Biology Now, 2e

106 ■ CHAPTER 06 Cell Division

CELLS

zygote inherits a haploid (n) set of chromo-

somes from each of the gametes, restoring the

complete diploid (2n) set of genetic information

to the offspring. Each homologous pair of chro-

mosomes in the zygote consists of one chromo-

some received from the father and one from the

mother. The zygote then divides by mitosis to

create a mass of cells that will eventually develop

into a mature organism (Figure 6.7). All cells

in a mature organism are diploid, containing

homologous pairs of chromosomes, except for

gametes, which are haploid and contain only one

of each homologous pair.

Meiosis occurs in two stages after inter-

phase—meiosis I and meiosis II—each involv-

ing one round of nuclear division followed by

cytokinesis, for a total of two cell divisions

(Figure 6.8). Meiosis I reduces the chromo-

some number to haploid by separating one of

each homologous pair into t wo different daugh-

ter cells. Each homologous chromosome lines

up with its partner and then separates to the

two ends of the cells, and cytokinesis occurs.

Meiosis II separates each sister chromatid

into two different daughter cells. This time, the

phases of the division cycle are almost exactly

like those of mitosis: sister chromatids sepa-

rate to the two ends of the cells, and cytoki-

nesis occurs, leading to an equal segregation

of chromatids into two new daughter cells. In

summary, meiosis I produces two haploid cells

(n) with one of each pair of duplicated chromo-

somes, and in meiosis II these two haploid cells

separate the sister chromatids and each divide,

giving rise to a total of four haploid cells (n)

with unduplicated chromosomes. Each haploid

gamete now has half of the chromosome set

found in the original diploid cell (2n) that

underwent meiosis.

BPA is toxic because it disrupts the process of

meiosis, hindering the ability of the chromosomes

to separate into four haploid cells. Hunt real-

ized that if BPA was disrupting meiosis in mice,

it could be doing the same in humans. And if a

human gamete (either the sperm or the egg) does

not contain the correct number of chromosomes,

fertilization typically results in a miscarriage.

Hunt was nervous about publishing the results

of her experiment. “We knew we were stepping

into a landmine,” she says. “We knew the paper

would get some press, because essentially we

were publishing that this chemical—used in

2 n

n n

2 n

2 n

Mother Father

Diploid parents

Diploid zygote

Diploid offspring (2n)

Meiosis

Fertilization

Mitosis

Diploid cells in the

ovary undergo

meiosis to produce

haploid egg cells.

Diploid cells in the

testes undergo

meiosis to produce

haploid sperm.

Gametes are

haploid: they have

only one copy of

each type of

chromosome and

therefore half the

chromosome set.

Fertilization

combines

chromosomes

from two haploid

gametes and

therefore restores

the diploid set.

Maternal

chromosome

Paternal

chromosome

Haploid

egg

Haploid

sperm

Figure 6.7

Fertilization creates a zygote from the fusion of two

gametes

In species with two sexes, female gametes are eggs and male gametes are

sperm. This figure shows only one of the 23 homologous pairs found in human

cells.

Q1: Is a zygote haploid or diploid?

Q2: Which cellular process creates a baby from a zygote?

Q3: If a mother or father was exposed to BPA prior to conceiving a

child, how might that explain potential birth defects in the fetus?