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U N I T 1
Cell and Tissue Function
Cell Division
Cells reproduce by duplicating their chromosomes and
dividing in two. There are two types of cell division:
mitosis and meiosis.
Mitosis
is the cell cycle process in
which nongerm cells are replicated. It provides a way
for the body to replace cells that have a limited life span,
such as skin and blood cells; increase tissue mass during
periods of growth; and repair tissue, such as in wound
healing.
Meiosis
is limited to replicating germ cells and
takes place only once in a cell line. It results in the
formation of gametes or reproductive cells (i.e., ovum
and sperm), each of which has only a single set of
23 chromosomes. Meiosis is typically divided into
two distinct phases: meiosis I and meiosis II (Fig. 5-7).
During meiosis I, homologous chromosomes pair up,
forming a double-structured chromosome containing
four chromatids (four strands) and therefore called a
tetrad (two chromatids per chromosome). They are
also sometimes called
bivalents.
The X and Y chro-
mosomes are not homologs and do not form biva-
lents. While in meiosis I, an interchange of chromatid
segments can occur (Fig. 5-8). This process, called
crossing over,
allows for new combinations of genes,
increasing genetic variability.
After cell division I, each of the two daughter cells
contains one member of each homologous pair of chro-
mosomes and a sex chromosome (23 double-stranded
chromosomes). No DNA synthesis occurs before mei-
otic division II. During cell division II, the 23 double-
stranded chromosomes (two chromatids) of each of the
two daughter cells from meiosis I divide at their centro-
meres (central regions where the chromatids meet). Each
subsequent daughter cell receives 23 single-stranded
chromatids. Thus, a total of four daughter cells are
formed by a meiotic division of one cell.
Chromosome Structure
Cytogenetics
is the study of the structure and numeric
characteristics of the cell’s chromosomes. Chromosome
studies can be done on any tissue or cell that grows and
divides in culture. Lymphocytes from venous blood are
frequently used for this purpose. After the cells have been
cultured, a drug called
colchicine
is used to arrest mito-
sis before the chromosomes separate. A chromosome
spread is prepared by fixing and spreading the chro-
mosomes on a slide. Subsequently, appropriate staining
techniques show the chromosomal banding patterns so
they can be identified. The chromosomes are imaged,
and the photoimages of each of the chromosomes are
cut out and arranged in 23 pairs according to a standard
classification system (see Fig. 5-6). The completed pic-
ture is called a
karyotype,
and the procedure for prepar-
ing the picture is called
karyotyping.
While the chromosomes are aligned on the equato-
rial plate of the cell, each chromosome takes the form
of chromatids to form an “X” or “wishbone” pat-
tern. Human chromosomes are divided into three types
according to the position of the centromere (Fig. 5-9).
Pairing
begins
Meiosis I
Meiosis II
First maturation
division
(23 double-structured
chromosomes)
Second maturation
division
(23 single chromosomes)
Pairing of
chromosomes
Chiasma
formation
Pulling apart of
double-structured
chromosomes
Anaphase of 1st
meiotic division
First meiotic
division
(cells have 23
double-structured
chromosomes)
Second
meiotic
division
(23 single
chromosomes)
These cells contain 46 double-structured chromosomes
Primary oocyte
after DNA replication
Secondary oocyte
Mature oocyte
(22 + X)
Spermatids (22 + Y)
Polar
bodies
(22 + X)
Secondary
spermatocyte
Primary spermatocyte
after DNA replication
A
A
B
B
C
D
E
F
G
Meiosis I
Meiosis II
FIGURE 5-7.
First and second meiotic divisions.
(
Top
)
Meiosis
I, during which homologous chromosomes
(A)
approach
each other and
(B)
pair;
(C)
intimately paired homologous
chromosomes interchange chromatid fragments (crossing
over) and
(D)
double-structured chromosomes pull apart.
(E)
Anaphase of first meiotic division. During meiosis II
(E, F)
, the double-structured chromosomes pull apart at
the centromere to form four single-stranded chromosomes
(reduction division).
(
Bottom
)
Events occurring during meiosis
I and II in female and male gametes.
(A)
The primitive female
germ cell (oocyte) produces only one mature gamete, the
mature oocyte.
(B)
The primitive male germ cell (primary
spermatocyte) produces four spermatids, all of which develop
into spermatozoa. (Adapted from Sadler RW. Langman’s
Medical Embryology, 9th ed. Philadelphia, PA: Lippincott
Williams &Wilkins; 2003.)
