Mutations are sudden, permanent changes in DNA. Single-gene mutations
include base substitutions, insertions, and deletions. Chromosomal
mutations include inversions, translocations, duplications, and deletions.
Changes in chromosome number include extra chromosomes, chromosome deficiencies,
and polyploidy.
- Mutations are sudden, lasting changes in
hereditary material. Many mutations are caused by thymine-thymine
dimers, by mismatching of nitrogenous bases, or by chromosome breakage.
- Mutation rates: Natural mutation rates are low (about 1 in a
million), but they vary from point to point along a chromosome. X-rays
or chemical mutagens (like dimethyl sulfoxide) can increase these rates.
- Types of single-gene mutations:
- Base-pair substitutions. Chemically, most of these are
transitions, substituting one pyrimidine (C,T) for another
or one purine (A,G) for another.
- Same-sense mutations: new codon codes for the same amino acid
- Mis-sense mutations: new codon codes for a different amino acid
- Nonsense mutations: new codon is a "stop" codon
- Frameshift mutations, resulting in misreading of many codons:
- Insertions (extra base added)
- Deletions (one or more bases removed)
- Types of chromosomal aberrations:
- Inversions: part of a chromosome reversed end-to-end
- Translocations: chromosome fragment attaches to
a different chromosome
- Duplications: part of a chromosome occurs twice
- Deletions: part of a chromosome is lost
- Changes in chromosome number:
- Aneuploidy, loss or gain of one chromosome at a time
- Monosomy: one chromosome lost from a diploid pair
- Trisomy: an extra chromosome added to a diploid pair
- Polyploidy, addition of entire haploid sets.
Levels of polyploidy include tetraploidy (4N), hexaploidy (6N),
octaploidy (8N), etc. Polyploidy is more common in
plants than in animals.
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