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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 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 mutation.
• 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 mutation.
• 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 (noncoding 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. Huntingdon’s 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- Sach`s disease, cystic
fibrosis, fragile X syndrome and Huntington`s
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.