Mutations are alterations in the nucleotide sequence of the DNA molecule that occur by chance and modify the genome at random. Once they occur, mutations can be transmitted from generation to generation when DNA replicates.
Mutations that affect phenotype occur naturally at a very low rate, which varies from gene to gene. Forward mutations usually occur more often than reversions.
The agents of spontaneously occurring mutations include chemical hydrolysis, radiation such as cosmic rays and ultraviolet light, and mistakes during DNA replication. Mutagens raise the frequency of mutation above the spontaneous rate. Cells have evolved enzyme systems that minimize mutations.
Mutations are the raw material of evolution. The action of natural selection on heritable mutations is a major agent of evolution.
What mutations tell us about gene structure:
Mutations within the same gene usually fail to complement each other. The concept of a complementation group thus defines the gene as a unit of function.
A gene is composed of a linear sequence of nucleotide pairs in a discrete, localized region of a chromosome. Recombination can occur within a gene, even between adjacent nucleotide pairs.
What mutations tell us about gene function:
The function of most genes is to specify the linear sequence of amino acids in a particular polypeptide.
The sequence of amino acids in a polypeptide determines the polypeptide's three-dimensional structure, which in turn determines its function. Each protein consists of one, two, or more polypeptides. One gene encodes one polypeptide. Proteins composed of two or more different subunits are encoded by two or more genes.
How genotype correlates with phenotype:
The functions of proteins produced within an organism determine phenotype.
Some mutations modify phenotype by altering the sequence of amino acids and thus the function of a protein. Other mutations do not affect a polypeptide's amino-acid sequence but instead affect the amount, time, or place of protein production.
Dominance relations between alleles depend on the relation between protein function and phenotype. Recessive alleles often produce nonfunctional proteins. Incomplete dominance arises when phenotype varies with the amount of functional protein produced. Dominant alleles reflect either situations in which small perturbations in the amount of protein produced disrupt normal phenotypes or other rare events that affect protein structure or synthesis.
