Mutations

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Key Components

Before we begin, here are some terms which you should familiarise yourself with.

  • Mitosis: the replication of somatic cells
  • Meiosis: the replication of gametes

Mutations

DNA frequently undergoes chemical changes, especially when it is being replicated in either mitosis or meiosis, resulting in errors or changes arising in the nucleotide sequence. Generally, these changes are quickly repaired. However, in the rare circumstances that they are not, then they form mutations. Therefore, mutations are a permanent change in the nucleotide sequence or genetic code, arising due to the failure of DNA repair. Spontaneous mutations arise naturally, largely resulting from random errors in DNA replication while induced mutations can occur because of deliberate or accidental exposure to mutagens e.g. UV, X-Rays.

Mutations are the only source of new alleles and can be passed onto the next generation if they occur within germline cells – the reproductive cells. Somatic mutations that occur in normal body cells are not passed down.

Point Mutations

Point mutations affect a single base in DNA either by substitution (silent, missense or nonsense mutations), insertion or deletion

  • Silent mutations lead to no change in the amino acid sequence, due to the redundancy of the genetic code
  • Missense mutations lead to a change in the amino acid sequence
    • Conservative, the altered amino acid can carry out normal function
    • Non-Conservative, the altered amino acid cannot carry out normal function
  • Nonsense mutations are mutations of a triplet to a stop codon, prematurely terminating the amino acid sequence
  • Frameshift mutations severely affect the amino acid sequence through the insertion or deletion of a base, affecting all codons from the point

Block Mutations

Block mutations are chromosomal changes affecting large segments of a chromosome, commonly arising due to spontaneous errors during recombination in meiosis, which may cause a substantial loss in genes. The substantial loss of genes can arise due to the location of the break in the chromosome. For example, if the break occurs in the middle of a gene or in the middle of a promoter region then the gene would be rendered inactive

  • Inversion: a segment of one chromosome is removed and then replaced in reverse order
  • Duplication: a segment of a chromosome is copied, resulting in duplicate segments
  • Translocation: segments of two chromosomes are exchanged
  • Deletion: a segment of a chromosome is lost

Block Mutations

Chromosomal Abnormalities

Aneuploidy is the abnormality of possessing extra or missing chromosomes within a cell. This can be caused by non-disjunction which is the inability for the chromosomes to separate correctly during anaphase in meiosis. If the spindle fibres are weak, therefore during anaphase, they cannot pull the chromosome to its relevant pole, thus, causing two chromosomes to go to the pole instead of just one, causing non-disjunction. An extra chromosome can result in down-syndrome (trisomy 21), while the deletion of a chromosome can result in turner syndrome (missing an X chromosome)

Polyploidy refers to a condition in which an organism has more than two matched sets of chromosomes (3n, 4n). This condition is much more prevalent in the plant kingdom than the animal kingdom. Scientists have hypothesised that because animals can move around in adverse weather conditions (drought, flood, etc) while plants cannot, therefore, over evolution, more chromosomes aid plants to survive. If one fails, then the others can take over


Aneuploidy


Polyploidy