Essential Cell Biology 5th edition

308 CHAPTER 9 How Genes and Genomes Evolve
Figure 9−14 The insertion of a mobile
genetic element helped produce modern
corn. Today’s corn plants were originally
bred from a wild plant called teosinte (A).
This wild ancestor produced numerous
ears that contained small, hard seeds.
(B) Modern corn, by contrast, produces
fewer cobs—but they contain numerous
plump, sweet kernels. The insertion of
a mobile genetic element near a gene
involved in seed development helped drive
the change. Here, the two plants are drawn
to the same scale; for simplicity, the leaves
are not shown.
ear
male flowers
ear
(A)
teosinte
(B) modern corn
Finally, mobile genetic elements ECB5 n9.300-9.14 provide opportunities for genome rearrangements
by serving as targets of homologous recombination (see
Figure 9−8). For example, the duplications that gave rise to the β-globin
gene cluster are thought to have occurred by crossovers between the
abundant mobile genetic elements sprinkled throughout the human
genome. Later in the chapter, we describe these elements in more detail
and discuss the mechanisms that have allowed them to establish a
stronghold within our genome.
QUESTION 9–2
Why do you suppose that horizontal
gene transfer is more prevalent
in single-celled organisms than in
multicellular organisms?
sex pilus
Genes Can Be Exchanged Between Organisms by
Horizontal Gene Transfer
So far we have considered genetic changes that take place within the
genome of an individual organism. However, genes and other portions of
genomes can also be exchanged between individuals of different species.
This mechanism of horizontal gene transfer is rare among eukaryotes
but common among bacteria, which can exchange DNA by the process of
conjugation (Figure 9–15 and Movie 9.1).
E. coli, for example, has acquired about one-fifth of its genome from other
bacterial species within the past 100 million years. And such genetic
exchanges are currently responsible for the rise of new and potentially
dangerous strains of drug-resistant bacteria. Genes that confer resistance
to antibiotics are readily transferred from species to species, providing
the recipient bacterium with an enormous selective advantage in evading
the antimicrobial compounds that constitute modern medicine’s
frontline attack against bacterial infection. As a result, many antibiotics
are no longer effective against the common bacterial infections for
which they were originally used; as an example, most strains of Neisseria
gonorrhoeae, the bacterium that causes gonorrhea, are now resistant to
penicillin, which is therefore no longer the primary drug used to treat this
disease.
flagellum
DNA
0.5 µm
Figure 9−15 Bacterial cells can exchange DNA through
conjugation. Conjugation begins when a donor cell captures a
recipient cell using a fine appendage called a sex pilus. Following
capture, DNA moves from the donor cell, through the pilus, into the
recipient cell. In this cryoelectron micrograph, the sex pilus is clearly
distinguished from the flagellum. Conjugation is one of several ways
in which bacteria carry out horizontal gene transfer. (From C.M.
Oikonomou and G.J. Jensen, Nat. Rev. Microbiol. 14:205–220, 2016.
With permission from Macmillan Publishers Ltd.)