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Gene Mutations


Gene Mutations Higher Human Biology Unit 1 Human Cells Reminder A gene mutation is the alteration of a nucleotide sequence in a gene`s DNA which leads to a change ... – PowerPoint PPT presentation

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Title: Gene Mutations

Gene Mutations
  • Higher Human Biology
  • Unit 1 Human Cells

  • 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.

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.

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

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

  • 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

  • e.g. Duchenne Muscular Dystrophy (DMD)

  • 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.

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

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

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.

  • e.g. Beta thalassemia

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.

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

  • 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.
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