Biology Now, 2e

Toxic Plastic ■ 99

Figure 6.2

The cell cycle

The eukaryotic cell cycle consists of two major stages: interphase and cell

division. M

Q1: When is DNA replicated during the cell cycle?

Q2: When in the cell cycle does DNA separate into the two

genetically identical daughter cells?

Q3: If a cell is not destined to separate into daughter cells, what

phase does it enter? Is this part of the cell cycle?

DNA replication

happens here.

Many cells in the human

body exit the cell cycle

and remain in a resting

stage for some time.

Some cells never leave

this phase.

G 2 ph

ase

Mi

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sis

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yt

o

ki

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is

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se

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G 0 phase

6

2

Checks are made to

ensure that conditions are

suitable for cell division.

3

DNA molecules

are divided equally

between the two

daughter cells.

5

The replicated

DNA is separated

with precision.

4

The cell has grown and is

ready to replicate its DNA.

1

Interphase

Cell

division

undergo a specialized type of cell division that

produces the eggs. She was shocked. The cells

were a mess, the chromosomes scrambled.

A whopping 40 percent of the resulting eggs

had chromosomal defects. “The controls were

completely bonkers,” says Hunt. “One week they

were fine, the next week they weren’t. That’s

when we knew something was going on.”

Like Soto and Sonnenschein, Hunt scru-

tinized every method and every piece of lab

equipment used in the experiment, looking for

the culprit. But as weeks passed, she couldn’t

figure out what had ruined her experiment.

Soto, Sonnenschein, and Hunt didn’t know

it at the time, but their botched experiments

would change the course of their scientific

careers forever. The three would spend the

next decade identifying, tracking, and inves-

tigating a toxic chemical that pervades our

environment.

Divide and Conquer

In Soto and Sonnenschein’s experiment, the

breast cells were multiplying under the wrong

circumstances. In Hunt’s experiment, the

mouse oocytes were not producing egg cells

correctly. In both cases, something was inter-

fering with the ability of the cells to divide—

disrupting the cell cycle. The cell cycle is a

sequence of events that make up the life of a

typical eukaryotic cell, from the moment of

its origin to the time it divides to produce two

daughter cells. The time it takes to complete

a cell cycle depends on the organism, the type

of cell, and the life stage of the organism.

Human cells, for example, typically have a

24-hour cell cycle, while Hunt’s mouse oocytes

can take days to complete a cycle. Some fly

embryos, on the other hand, have cell cycles

that are only 8 minutes long.

There are two main stages in the cell cycle

of eukaryotes: interphase and cell division—

each marked by distinctive cell activities

(Figure 6.2). Interphase is the longest stage

of the cell cycle; most cells spend 90 percent or

more of their life span in interphase. During this

phase, the cell takes in nutrients, manufactures

proteins and other substances, expands in size,

and conducts special functions depending on

the cell type. Neurons in the brain, for example,

transmit electrical impulses, while beta cells in

the pancreas release insulin.

Interphase can be divided into three main

intervals: G 1 , S, and G 2. The G 1 phase (for

“gap 1”) is the first phase in the life of a newborn

cell. In cells that are destined to divide, prepara-

tions for cell division begin during the S phase