Download I. Mutations: primary tools of genetic analysis

Chapter 6: Anatomy and Function of a Gene: Dissection through Mutation
Outline
I.
Mutations: primary tools of genetic analysis
A. Mutations are heritable changes in base sequence that modify the information content
of DNA ð one way geneticists classify mutations is by their effect on the DNA molecule
B. Spontaneous mutations affecting genes occur at a very low rate
1. The mutation rate varies from gene to gene
2. Forward mutations occur more often than reverse mutations
C. Mutations arise from many kinds of random events
1. Mutations are chance occurrences modifying the genome at random
2. Hydrolysis, radiation, ultraviolet light, and oxidation can alter the information stored in
DNA
3. Mistakes during DNA replication can also alter genetic information
4. Unequal crossing over and transposons cause DNA changes that are not susceptible to
excision or mismatch repair
5. Mutagens induce mutations
D. Impact: mutations have consequences for the evolution of species and the survival of
organisms
II. What mutations tell us about gene structure
A. Complementation testing reveals whether two mutations are in the same or different
genes
B. A gene is a linear sequence of nucleotides that can mutate independently and
recombine with each other
1. The experimental system: Analyzing multiple mutations in the rIIA gene of
bacteriophage T4
2. The rII region has two genes
3. A gene is composed of mutable elements that can recombine
C. A gene is a discrete linear set of nucleotides
1. Using deletions to map mutations and define a gene
2. Hot spots are more prone to mutation
III. What mutations tell us about gene function
A. The one-gene, one-enzyme hypothesis: a gene contains the information for producing
a specific enzyme
1. The experimental evidence for “one gene, one enzyme”
2. Interpretation of results: genes encode enzymes
B. Genes direct the synthesis of proteins by specifying the identity and order of amino
acids in a polypeptide chain
1. Proteins are linear polymers of amino acids linked by peptide bonds
2. The primary business of most genes is to specify the amino acid sequence of the
proteins they encode
3. The sequence of amino acids in a polypeptide determines a protein’s three-dimensional
shape and thus its function
4. Some proteins consist of more than one polypeptide
5. Summation: Each gene directs the synthesis of a particular polypeptide
IV. How genotype correlates with phenotype
A. Dominance relations between alleles depend on the relation between protein function
and phenotype
1. Alleles that produce nonfunctional proteins are usually recessive
2. Incomplete dominance can arise when the phenotype varies in proportion to the
amount of functional protein
3. Dominance can reflect several kinds of phenomena
V. How gene mutations affect light-receiving proteins and vision : a comprehensive
example
A. The cellular and molecular basis of vision
1. The cells
2. Four genes, four polypeptides
3. The rhodopsin gene family evolved by duplication and divergence
B. How mutations in the rhodopsin family influence the way we see
1. Many amino acid substitutions in rhodopsin result in partial or complete blindness
2. Mutations in the genes for the cone-cell pigments alter color vision in predictable ways
3. Unequal crossing over between the red and green genes produces most of the
variations in red-green perception
4. Some mutations knock out the ability to see both red and green
Essential Concepts

Mutations are alterations in the DNA molecule that occur by chance and modify the
genome at random. Once they occur, they can be transmitted from generation to
generation.
a. Mutations that affect phenotype occur naturally at a very low rate, which varies
from gene to gene. Forward mutations occur more often than reversions.
b. 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.
c. Mutations are the raw material of evolution. The action of natural selection on
heritable mutations is one agent of evolution.

What mutations tell us about gene structure:
a. 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.
b. A gene is composed of a linear sequence of nucleotides in a discrete, localized
region of a chromosome. Recombination can occur within a gene, even between
adjacent nucleotides

What mutations tell us about gene function:
a. The function of most genes is to specify the linear sequence of amino acids in a
particular polypeptide.
b. The sequence of amino acids in a polypeptide determines the polypeptide’s threedimensional structure, which in turn determines its function. Each protein consist 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:
a. The functions of proteins produced within an organism determine phenotype.
b. 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.
c. 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 either reflect situations in which small
perturbations in the amount of protein produced disrupt normal phenotypes or other
rare events that affect protein structure or synthesis.