Transcript Mutations - year13bio

Mutations
Year 11
Mutations
• A mutation is a permanent genetic mistake in a gene or a
chromosome.
• Mutations can occur spontaneously or be induced.
Spontaneous mutations arise from errors in replication.
Induced mutations are caused by mutagens.
• Mutations are the only way that new alleles can be created
Mutagens
• Mutagens are chemicals or radiation that
can induce mutations eg. Nuclear radiation,
UV rays, x-rays, tobacco, agent orange,
asbestos.
• http://www.youtube.com/watch?v=abcsZZ9
Duxw
• http://www.youtube.com/watch?v=8RdvlNs
Qx-A
The effect of mutations
• When mutations occur in the testes and
ovaries they will be inherited by future
generations (GAMETIC MUTATIONS).
• Somatic mutations in body cells are not
inherited but can effect the person during
their life time.
Not all mutations are bad
• Beneficial mutations:
Examples: 1) bacteria becoming resistant to
antibiotics
2) pesticide resistance
3) tolerance to high cholesterol
levels (see page 143)
4) fast rates of mutations in the
protein coat of viruses
Harmful mutations
• Cystic fibrosis
• Sickle cell disease
• Albinism
Neutral mutations
• Are neither harmful or beneficial but may
have an effect in future generations.
• Mutations do not always result in variation,
but when they do, the variation is often in
the form of entirely new alleles.
Types of mutations
• 1) Gene mutation – this is when a base
change affects the DNA sequence of a
single gene.
2) Chromosomal rearrangements
= block mutations
• This is when blocks of genes within a
chromosome are rearranged.
3) Changes in chromosome
number
Aneuploidy is the loss or gain of whole
chromosomes.
Polyploidy is the loss or gain of complete
sets of chromosomes.
1. Gene mutations
• Point mutations – changes in a single
nucleotide. A nucleotide can be replaced by
another (substitution), it can be removed
(deletion) or an extra nucleotide can be
added (insertion).
• Tautomerism – abnormal base pairing.
Insertion mutations
• When a single extra base is inserted into the
DNA sequence a new sequence of codons
can result due to a reading frame shift.
• The protein that is then made is usually
non-functional. The closer the insertion is
to the start codon the more the protein will
be affected.
Deletion mutations
• A deletion of a base in a DNA sequence can
have the same effect as an insertion
mutation due to a reading frame shift.
Substitution mutations
• Occurs when a base is substituted for another
base.
• Mis-sense substitution – change in codon leads to
formation of protein but not the right protein. If
the third base in a triplet is substituted, the amino
acid may not actually be changed.
• Non-sense substitution- the amino acid is changed
to a stop codon resulting in a shorter, usually nonfunctional protein.
Mis-sense
substitution
Non-sense
substitution
Tautomerism
Some point mutations
may result from bases
with an abnormal
number of hydrogenbonding sites.
This results in abnormal
base pairing. Pairs are
called tautomers.
Inherited metabolic disorders –
Page 146 biozone
More than 6000 diseases attributed to diseases
in single genes.
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Sickle cell disease
B- Thalassaemia
Cystic Fibrosis
Huntington Disease
Sickle Cell Disease
Single nucleotide substitution in HBB gene that
codes for beta chain of haemoglobin.
Autosomal recessive mutation.
Animation – sickle cell
• http://www.hhmi.org/biointeractive/dna/DN
Ai_sicklecell.html
People that are heterozygous for sickle cell mutation are
often resistant to malaria.
Studies have shown that African Americans, who have
lived in malaria-free areas for as long as ten generations,
have lower sickle cell gene frequencies than Africans -and the frequencies have dropped more than those of
other, less harmful African genes. Similarly, the sickle
cell gene is less common among blacks in Curacao, a
malaria-free island in the Caribbean, than in Surinam, a
neighboring country where malaria is rampant -- even
though the ancestors of both populations came from the
same region of Africa.
Cystic fibrosis
Over 500 different recessive mutations of the CFTR gene
have been identified.
Most common mutation (70% of sufferers) is a triplet
deletion (AAA), which means the 508th amino acid of the
CFTR gene is missing (deletion mutation).
Autosomal recessive mutation.
How does CF affect the body?
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Cystic fibrosis (CF) is a chronic, life-shortening disease that occurs as a result
of a genetic defect. The defective gene interferes with the body’s ability to
transfer water and salt to and from cells. This causes secretions, which are
normally thin and watery in healthy people, to become very thick and sticky.
