Biology Now, 2e

100 ■ CHAPTER 06 Cell Division

CELLS

cell simply copies the circular chromosome and

each daughter cell receives one copy of the DNA

loop—resulting in two cells that are genetically

identical (Figure 6.3).

Soto, Sonnenschein, and Hunt, however,

were studying eukaryotic cells, where cell

(“S” stands for “synthesis”). A critical event

during the S phase is the copying, or replication,

of all the cell’s DNA molecules, which contain

the organism’s genetic information. The G 2

phase (for “gap 2”) occurs after the S phase but

before the start of cell division.

Early cell biologists bestowed the term “gap”

on the G 1 and G 2 phases because they believed

those phases to be less significant periods in

the life of a cell than are the S phase and cell

division. We now know that the “gap” phases

are often periods of growth during which

both the size of the cell and its protein content

increase. Furthermore, each “gap” phase serves

as a checkpoint to prepare the cell for the phase

immediately following it, ensuring that the cell

cycle does not progress unless all conditions are

suitable.

Cell division is the last stage in the cell cycle

of an individual cell. As cell division begins, the

cell contains twice the usual amount of DNA

because of DNA replication during the S phase.

Not all cells complete the cell cycle. Many types

of cells—neurons and beta cells, for example—

become specialized shortly after enter ing G 1 , and

they pull out of the cell cycle to enter a nondivid-

ing state called the G 0 phase. The G 0 phase can

last for a period ranging from a few days to the

lifetime of the organism.

Cells begin cell division for two basic reasons:

(1) to reproduce and (2) to g row and repair a

multicellular organism. Most single-celled

organisms use cell division to produce offspring

through asexual reproduction. Asexual repro-

duction generates clones, offspring that are

genetically identical to the parent.

Cell division occurs in all living organisms

(e u k a r y o t e s a nd pr ok a r y o t e s) a nd i nv ol v e s c op y-

ing the DNA in the parent cell, then delivering

one copy of it to each of two daughter cells. Most

prokaryotes carry their genetic material in just

one loop of DNA. These cells reproduce through

binary fission, a type of cell division in which a

DNA replication and segregation

Cell separation

Cell division

Parent cell

Two daughter cells

Circular DNA

molecule

New cell wall

Plasma

membrane

Cell wall

DNA is replicated,

giving rise to two

circular DNA

molecules.

The cell expands,

and a partition is

created that

isolates the two

DNA molecules

into separate

compartments.

Figure 6.3

Cell division in a prokaryote

Many prokaryotes reproduce asexually in a type of

cell division known as binary fission.

Q1: Name one similarity between cell

division in prokaryotes and eukaryotes.

Q2: Why is binary fission referred to as

“asexual reproduction”?

Q3: Name one difference between cell

division in prokaryotes and eukaryotes.

Patricia Hunt is a reproductive biologist at Washington

State University in Pullman, Washington, who studies

meiosis in mammalian cells.

PATRICIA HUNT