrong amino acids being
picked up from the point of mutation
onwards. These mutations are usually
lethal.
Insertion
Deletion
Chromosome Mutations
Rearrangement of Genes
this is an alteration of the genes either in
number or sequence.
Deletion
a piece of the middle of the chromosome
breaks off, and the two ends join up.
Deletion
Chromosome Mutations
Inversion
a piece of the chromosome falls out, is
rotated 180 before it moves back into the
chromosome.
Chromosome Mutations
Translocation
moves a segment of one chromosome to another,
non-homologous one.
Reciprocal translocation
2 non-homologous chromosomes exchange
fragments
Non-reciprocal translocation
1 fragment breaks off and joins on to a nonhomologous chromosome without receiving a
fragment in return.
Translocation
Chromosome Mutations
Duplication
a segment of the chromosome is repeated.
This can have a mild or severe effect
depending on how many repeats there are.
Duplication