Essential Cell Biology 5th edition

Generating Genetic Variation
299
ORIGINAL GENOME
gene
MUTATION
WITHIN A GENE
ALTERED GENOME
mutation
Figure 9–2 Genes and genomes
can be altered by several different
mechanisms. Small mutations, duplications,
rearrangements, and even the infusion
of fresh genetic material all contribute to
genome evolution.
regulatory
DNA
MUTATION IN
REGULATORY DNA
gene
mutation
mRNA
GENE
DUPLICATION
AND DIVERGENCE
gene
introns
exon
gene A
EXON
SHUFFLING
+ +
gene B
mobile genetic element
TRANSPOSITION
+ +
regulatory
DNA
gene
insertion
organism A
HORIZONTAL
TRANSFER
organism B
organism B with new
gene from organism A
genome evolution. But first, we pause to consider the contribution of
sex—the mechanism that many organisms use to pass genetic information
on to future generations.
In Sexually Reproducing Organisms, Only Changes to
the Germ Line Are Passed On to Progeny
For bacteria and unicellular organisms that reproduce asexually, the
ECB5 e9.02/9.02
inheritance of genetic information is fairly straightforward. Each individual
duplicates its genome and donates one copy to each daughter cell
when the individual divides in two. The family tree of such unicellular
organisms is simply a branching diagram of cell divisions that directly
links each individual to its progeny and to its ancestors.
For a multicellular organism that reproduces sexually, however, the family
connections are considerably more complex. Although individual cells
within that organism divide, only the specialized reproductive cells—the
gametes—carry a copy of its genome to the next generation of organisms
(discussed in Chapter 19). All the other cells of the body—the somatic
cells—are doomed to die without leaving evolutionary descendants of
QUESTION 9–1
In this chapter, we argue that
genetic variability is beneficial for
a species because it enhances that
species’ ability to adapt to changing
conditions. Why, then, do you think
that cells go to such great lengths
to ensure the fidelity of DNA
replication?