Biology Now, 2e

104 ■ CHAPTER 06 Cell Division

CELLS

Each chromosome is replicated

before mitosis begins.

As mitosis begins, the chromatin

packs even more tightly, and the

chromosome becomes easily

visible with a light microscope.

Centromere

Centromere

One

replicated

chromosome

Two sister

chromatids

Two sister

chromatids

One chromosome

During gap phase Synthesis phase At the beginning of mitosis One

replicated

chromosome

Figure 6.5

Chromosomes are copied and condensed in preparation for cell division

Chromosomes spend the majority of the cell cycle unpackaged (left). They are copied (replicated) during synthesis, then tightly

packaged, or condensed, during early mitosis.

Q1: Why is it important for a chromosome to be copied before mitosis?

Q2: Are sister chromatids attached at the centromere considered to be one or two chromosomes?

Q3: Why is the chromosome’s DNA tightly packed for mitosis and cytokinesis? (Hint: Think about what would happen if it

were unpackaged, as during interphase.)

next (Figure 6.6). For example, upon receiving

the appropriate signals, cell cycle regulatory

proteins advance a cell from the G 1 phase to the

S phase by triggering chromosome replication

and other processes associated with it.

Cell cycle regulatory proteins also respond

to negative internal or external control signals.

Internal signals will pause a cell in the G 1 phase

under any of the following conditions: the cell

is too small, the nutrient supply is inadequate,

or the cell’s DNA is damaged. G 2 pauses in the

same circumstances, as well as when chromo-

some duplication in the S phase is incomplete

for any reason.

Nonylphenol interferes with the G 0 and G 1

checkpoints. In essence, it gives the cell a green

light to enter the cell cycle at a time when the cell

would not normally divide. So, when Soto and

Sonnenschein realized that nonylphenol enables

human breast cells and rat uterine cells to divide

inappropriately, they became concerned. If

nonylphenol was being used in everyday plastics,

it was possible that healthy human cells were

exposed to it on a regular basis.

In a 1991 paper detailing their discovery, Soto

and Sonnenschein wrote that nonylphenol might

be interfering with science experiments like

theirs and that, even more important, it could be

harmful to humans. “From the very beginning,

we realized that this could be a health problem,”

says Sonnenschein.

Unequal Division

In Ohio, Patricia Hunt was having no success

determining why her control mouse eggs divided

abnormally, with either too many or too few chro-

mosomes. After searching in vain for months, one

day she noticed that something was wrong with