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Mutations

This overview delves into mutations, defined as changes in DNA sequences that can occur in somatic or gametic cells. It explores spontaneous mutations, those caused by mutagens like UV light and chemicals, and categorizes them into gene (point) mutations and block mutations. We discuss their effects, including harmful, beneficial, or neutral outcomes, and examine types of point mutations—substitutions, insertions, and deletions—along with block mutations like deletions and translocations. Aneuploidy and polyploidy are addressed, highlighting their implications in human genetics.

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Mutations

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  1. 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

  2. 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)

  3. 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.

  4. 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.

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

  6. 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:

  7. 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.

  8. A bird flys thru the trees • A flys thru the trees • A drib flys through the trees • A flys thru bird the trees • A bird bird flys thru the trees

  9. 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.

  10. 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

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

  12. Polyploidy • 3+ complete sets of chromosomes, common in plants and rare in humans (spontaneously aborts). • Autopolyploidy – within a single species • Allopolyploidy – within hybrid

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