Unit A Reproduction

3.1
Meiosis
LEARNING TIP
Scan the titles, headings, and
subheadings, as well as the words
in bold type. Make predictions about
what you expect to learn in Chapter 3.
egg
sperm
Figure 1 Sperm cells are much
smaller than egg cells and have a
very different shape.
In Chapter 2, you learned that the offspring of parents that reproduce
asexually are genetically identical to each other and to the parents. You also
learned that asexual reproduction involves cell division and sexual reproduction
involves sex cells joining to produce a zygote. In this section, you will find
out how the sex cells are formed.
Multicellular organisms that reproduce sexually have two types of cells:
somatic cells and sex cells. Somatic cells, also called body cells, reproduce
by cell division and make up the vast majority of an organism’s cells. Human
somatic cells have 23 pairs of chromosomes, for a total of 46 chromosomes.
The sex cells are also called gametes. Gametes have half the chromosomes of
the parent cell. Gametes make up an extremely tiny fraction of an organism’s
cells. Male gametes are called sperm, and female gametes are called eggs or
ova (singular: ovum) (Figure 1).
Chromosome Numbers
In general, individuals of the same species have the same number of
chromosomes. Goldfish have 94 chromosomes, fruit flies have 8, and humans
have 46. If sexual reproduction involves the fusing of genetic material from
two individuals, then why doesn’t the resulting offspring contain twice the
number of chromosomes as the parents? To maintain the same number of
chromosomes from generation to generation, there needs to be a way to
reduce the number of chromosomes that are passed on from each parent.
The process that produces gametes, which have half the number of
chromosomes as the parent, is called meiosis.
Meiosis happens only in the cells that produce gametes. These cells are called
reproductive cells. Meiosis is sometimes called “reduction division” because it
reduces the chromosome number by half. Each fruit fly gamete (Figure 2)
contains four chromosomes—half of the genetic material of the parent. Cells
with half the chromosome number of the parent are called haploid, symbolized
with a single n. Cells that have a complete set of chromosomes are called
diploid symbolized as 2n.
75X
n
n
4
2n
2n
4
4
Figure 2 Fruit fly somatic cells have
eight chromosomes (diploid or 2n) and
are produced by mitosis. Fruit fly gametes
have four chromosomes (haploid or n)
and are produced by meiosis.
8
8
4
4
4
74 Unit A Reproduction
NEL