Transcript DNA and the Genome - Speyside High School

Key area 6: Mutations
Unit 1: DNA and the Genome
Mutations
Mutations are a change in the structure
or amount of an organism’s DNA.
Mutations arise spontaneously and at
random.
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Frequency of mutations
Mutations are rare.
In humans it is estimated that there is 1
mutation per 15-30 million base pairs.
Mutation rates differ in different genes
and different organisms.
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Types of mutation
1. Single gene mutations
2. Chromosome structure mutations
3. Polyploidy
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1. Single gene mutations
These mutations involve a change in one
of the base pairs in the DNA sequence of
a single gene. Also known as point
mutations.
These may occur in the protein-coding
sequence or the regulatory sequences
which control expression of the gene.
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(a) Deletion
mRN
A
Protein
Met
Gln
mRN
A
Protein
Met
Gln
Thr
Ser
His
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STOP
Leu
DNA and
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Genome
(b) Insertion
mRN
A
Protein
Met
Gln
Thr
Ser
STOP
mRN
A
Protein
Met
Ala
Asp
Ile
Leu
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(c) Substitution
mRN
A
Protein
Met
Gln
Thr
Ser
STOP
mRN
A
Protein
Met
Gln
Thr
Ser
STOP
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Write a description of each type of
mutation:
Insertion, deletion, substitution.
Use the following sequences as your
original DNA sequence:
TAGCCGTATCGTA
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The impact of point mutations
Research and produce a A3 poster on what
is meant by:
•
•
•
•
•
Silent mutations
Neutral mutations
Missense mutations
Nonsense mutations
Frameshift mutations
Use diagrams to demonstrate the effects of each
mutation on the resulting protein sequence.
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Case Study: Sickle cell anaemia
• What type of mutation causes this
disease?
• Are any of the mutations confined to a
specific area or ethnic race?
• What are the symptoms associated with
each mutation?
• Are any of the mutations beneficial or
are they detrimental?
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Splice site mutations
RNA splicing is important in creating the
mature mRNA transcript.
Mutations in the area that marks the
start or end of an intron, may lead to one
or more introns not being removed. This
will lead to a creation of a protein that
does not function properly.
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Thalassemia, a disease caused by a defect in
haemoglobin synthesis, is caused by a splice
site mutation.
Intron
Exon
Normal splicing
Normal
functional
haeomoglobin
with 4 oxygen
carrying
subunits
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Abnormal splicing
Haeomoglobin
with only 2
oxygen
carrying
subunits
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2. Chromosome structure
mutations
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(a) Deletion
Deletion mutations have a
drastic effect on the
organism – as genes are
lost.
e.g. Deletion of part of
chromosome 5 leads to Cri
du Chat syndrome.
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(b) Duplication
This results in a set of genes
being repeated. Some
duplications can be
detrimental.
e.g. Duplication of oncogenes
is a common cause of cancer.
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Evolutionary importance of
gene duplication
Duplication of a gene produces a second
copy of the gene which is free from
selection pressure (it can become altered
with out affecting the original gene’s
function).
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This means it can mutate to produce new
DNA sequences.
This may confer advantages to the
organism and possibly increase it fitness
and chances of survival.
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(c) Inversion
Reverses the normal
sequence of genes.
This often results in the
formation of non-viable
gametes.
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Non-reciprocal translocation
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Reciprocal translocation
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Translocation involves transfer of genes
from one chromosome to another.
This results in problems pairing
chromosomes during gamete formation
and leads to non-viable gametes.
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3. Polyploidy
Polyploidy is duplication of all the
chromosomes – resulting in extra sets of
chromosomes.
It is the result of an error during gamete
formation.
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Polyploidy in plants
Roughly 50% of plant species are
polyploid.
Polyploid plants are normally larger, and
have increased seed and fruit size. This is
of economic importance.
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Polyploid plants with
an uneven number of
sets of chromosomes
are sterile. This
allows the production
of seedless fruits.
e.g. bananas
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Polyploidy in animals
It is thought that polyploidy contributed
to the evolution of vertebrate animals.
However, mammals which are polyploid fail
to survive.
Except: Plains
Viscacha rat of
Argentina.
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