Essential Cell Biology 5th edition

654 CHAPTER 19 Sexual Reproduction and Genetics
haploid egg
diploid parents
MEIOSIS
haploid sperm
FERTILIZATION
diploid zygote
MITOSIS
diploid organism
somatic cells
germ-line
cells
somatic cells
germ-line cells
Figure 19−4 Germ-line cells and somatic
cells carry out fundamentally different
functions. In sexually reproducing animals,
diploid germ-line cells, which are specified
early in development, give rise to haploid
gametes by meiosis. The gametes
propagate
ECB5
genetic
E19.05/19.04
information into the
next generation. Somatic cells (gray) form
the body of the organism and are therefore
necessary to support sexual reproduction,
but they themselves leave no progeny.
Sexual Reproduction Gives Organisms a Competitive
Advantage in a Changing Environment
The processes that generate genetic diversity during meiosis operate at
random, and so the collection of alleles an individual receives from each
parent is just as likely to represent a combination that is inferior as it is
an improvement. Why, then, should the ability to try out new genetic
combinations give organisms that reproduce sexually an evolutionary
advantage over those that “breed true” through an asexual process? This
question continues to perplex evolutionary geneticists, but one advantage
seems to be that reshuffling genetic information through sexual
reproduction can help a species survive in an unpredictably variable
environment. If two parents produce many offspring with a wide variety
of gene combinations, they increase the odds that at least one of their
progeny will have a combination of features necessary for survival in a
variety of environmental conditions. They are more likely, for example, to
survive infections by bacteria, viruses, and parasites, which themselves
continually change in a never-ending evolutionary battle. This genetic
gamble may explain why even unicellular organisms, such as yeasts,
intermittently indulge in a simple form of sexual reproduction. Typically,
they switch on this behavior as an alternative to ordinary cell division
when times are hard and starvation looms. Yeasts with a genetic defect
that makes them unable to reproduce sexually show a reduced ability to
adapt when they are subjected to harsh conditions.
Sexual reproduction may also be advantageous for another reason. In
any population, new mutations continually occur, giving rise to new
alleles—and many of these new mutations may be harmful. Sexual reproduction
can speed up the elimination of these deleterious alleles and help
to prevent them from accumulating in the population. By mating with
only the fittest males, females select for good combinations of alleles and
allow bad combinations to be lost from the population more efficiently
than they would otherwise be.
Whatever its advantages, sex has clearly been favored by evolution. In
the following section, we review the central features of this popular form
of reproduction, beginning with meiosis, the process by which gametes
are formed.
MEIOSIS AND FERTILIZATION
Our modern understanding of the fundamental cycle of events involved
in sexual reproduction grew out of discoveries made in the late 1800s,
when biologists noted that the fertilized eggs of a parasitic roundworm
contain four chromosomes, whereas the worm’s gametes (sperm and
eggs) contain only two. Gametes must be therefore produced by a special
kind of “reductive” division in which the number of chromosomes is precisely
halved (see Figure 19−3). The term meiosis was coined to describe
this form of cell division; it comes from a Greek word meaning “diminution,”
or “lessening.”
From these early experiments on roundworms and other species, it
became clear that the behavior of the chromosomes, which at that time
were simply microscopic bodies of unknown function, matched the pattern
of inheritance, in which the two parents make equal contributions
to the character of the progeny despite the enormous difference in size
between egg and sperm (see Figure 19−2). These observations were
among the first clues that chromosomes contain the material of heredity.
The study of sexual reproduction and meiosis therefore has a central
place in the history of cell biology.