Gene Mutations

1
Gene Mutations

• Higher Human Biology
• Unit 1 Human Cells

2
Reminder

• A gene mutation is the alteration of a nucleotide
  sequence in a genes DNA which leads to a change
  in the genetic code. This can occur in several
  different ways.

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Point Mutations

• This involves a change in one nucleotide in the
  DNA sequence of a single gene. This results in
  one or more codons for one or more amino acids
  becoming altered. A single nucleotide is
  substituted, inserted or deleted.

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Substitution Mutations

• A substitution can change the triplet code on a
  codon so that it still makes sense but produces a
  code for a different amino acid creating a
  different protein. This is called a missense
  mutation.

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• e.g. Phenylketonuria (PKU)
• e.g. Sickle Cell Anaemia

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• A substitution can sometimes cause a triplet code
  on a codon to change from coding for a particular
  amino acid to becoming a stop codon. This causes
  the premature halt to the production of a
  particular protein. This is called a nonsense
  mutation.

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• e.g. Duchenne Muscular Dystrophy (DMD)

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• The genetic code is read as a series of triplet
  bases during translation. The insertion or
  deletion of a nucleotide will affect the triplet
  grouping of the genetic code. It becomes shifted
  in a way that alters every subsequent triplet
  code (codon) from the point of mutation onwards.

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• This is highly likely to produce a non-functional
  protein. Such mutations are called frameshift
  mutations (see the example below).

10

• e.g. Tay-Sachs Disease
• e.g. Cystic Fibrosis

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Splice Site Mutations

• Before mRNA leaves the nucleus, introns
  (non-coding regions) are removed and exons
  (coding regions) are joined together. This is
  called RNA splicing. This process is controlled
  by the base sequences of the genetic code.
  Mutations can occur which cause mistakes in the
  splicing process this can cause an intron to be
  retained in error resulting in modified mRNA.
  This may in turn result in the production of an
  altered protein.

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• e.g. Beta thalassemia

13
Nucleotide Sequence Repeat Expansion

• A gene mutation can sometimes be the result of
  certain triplet codes being repeated by the
  insertion of a large number of copies of a
  nucleotide sequence. This is called a nucleotide
  sequence repeat expansion. It can result in the
  production of a defective protein with a string
  of extra copies of one particular amino acid or
  it may result in the failure to express any
  protein at all.

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• e.g. Huntingdons disease
• e.g. Fragile X syndrome

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• There are many human genetic disorders caused by
  single gene mutations. Sickle cell disease,
  phenylketonuria, Duchenne muscular dystrophy,
  beta thalassemia, Tay- Sachs disease, cystic
  fibrosis, fragile X syndrome and Huntingtons
  disease are all examples of such disorders.
• Carry out a case study on two of the genetic
  disorders listed above. For each chosen disorder
  describe the symptoms, the type of mutation that
  causes the disorder and any treatment that can be
  given to help sufferers. The internet or Torrance
  pages 55-59 will be useful.