Gene mutation AQA A2 biology notes

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Gene mutation
Mutation: any change to the quantity of the structure of the DNA of an organism
Mutations in body cells are not passed to next generation
Mutations occurring during formation of gametes may be inherited, which is the basis of
discontinuous variation.
Discontinuous variation: Variation within a population in which few or no intermediate phenotypes fall
between the extremes ( eg: blood groups)
Any changes to 1+ base in DNA triplet code= change in AA sequence of polypeptide
DNA bases can change by substitution or deletion of bases
Substitution of bases
Substitution: mutation in which a nucleotide in a DNA molecule is replaced by another nucleotide that
has a different base
Result of change to a single base could result in:
A NON-SENSE MUTATION ­ occurs if base change results in a stop codon being formed. The
production of a prematurely stopped polypeptide, so the final protein would be significantly
different and not perform its normal function.
A MIS- SENSE MUTATION ­ occurs when base change results in a different AA being coded
for. The polypeptide made will differ in one AA. The significance of this difference will
depend upon the precise role of the AA, if it importance in forming tertiary structure of
protein shape or not.
A SILENT MUTATION ­ occurs when the substituted base codes for the same AA as before,
due to the degenerative nature of the genetic code. As both DNA triplets code for the same
AA, there is no change to polypeptide produce d and so the mutation will have no effect.]
Deletion of bases
Deletion of bases arises when a nucleotide is lost from the normal DNA sequence. The AA
sequence of the polypeptide made will be significantly different.
Deleted nucleotide creates a "frame shift" because it alters the triplet code and is shifted
one letter to the left.
One deleted base at the start could alter every triplet code in the sequence.
Deletion of bases near the end of sequence will have a smaller impact.

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Cause of mutations
Gene mutations can arise spontaneously during DNA replication, and are permanent. They
occur without outside influence.
Basic mutation rate is increased by outside factors ­ MUTAGENIC AGENTS/ MUTAGENS:
High energy radiation that can disrupt DNA molecule
Chemicals that alter the DNA structure or interfere with transcription
Mutations produce genetic diversity for natural selection and speciation, nut often produce an
organism less well suited to the environment.…read more


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