Page 60 - Genetics_From_Genes_to_Genomes_6th_FULL_Part2

PART II What Genes Are and What They Do
7

chapter

Anatomy and

Function of a Gene:

Dissection Through

A scale played on a piano keyboard and a gene on a chromosome
Mutation are both a series of simple, linear elements (keys or nucleotide
pairs) that produce information. A wrong note or an altered
nucleotide pair calls attention to the structure of the musical
scale or the gene.
© Ingram Publishing RF

chapter outline

• 7.1 Mutations: Primary Tools of Genetic Analysis
• 7.2 Molecular Mechanisms That Alter DNA
Sequence
• 7.3 DNA Repair Mechanisms
HUMAN CHROMOSOME 3 CONSISTS of approximately • 7.4 What Mutations Tell Us About Gene Structure
220 million base pairs and carries about 1600 genes • 7.5 What Mutations Tell Us About Gene Function
(Fig. 7.1). Somewhere on the long arm of the chromosome • 7.6 A Comprehensive Example: Mutations That
resides the gene for rhodopsin, a light-sensitive protein Affect Vision
active in the rod cells of our retinas. The rhodopsin gene
determines perception of low-intensity light. People who
carry the normal, wild-type allele of the gene see well in
a dimly lit room and on the road at night. One simple change—a mutation—in the
rhodopsin gene, however, diminishes light perception just enough to lead to night
blindness. Other alterations in the gene cause the destruction of rod cells, resulting
in total blindness. Medical researchers have so far identified more than 30 mutations
in the rhodopsin gene that affect vision in different ways.
The case of the rhodopsin gene illustrates some very basic questions. Which of the
220 million base pairs on chromosome 3 make up the rhodopsin gene? How are
the base pairs that constitute this gene arranged along the chromosome? How can a
single gene sustain so many mutations that lead to such divergent phenotypic effects?
In this chapter, we describe the ingenious experiments performed by geneticists during
the 1950s and 1960s as they examined the relationships among mutations, genes, chro-
mosomes, and phenotypes in an effort to understand, at the molecular level, what genes
are and how they function.
We can recognize three main themes from the elegant work of these investigators.
The first is that mutations are heritable changes in base sequence that can affect pheno-
type. The second is that physically, a gene is usually a specific protein-encoding seg-
ment of DNA in a discrete region of a chromosome. (We now know that some genes
encode various kinds of RNA that do not get translated into protein.) Third, a gene is not
simply a bead on a string, changeable only as a whole and only in one way, as some had
thought. Rather, genes are divisible, and each gene’s subunits—the individual nucleo-
tide pairs of DNA—can mutate independently and can recombine with each other.
Knowledge of what genes are and how they work deepens our understanding of
Mendelian genetics by providing a biochemical explanation for how genotype

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