Mutations

• Any change in DNA sequence which is not immediately and properly repaired.

• If they occur in somatic cells then they are non-inheritable, if in gametes then can be passed on to offspring.

• Can be due to mistakes in DNA replication (spontaneous) or caused by mutagenic agents e.g. UV light, ionising radiation, Xrays, chemicals, viruses

• May be harmful, beneficial or neutral.

• Usually recessive in diploids, can pass through many generations with no further change.

• Two main types: gene mutations (point) and chromosome mutations (block, number of chromosomes or sets of chromosomes)

Point Mutations

• These are single gene mutations. There is a change in the DNA sequence of one base. This affects the amino acid coded for and therefore the protein. There are 3 types of point mutation.

• Substitution: a base is swapped for another base, the code for the amino acid may be changed resulting in a non-functioning protein. E.g. sickle cell anaemia.

• Insertion: an extra base is added resulting in a frame shift in the reading of the code making a new sequence of amino acids.

• Deletion: a base is removed resulting in a frame shift in the reading of the code making a new sequence of amino acids. E.g. cystic fibrosis.

• Tautomerism is where the wrong base pairing occurs which results in the amino acid sequence being misread.

Block mutations

• A change in structure of the chromosome. Groups of genes are affected and often occurs during crossing over or due to mutagens. Four types:

• Deletion: loss of a section of chromosome

• Inversion: part of a chromosome turned around.

• Translocation: part of one chromosome breaks off and is joined to another chromosome.

• Duplication: part of a chromosome is copied and added to the chromosome.

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Chromosome numbers

• Aneuploidy: loss or gain of whole chromosomes

• Polyploidy: loss or gain of sets of chromosomes.

• All cause diseases with multiple effects which are called syndromes.

Aneuploidy

• Normally a human has pairs of homologous chromosomes (disomy).

• Aneuploidy is where one pair of chromosomes either has monosomy (only 1 chromosome) or trisomy (three chromosomes).

• Can occur in autosomes (nonsex chromosomes) e.g. downs, patau, edward syndrome

• Can occur in sex chromosomes e.g. Klinefelter (XXY) or Turner (XO) syndrome.

Polyploidy

• 3+ complete sets of chromosomes, common in plants and rare in humans (spontaneously aborts).

• Autopolyploidy – within a single species

• Allopolyploidy – within hybrid