HUMAN BIOLOGY

362 Chapter 18

Meiosis: the beginnings of eggs and sperm

n Meiosis divides the nuclei of germ cells in a way that halves

the number of chromosomes in daughter cells. it is the first

step toward the gametes required for sexual reproduction.

n Links to Oocyte formation 16.2, sperm formation 16.4

in meiosis the parent cell

nucleus divides twice

As described in Sections 18.1 and

18.2, meiosis is like mitosis in some

ways, but the outcome is different.

During interphase, a germ cell copies its DNA, forming

duplicated chromosomes. Each duplicated chromosome

consists of two sister chromatids attached to one another:

one chromatid

centromere

its sister chromatid

one chromosome in the duplicated state

Prophase I

Metaphase I

Anaphase I

Telophase I

Meiosis I

DNA is

replicated

prior to

meiosis I

Interphase

Prophase II

Metaphase II

Anaphase II

Telophase II

Meiosis II

As in mitosis, a spindle moves chromosomes into the

proper position for the formation of daughter nuclei. In

meiosis, however, there are two consecutive divisions of the

chromosomes. The result will be four haploid nuclei. There

is no interphase between the two nuclear divisions, which

we call meiosis I and meiosis II:

During meiosis I, each duplicated chromosome lines up

with its partner, homologue to homologue. Then the two

partners are moved apart:

The cytoplasm typically divides after each homologue

is separated from its partner. The two daughter cells are

haploid, with only one of each type of chromosome.

Later, during meiosis II, the two sister chromatids of

each chromosome are separated from each other:

18.6

zygote

multicellular

body

gametes

Diploid

Haploid

mitosis

fertilization meiosis

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Each sister chromatid is now a separate chromosome.

After the four nuclei form, the cytoplasm divides again.

The result is four haploid cells, either sperm or oocytes.

Meiosis leads to the formation of gametes

The human life cycle starts with meiosis. Next come the

formation of gametes, fertilization, then growth of the new

individual by way of mitosis:

In a male, meiosis and the formation of gametes are

called spermatogenesis because the forthcoming gam-

etes will be sperm (Figure 18.9). First, a diploid germ cell

increases in size. The resulting large, immature cell (a pri-

mary spermatocyte) undergoes meiosis. The resulting four

haploid spermatids then change in form, develop tails, and

become sperm—mature male gametes.

In females, meiosis and gamete formation are called

oogenesis (Figure 18.10). As you might expect, oogenesis

differs from spermatogenesis in some important ways. For

example, compared to a primary spermatocyte, many more

components of the cytoplasm form in a primary oocyte, the

female germ cell that undergoes meiosis. Also, in females

the cells formed after meiosis are of different sizes and

have different functions.

oogenesis The formation
of a secondary oocyte.

spermatogenesis The
formation of sperm.

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