Mutations

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Mutations

• Year 11

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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

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Mutagens

• Mutagens are chemicals or radiation that can
  induce mutations eg. Nuclear radiation, UV rays,
  x-rays, tobacco, agent orange, asbestos.

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• http//www.youtube.com/watch?vabcsZZ9Duxw
• http//www.youtube.com/watch?v8RdvlNsQx-A

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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.

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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

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Harmful mutations

• Cystic fibrosis
• Sickle cell disease
• Albinism

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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.

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Types of mutations

• 1) Gene mutation this is when a base change
  affects the DNA sequence of a single gene.

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2) Chromosomal rearrangements block mutations

• This is when blocks of genes within a chromosome
  are rearranged.

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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.
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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.

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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.

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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.

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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
  non-functional protein.

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Mis-sense substitution
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Non-sense substitution
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Tautomerism
Some point mutations may result from bases with
an abnormal number of hydrogen-bonding
sites. This results in abnormal base pairing.
Pairs are called tautomers.
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Inherited metabolic disorders Page 146 biozone

• More than 6000 diseases attributed to diseases in
  single genes.
• Sickle cell disease
• B- Thalassaemia
• Cystic Fibrosis
• Huntington Disease

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Sickle Cell Disease
Single nucleotide substitution in HBB gene that
codes for beta chain of haemoglobin. Autosomal
recessive mutation.
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Animation sickle cell

• http//www.hhmi.org/biointeractive/dna/DNAi_sickle
  cell.html

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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.
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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.
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How does CF affect the body?

• Cystic fibrosis (CF) is a chronic,
  life-shortening disease that occurs as a result
  of a genetic defect. The defective gene
  interferes with the bodys 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
  cant 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 cant get past the obstruction.
• Food is not digested properly and the body cant
  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.

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Chromosome mutations

• Causes
• Errors in crossing over at meiosis
• mutagens

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Types of chromosomal (block) mutations

• Inversion pieces of chromosomes are flipped
  over so the genes appear in the reverse order.
• There is no loss of genetic material

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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.
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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.
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4) Deletion

• Pieces of chromosome are lost when
• a middle piece of the chromosome falls out and
  the two ends rejoin, so some genes are lost.
• The end of a chromosome may break off and is
  lost.

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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.
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Some terminology

• Disomy 2n (normal)
• Monosomy 2n 1 eg. Turners syndrome
• Trisomy 2n 1 eg. Downs syndrome

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Trisomy in human autosomes

• 1) Downs syndrome Trisomy 21

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Causes of Downs syndrome

• 92 of cases due to non-disjunction of chromosome
  21 during meiosis
• 5 result from translocation of chromosome 21
  (usually onto chromosome 14).

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Non-disjunction in meiosis I
Non-disjunction in meiosis II
Normal meiosis
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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.
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3) Edward syndrome Trisomy 18
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Many aneuploidies show a maternal age effect
with incidence increasing with age of mother.
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Maternal age effect probably because1) 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.
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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.
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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.
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Faulty egg production
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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.

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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.

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Autopolyploidy
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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).

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Evolution of wheat