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NEL Cell Division 559

Section17.

M:

mitosis and

cytokinesis

cell

cycle

S:

synthesis of DNA

for duplication of

chromosomes

G1:

phase of

rapid cell

growth

G2:

growth and

preparation for

cell division

INT

ERPHASE

DI

VISI

ON PHA

SE

Figure 2

The cell cycle. The circle represents

the entire life cycle of the cell,

which can be divided into two

major phases: interphase and the

division phase. Most cells spend the

majority of their time in interphase.

This combination of DNA and proteins is called chromatin. As the cell moves through

the cell cycle, chromosomes may be either uncondensed or condensed. Uncondensed

chromosomes are long, thin strands that cannot be seen under a light microscope. A

condensed chromosome can be seen under a light microscope and may resemble the

diagram in Figure 3. Condensed chromosomes may be either unduplicated or dupli-

cated. In a duplicated chromosome, the original chromosome and its duplicate are

attached to each other by a structure called the centromere. While attached to one

another, the two chromosome duplicates are called sister chromatids. Since sister chro-

matids contain identical genetic information, the pair, attached at the centromere, is

still considered to be one chromosome.

chromatinthe complex of DNA

and protein that make up

chromosomes

centromerethe structure that

holds chromatids together

sister chromatidsa chromosome

and its duplicate, attached to one

another by a centromere until

separated during mitosis

one chromosome
(unduplicated)

one chromosome

(duplicated)

centromere

sister chromatids

Figure 3

An unduplicated and a duplicated

chromosome