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Chromosome mutations and gene mutations
Alterations in the number or structure of chromosomes are called chromosome mutations. Chromosome mutations can happen during mitosis and meiosis when chromosomes are being condensed and pulled apart. Homologous chromosomes may fail to separate, resulting in non-disjunction. Chromosome mutations also occur during interphase when DNA replicates, and during crossing over when sections of chromosomes are exchanged.
Gene mutations are changes in the nucleotide base sequence in a cistron (the portion of DNA that makes up a single gene). A change of a single nucleotide base pair is called a point mutation. There are a number of types of point mutation, including:
▪ substitution - the replacement of one nucleotide with another containing a different base
▪ deletion - the loss of a nucleotide
▪ insertion or addition - addition of an extra nucleotide.
Sickle-cell anaemia is an example of an inherited condition that results from a substitution. Gene mutations may also result from duplication (repetition of a portion of a nucleotide sequence within a cistron) and inversion (reversal of the portion of the nucleotide sequence in the cistron).
Most mutations, if expressed, are harmful. Note, however, that in diploid organisms such as ourselves, mutations usually result in recessive alleles. These are expressed only in the homozygous condition unless the mutation is on the X chromosome. Many mutations result in a change in the shape of a protein so that the protein cannot function properly (for example, the mutation that causes sickle-cell anaemia). Mutations that affect large sections of a gene, and chromosome mutations are often lethal. However, some mutations have no effect: a mutation may occur in a non-coding part of DNA; it may produce a different codon for the same amino acid; or the altered amino acid sequence may not affect the protein's shape or function. Occasionally, a mutation is beneficial, changing the phenotype so that an organism has a better chance of surviving and reproducing. Although beneficial mutations are very rare events, they are bound to happen sooner or later if there is a large number of individuals in a population. These mutations are of immense importance because they are the ultimate source of all variation: the raw material for the evolution of new species by natural selection.


■ Glossary of essential terms for you to know

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