HUMAN BIOLOGY

CeLL reprOduCtIOn 357

a chromosome undergoes changes

in preparation for cell division

As already mentioned, a chromosome consists of DNA and

some proteins that are attached to it. (Together, the DNA

and protein are called chromatin.) Before a cell begins to

divide by either mitosis or meiosis, its chromosomes are

duplicated:

For much of the cell division process, each chromosome

and its copy stay together. During this time they are called

sister chromatids. A sister chromatid has at least one

“pinched in” region called a centromere. It will provide

attachment sites for microtubules that move chromosomes

when a cell nucleus is dividing.

With this background we now begin a closer look at cell

division, starting with mitosis.

What are the cell cycle, mitosis, and meiosis?

• The cell cycle is the sequence of events in a somatic cell during
  which the cell grows, doubles the components of its cytoplasm,
  then copies its DNA, forming duplicates of its chromosomes.


• A somatic cell divides its nucleus and cytoplasm at the end
  of the cycle. The division of a somatic cell’s nucleus is called
  mitosis. Mitosis gives each daughter cell the same diploid
  number of chromosomes as were in the parent cell.


• Meiosis occurs in germ cells that give rise to gametes. Meiosis
  provides each gamete with a haploid number of chromosomes,
  one half the number that were in the parent cell.

taKe-hoMe Message

The length of the cell cycle
varies depending on the type of
cell. For instance, the cycle lasts
25 hours in epithelial cells in
your stomach lining and 18 hours
in bone marrow cells. New red
blood cells form and replace your
worn-out ones at an average rate
of 2 to 3 million each second.
Mitosis occurs in cells that
are dividing as the body grows
or repairs tissues. It assures that
each new “daughter” cell is diploid, with a full comple-
ment of 46 parental chromosomes. By contrast, meiosis
(my-oh-sis) occurs in germ cells in the ovaries and testes,
and it halves the number of parental chromosomes (Figure
18.5). Two rounds of division, called meiosis I and meiosis
II, produce gametes that are haploid cells, with half the
diploid number of chromosomes. Table 18.1 summarizes
the differences between mitosis and meiosis, which we’ll
look at in more detail in the following sections.

Figure 18.5 Meiosis halves the chromosome number.

A In meiosis I, each duplicated

chromosome in the nucleus pairs

with its homologous partner.

C Sister chromatids separate in meiosis II. The now undupli-

cated chromosomes are packaged into four new nuclei.

B Homologous partners separate.

The still-duplicated chromosomes are

packaged into two new nuclei.

Mitosis Meiosis

Function Growth, including Gamete production

repairing and maintaining (sperm/eggs)

tissues

Occurs in Somatic (body) cells Germ cells in gonads

(testes and ovaries)

Mechanism Chromosomes are Chromosomes are

duplicated once, then duplicated twice,

the cytoplasm is divided then the cytoplasm

is divided

Outcome Maintains the diploid Halves the diploid

chromosome number chromosome

(2n → 2 n) number (2n → n)

Effect Two diploid Four haploid

daughter cells daughter cells

Table 18.1 Overview of Mitosis and Meiosis

one chromatid

centromere

its sister chromatid

a chromosome

(unduplicated)

a chromosome

(duplicated)

© Cengage Learning

© Cengage Learning

centromere Constricted
area where sister chroma-
tids attach to each other.

meiosis Process that
divides the nucleus in a
dividing germ cell in a way
that halves the chromosome
number in daughter cells.

sister chromatids A dupli-
cated chromosome and its
copy.

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