Gene Mutations

Causes of Mutations Mutations result from both internal and external factors Spontaneous Mutations : Those that are a result of natural changes in DNA structure Induced Mutations : result from changes caused by environmental chemicals or radiation

Categories of Mutations In multicellular organisms there are two broad categories of mutations: Somatic mutations: Arise in somatic tissues which do not produce gametes Germline mutations: Arise in cells that ultimately produce gametes These mutations can be passed to future generations

There are two basic classes of mutations: somatic mutations and germ-line mutations

Types of Gene Mutations Number of ways to classify gene mutations Some classification schemes are based on the nature of the phenotypic effect Base substitutions Insertions and deletions

Base Substitutions Alternation of a single nucleotide in the DNA Complementary nature of the two DNA strands When the base of one nucleotide is altered, corresponding nucleotide on the opposite strand will be altered in the next round of replication Leads to a base-pair substitution

Types of Base Substitution Base substitutions are of two types: Transition: A purine is replaced by a different purine or, alternatively a pyrimidine is replaced by a different pyrimidine Transversion: A purine is replaced by a pyrimidine or a pyrimidine is replaced by a purine. Number of possible transversions is twice the number of possible transitions, but transitions usually arise more frequently

A transition is the substitution of a purine for a purine or a pyrimidine for a pyrimidine; a transversion is the substitution of a pyrimidine for a purine or a purine for a pyrimidine

Insertions and Deletions Second major class of gene mutations contains insertions and deletions Addition or the removal of one or more nucleotide pairs Insertions and deletions are more frequent

Insertions and Deletions Frameshift mutations: Insertions and deletions often lead to a change in the reading frame of a gene Sequences that encode proteins may lead to frameshift mutations changes in the reading frame Frameshift mutations generally have drastic effects on the phenotype Addition or removal of one or more amino acids may still affect the phenotype These mutations are called in-frame insertions and deletions

Phenotypic Effects of Mutations The effect of a mutation must be considered with reference to a phenotype against which the mutant can be compared Forward Mutation: A mutation that alters the wild-type phenotype Reverse Mutation (a reversion): changes a mutant phenotype back into the wild type

Phenotypic Effects of Mutations Missense Mutation: base substitution that alters a codon in the mRNA, resulting in a different amino acid in the protein Nonsense Mutation: changes a sense codon (one that specifies an amino acid) into a nonsense codon (one that terminates translation)

Phenotypic Effects of Mutations Silent Mutation: Alters a codon but due to the redundancy of the genetic code, the codon still specifies the same amino acid Neutral Mutation: is a missense mutation that alters the amino acid sequence of the protein but does not change its function. It occurs when one amino acid is replaced by another that is chemically similar or when the affected amino acid has little influence on protein function

Phenotypic Effects of Mutations Loss-of-function Mutations: cause the complete or partial absence of normal function. A loss-of-function mutation so alters the structure of the protein Gain-of-function Mutation: produces an entirely new trait or it causes a trait to appear inappropriate tissues or at inappropriate times in development

Base substitutions can cause (a) missense, (b) nonsense, and (c) silent mutations

Phenotypic Effects of Mutations Conditional mutations: which are expressed only under certain conditions Lethal Mutations: which cause premature death

Phenotypic Effects of Mutations Suppressor Mutation: is a genetic change that hides or suppresses the effect of another mutation. This type of mutation is distinct from a reverse mutation, in which the mutated site changes back into the original wild-type sequence

Relation of forward, reverse, and suppressor mutations

Mutation Rates The frequency with which a gene changes from the wild type to a mutant Expressed as the number of mutations per biological unit, which may be mutations per cell division, per gamete, or per round of replication