Transcript Higher Biology

Higher Biology
Mutations
G Davidson
Mutations
Mutations are changes which alter the
amount or structure of the genetic
material in an organism.
This can result in a change in the
characteristics of an organism.
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Mutations
Mutations are rare although most of the
genes in existence today are probably the
result of mutations.
If mutations occur in gamete producing cells
or gametes themselves, then the mutations
can be passed on to the next generation.
However, if a mutation was to occur in one of
your skin cells, it might affect a small part of
your skin but won’t be passed on to your
children.
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Mutations
Mutations can be very small changes,
such as one base changing in the DNA,
or can be large, such as increasing or
decreasing the chromosome number.
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Mutations
Considering the complexity of DNA, it is
amazing that mutations are so
infrequent.
However, when a mutation does occur,
resulting in a change in phenotype, the
resulting individual is called a mutant.
In a real sense therefore we are all
mutants!
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Chromosome Numbers and
Polyploidy
Mutations can be brought about by a
process called non-disjunction which
sometimes happens during meiosis.
To cause non-disjunction a spindle fibre
fails and as a result, some gametes get
an extra chromosome, while others lack
one.
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Chromosome Numbers and
Polyploidy
One specific example of this is Down’s
Syndrome in humans, caused by the egg
having 24 chromosomes and a normal
sperm with 23 chromosomes, producing a
zygote with 47 chromosomes.
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Polyploidy
Polyploidy is an extreme case of nondisjunction can result in the effect
called Polyploidy.
This results in a cell containing at least
three times the normal chromosome
compliment, (e.g. triploid = 3n and
tetraploid = 4n).
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Polyploidy
Polyploidy can be the result of a haploid
gamete fertilising a diploid gamete or a
diploid gamete fertilising another
diploid gamete.
This tends to be more common in plants
than in animals and can often result in
the formation of new species.
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Polyploidy
Parental cells
AA or 2n
Parental Gametes
AA or 2n
Triploid Offspring
diploid
AA or 2n
AAA or 3n
Sterile
A or n
haploid
AA or 2n
AA or 2n
Tetraploid Offspring
diploid
AA or 2n
AAAA or 4n
Fertile
AA or 2n
diploid
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Polyploidy
Mutant or polyploid plants tend to show
an increase in size, vigour, and disease
resistance, and this is of great economic
importance.
Most cereals and many other crop
plants are polyploid
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Polyploidy
Scientists have developed a method of
preventing spindle formation at mitosis
using a chemical called colchinine.
This leads to polyploid plants which,
after the chemical is removed, divide
normally, and a new species of crop
plant can be produced.
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Chromosome mutations
Chromosome mutations are normally
large changes which are usually
detectable under the microscope during
cell division.
They usually occur during crossing over
when the number or sequence of genes
may be altered.
There are several ways this can happen:
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Chromosome mutations
Type of
Mutation
Description
Significance
Deletion
Genes chopped out
Genes lost
Inversion
Genes rotate 180o
Formation of nonviable gametes
Duplication
Genes copied twice
Duplicated gene
could mutate
Translocation
Genes stick to
different
chromosomes
Problems of
homologous
pairing.
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Deletion
1
1
2
2
New chromosome
3
3
7
4
5
8
BREAK
6
4
7
5 Deleted genes
8
6
Original chromosome
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Inversion
1
1
2
2
3
3
4
4
5
6
BREAK
7
8
8
Original chromosome
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1
1
2
2
3
3
4
4
5
6
BREAK
7
8
8
Original chromosome
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1
1
2
2
3
3
4
4
5
6
BREAK
7
8
8
Original chromosome
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1
1
2
2
3
3
4
4
5
6
BREAK
7
8
8
Original chromosome
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1
1
2
2
3
3
4
4
5
6
BREAK
7
8
8
Original chromosome
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1
1
1
2
2
2
3
3
3
4
4
4
5
6
7
BREAK
6
7
8
5
8
Original chromosome
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New chromosome with
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Inverted genes
Duplication
1
1
2
2
3
3
4
3
Duplicated
4
Genes from
4
BREAK
5
5
chromosome
6
5
7
6
7
8
8
Original chromosome
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Homologous
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New chromosome
22
Translocation
1
1
2
2
3
Chromosome A
3
4
4
5
5
21
Translocated
genes
21
22
22
Chromosome B
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BREAK
23
23
24
24
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Gene mutations
Gene mutations are relatively small,
usually involving one or more nucleotides
being changed.
This leads to a different sequence of
bases, which may result in one particular
amino acid being altered and therefore
cause a slightly different protein to be
made.
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Gene mutations
E.g. a single base change in DNA can
cause the haemoglobin molecule to have
one altered amino acid and this results
in sickle cell anaemia.
Some genes can cause death if they
mutate and are called lethal genes.
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Gene mutations
Type of
Mutation
Description
Significance
Substitution
e.g. A swapped for T
Changed amino acids lead
to changed proteins
Insertion
Extra base in DNA
All amino acids altered
after inserted base
Deletion
Base chopped out
All amino acids altered
after deleted base
Inversion
Base order rotates
through 180o
One or two amino acids
altered at inversion
site, no more
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Substitution
Normal
DNA
mRNA
Amino
acid
Mutant
C
T
T
C
A
T
G
A
A
G
U
A
Glutamic
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Valine
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Deletion
Normal
DNA
A
G
U
C
A
G
U
Mutant
T
C
A
C
G
T
A
A
G
U
C
A
U
G
C
C
G
T
A
mRNA
Amino
acid
Serine
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Glutamine
Serine
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Arginine
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Insertion
Normal
DNA
A
G
U
C
A
G
U
Mutant
T
C
A
C
G
A
G
U
C
A
U
G
G
C
T
C
C
A
G
mRNA
Amino
acid
Serine
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Glutamine
Serine
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Proline
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Inversion
Normal
DNA
A
G
U
C
A
G
U
Mutant
T
C
A
C
G
A
G
U
C
A
U
T
A
G
C
C
G
mRNA
Amino
acid
Serine
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Glutamine
Serine
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Threonine
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Mutation Frequencies
Mutations, which cause defects,
illnesses or death, are termed
deleterious.
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Mutation Frequencies
At this stage of evolutionary history, most
genes are necessary and useful in their
current forms, therefore a mutation is like to
be deleterious and unlikely to give the
organism any survival advantage.
As a result, these harmful mutations tend to
be lost quickly from a population, particularly
if they are dominant alleles and lethal in their
effect.
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Mutation Frequencies
Recessive lethals can stay in a large
population for longer as the chances of two
recessive lethals of the same gene meeting in
a zygote are very small, therefore the
recessive will not be wiped out so quickly.
The rate of mutations can be increased by
mutagenic agents, e.g.
Radiation such as X-rays, UV light, gamma rays
Chemicals like colchinine, mustard gas etc.
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Mutation Frequencies
These are termed induced mutations.
Mutations are useful as they are the
only source of new variation and without
them there would be no source of
variation.
Without variation evolution would not be
possible.
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