Mutations

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					          Mutations




Year 13
                     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.

• An example of a neutral mutation is when
  the codon AUU is changed to AUC. They
  both code for the amino acid leucine, so the
  protein is unlikely to be affected.
         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 non-
  functional protein.
Mis-sense
substitution
Non-sense
substitution
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.
                Inversion
• When two nucleotides are inverted. If this
  happens within a codon, only the amino
  acid will be affected.
Do point mutation problems page
              148
         Then 141-144
    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
    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.
          Pages 146-148
• http://www.youtube.com/watch?v=FMAOE
  OmLoUE
       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.
Do pages 150 and 151
           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).
                 Non-
                 disjunction
                 in meiosis I




                 Non-
                 disjunction
Normal meiosis   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.
• Page 140
       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
• Video – the struggle to be male
Previous slides covered in 153-
         157 Biozone
               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

				
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