The thick secretions clog up organs and prevent them from working properly.
Lungs
Cystic fibrosis causes problems in the lungs when the thick mucus builds up
and gets stuck in the airways.
When this happens:
Breathing passageways become blocked and air can’t get through.
Bacteria grow in the mucus collections and causes infection in the lungs, nose,
and sinuses.
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Pancreas
The pancreas is part of the digestive system. Its job is to secrete enzymes that
are needed to digest food, and a hormone called insulin that controls blood
sugar. Cystic fibrosis also causes these secretions to become thick.
When this happens:
The pancreatic ducts become clogged.
Enzymes can’t get past the obstruction.
Food is not digested properly and the body can’t absorb nutrients.
Eventually, the obstruction of the pancreas may cause scarring that damages
the insulin-producing cells and prevents them from producing insulin. Insulin
is not available to the cells, which causes blood sugar levels to rise. This
condition is called insulin-dependent diabetes, which happens in about 15% of
all CF patients.
Chromosome mutations
Causes:
1) Errors in crossing over at meiosis
2) mutagens
Types of chromosomal (block) mutations
1) Inversion – pieces of chromosomes are flipped over so
the genes appear in the reverse order.
There is no loss of genetic material:
2) Translocation
Pieces of chromosome are moved from one
chromosome to another.
Can cause major problems when the
chromosomes are passed to gametes. Some will
receive extra genes, some will be deficient.
3) Duplication
Pieces of chromosomes are repeated so there
are duplicate segments.
One chromosome “donates” a segment of
chromsome to another.
Some gametes will receive double the genes,
others will have no genes for the affected
segment.
4) Deletion
Pieces of chromosome are lost when:
1) a middle piece of the chromosome falls
out and the two ends rejoin, so some
genes are lost.
2) The end of a chromosome may break off
and is lost.
Aneuploidy
The diploid (2n) number of chromosomes in
humans is 46.
The haploid (n) number of chromosomes in
humans is 23.
Aneuploidy is the loss or gain of whole
chromosomes. Extra or lost chromosomes
can either be autosomes or sex chromsomes.
Some terminology
• Disomy = 2n (normal)
• Monosomy = 2n – 1 eg. Turner’s syndrome
• Trisomy = 2n + 1 eg. Down’s syndrome
Trisomy in human autosomes
• 1) Down’s syndrome:
Trisomy 21
Causes of Down’s syndrome
• 92% of cases due to
non-disjunction of
chromosome 21
during meiosis
• 5% result from
translocation of
chromosome 21
(usually onto
chromosome 14).
Nondisjunction
in meiosis I
Normal meiosis
Nondisjunction
in meiosis II
2) Patau Syndrome: Trisomy 13
Usually die before 3 months
A newborn male with
full trisomy 13 (Patau
syndrome). this baby
has a cleft palate, atrial
septal defect, inguinal
hernia, and postaxial
polydactyly of the left
hand.
3) Edward syndrome: Trisomy 18
Many aneuploidies show a “maternal age effect”
with incidence increasing with age of mother.
Maternal age effect probably because:
1) all eggs are present at birth but meiosis occurs in stages,
with meiosis not being complete until after fertilisation.
Therefore, the eggs present in an older woman are old and
there is a greater chance that errors in meiosis will occur.
Anueploidy in Human Sex
chromosomes
The human sex chromosomes are XX for female
and XY for male.
Abnormal sex chromosome configurations can arise
when the sex chromosomes fail to separate properly
during meiosis.
Faulty Sperm Production
Aneuploidy in
human sex
chromosomes
may result from
faulty sperm
production. This
results from the
failure of the X
and Y
chromosomes to
separate during
meiosis.
Faulty egg production
Polyploidy
• Is when a cell or organism contains three or
more times the haploid number of
chromosomes (3n or more).
• Rare in animals, common in plants.
• Polyploid animals include earthworms,
shrimps and aphids.
Autopolyploidy
• Is a type of polyploidy.
• Involves a multiple of identical sets of
chromosomes from the same species.
• Hybrid may be fertile or sterile depending
on the number of chromosome sets.
Hybrids with an even number of of
chromosome sets will be fertile because
chromosome pairing can occur at meiosis.
Autopolyploidy
Allopolyploidy
• Involves the combination of chromosomes
from two or more different species to form a
hybrid.
• Fertile polyploids may arise from doubling
of the chromosome complement in the
infertile hybrid (a process called
amphiploidy).
Evolution of wheat