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Summary Mutations
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MUTATIONSGENERAL
― Mutation is any change in the genetic composition of an organism.
― Changes in the genetic material [DNA] by changing the order of the nitrogen
bases. This alters the sequence of the nucleotides and disrupts protein formation.
― May occur by chance spontaneously / as a result of mutagens. Mutagens are
physical / chemical agents that cause mutations.
― Examples of mutagens;
o X-rays.
o UV light.
o Toxic chemicals.
o Viral infections.
o Extreme heat.
― Not all mutations are hereditary; only those that occur in the sex organs
during gametogenesis are inherited.
― Mutations can occur in two different types of cells: somatic [body] cells and
gamete [sex] cells.
― Mutations that occur in somatic cells usually result in killing that body cell only. An
exception to this is if the mutation occurs to the DNA that controls regulation of
the cell cycle. This can result in cancer.
― Somatic mutations usually do not affect the individual and cannot be passed on to
the next generation.
― Two types of mutations in gamete cells;
o Chromosome mutations / chromosomes aberrations.
o Gene mutations.
GENE MUTATIONS
― As a result of a change in the nucleotide sequence in the DNA molecule.
― Consequently, the code for protein synthesis changes and results in the formation
of a faulty protein / no protein at all.
― Can occur during;
o DNA replication.
o Transcription of DNA to mRNA.
o Crossing over during metaphase I of meiosis.
TYPES OF GENE MUTATIONS
― Four types of gene mutations;
o Substitution
o Deletion
o Inversion
o Insertion
― These mutations may be classified as either point mutations / frameshift
mutations.
― A change in the nucleotide sequence of DNA affects the mRNA as well as the
protein.
The following normal DNA sequence and the complementary mRNA formed during
transcription is used to explain mutations:
DNA: TAC – TTC – GAA – CGG - ACG
mRNA: AUG – AAG – CUU – GCC - UGC
, Substitution
― Mutation where one nitrogenous base is replaced by another. This results in the
alteration of only one amino acid.
“G” is replaced by “A”
― Point mutation.
DNA: TAC – TTC – GAA – CAG - ACG
mRNA: AUG – AAG – CUU – GUC - UGC
Amino acids: methionine – lysine – leucine – valine - cysteine
Inversion
― A gene mutation where one / more base triplets are inverted. This results in the
alteration of one / more amino acids.
― Point mutation. Sequence of base
triplets is inverted
DNA: TAC – TTC – AAG – CGG - ACG
mRNA: AUG – AAG – UUC – GCC - UGC
Amino acids: methionine – lysine – phenylalanine – alanine – cysteine
Deletion
― A gene mutation where one / more nitrogenous bases are lost may result in a
frame shift. After such a deletion all the base triplets are altered and
consequently also the amino acids.
“C” was
lost
DNA: TAT – TCG – AAC – GGA - CG
mRNA: AUA – AGC – UUG – CCU - GC
Amino acids: isoleucine – serine – leucine – proline – one amino acid is missing
Insertion
― A gene mutation where one / more nitrogenous bases are inserted may result in a
frame shift. After an insertion, all the base triplets are altered and consequently
also the amino acids.
“C” was inserted
DNA: TAC – CTT – CGA – ACG – GAC - G
mRNA: AUG – GAA – GCU – UGC – CUG - C
Amino acids: methionine – glutamic acid – alanine– cysteine – leucine – one amino acid is
missing
A change in only one base can change the function of the protein. This type mutation
called point mutation.
Deletions and insertions of bases that are not multiples of three have a greater effect,
because they cause a frame shift and many amino acids change. The affect is more
dramatic compared to point mutations. They may change every amino acid that
follows the point of the mutation. They can alter a protein so much that it is unable to
perform its normal functions.
CHROMOSOME MUTATIONS / ABERRATIONS
― Whole chromosomes / portions of chromosomes containing more than one gene
are involved.
TYPES OF CHROMOSOMAL MUTATIONS
― Deletion: involves the loss of all / part of a chromosome.
― Duplication: involves the production of extra copies of parts of the chromosome.
― Inversion: reverses the direction of parts of a chromosome.
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