MUTATIONS
Heritable Changes in Genes
• Mutations are heritable changes in DNA—changes that are passed on to daughtercells. In single-celled organisms, any mutations that occur are passed to the daughter cells at the time of cell division.
• Multicellular organisms have two types of mutations:
• Somatic mutations are passed on during mitosis, but the affected cells never become
gametes and so do not pass to subsequent generations
• Germ-line mutations are mutations that occur in cells that might give rise to gametes.
• Some mutations cause visible phenotypic change. Others cause metabolic changes
that might not yet be detectable.
• Some mutations exert their effect only under certain restrictive conditions. These
are called conditional mutants. They are unaffected under permissive conditions,
but express the mutant phenotype at the restrictive condition. Temperature-sensitive
mutants are an example.
• All mutations are alterations of the DNA nucleotide sequence and are of two types:
Point mutations are mutations of single genes and Chromosomal mutations are
changes in the arrangements of chromosomal DNA segments.
Point mutations are changes in single bases
• Point mutations result from the addition or subtraction of a nucleotide base or the
substitution of one base for another. Point mutations can occur as a result of mistakes
during DNA replication, or by environmental mutagens, such as chemicals and radiation.
Because of redundancy in the genetic code, some point mutations result in no change in the amino acids in the protein. These are called silent mutations.
• Some mutations cause an amino acid substitution. These are called missense
mutations. An example in humans is sickle-cell anemia, a defect in the b-globinsubunits of hemoglobin. The red blood cells collapse when oxygen levels are low.
Missense mutations might reduce the functioning of a protein or disable it completely.
• Nonsense mutations are base substitutions that cause a change from a codon that
instructs the incorporation of an amino acid to a codon that terminates translation.
• A frame-shift mutation is when a single base is inserted or deleted in a gene. This
causes the most disruption when the event occurs at or near the beginning of the
template. This type of mutation shifts the code, changing many of the codons to
different codons. These shifts almost always lead to the production of nonfunctional
proteins.
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