Essential Cell Biology 5th edition

The Benefits of Sex
653
Figure 19−2 Despite their tremendous
difference in size, sperm and egg
contribute equally to the genetic
character of the offspring. This difference
in size between male and female gametes
(in which eggs contain a large quantity of
cytoplasm, whereas sperm contain almost
none) is consistent with the fact that the
cytoplasm is not the basis of inheritance.
If it were, the female’s contribution to the
makeup of the offspring would be much
greater than the male’s. Shown here is a
scanning electron micrograph of an egg
with human sperm bound to its surface.
Although many sperm are bound to the
egg, only one will fertilize it. (David M.
Philips/Science Source.)
25 µm
The sexual reproductive cycle thus involves an alternation of haploid
cells, each carrying a single set of chromosomes, with generations of
diploid cells, each carrying two sets of chromosomes. One benefit of this
arrangement is that it allows sexually reproducing organisms to produce
offspring that are genetically ECB5 diverse, e19.03/19.02 as we discuss next.
mother
diploid parents
father
Sexual Reproduction Generates Genetic Diversity
Sexual reproduction produces novel chromosome combinations. During
meiosis, the maternal and paternal chromosome sets in the diploid germline
cells are partitioned into the single chromosome sets of the gametes.
Each gamete will receive a mixture of maternal homologs and paternal
homologs; when the genomes of two gametes combine during fertilization,
they produce a zygote with a unique chromosomal complement.
If the maternal and paternal homologs carry the same genes, why should
such chromosomal reassortment matter? One answer is that although
the set of genes on each homolog is the same, the paternal and maternal
version of each gene is not. Genes occur in variant versions, called
alleles, with slightly different DNA sequences. For any given gene, many
different alleles may be present in the “gene pool” of a species. The existence
of these variant alleles means that the two copies of any given gene
in a particular individual are likely to be somewhat different from each
other—and from those carried by other individuals. What makes individuals
within a species genetically unique is the inheritance of different
combinations of alleles. And with its cycles of diploidy, meiosis, haploidy,
and cell fusion, sex breaks up old combinations of alleles and generates
new ones.
Sexual reproduction also generates genetic diversity through a second
mechanism—homologous recombination. We discuss this process, which
scrambles the genetic information on each chromosome during meiosis,
a bit later.
Figure 19−3 Sexual reproduction involves both haploid and diploid cells.
Sperm and egg are produced by meiosis of diploid germ-line cells. During
fertilization, a haploid egg and a haploid sperm fuse to form a diploid zygote.
For simplicity, only one chromosome is shown for each gamete, and the sperm
cell has been greatly enlarged. Human gametes have 23 chromosomes, and
the egg is much larger than the sperm (see, for example, Figure 19−2).
haploid egg
one pair
of homologs
MEIOSIS
FERTILIZATION
diploid zygote
MITOSIS
haploid sperm
genetically unique
diploid organism
composed of many cells
maternal
homolog
paternal
homolog