Fundamentals of Anatomy and Physiology

80

Anaphase I
The microtubules of the spindle shorten and pull the cen-
tromeres toward the poles, dragging both sister chroma-tids
with it. Thus, unlike mitosis, the centromere does not
divide in this stage. Because of the random orientation of
the homologous chromosomes on the equatorial plate, a
pole may receive either homologue of each pair. Thus, the
genes on different chromosomes assort independently.

Telophase I
The homologous chromosome pairs have separated and
now a member of each pair is at the opposite ends of the
spindle. Now at each pole is a cluster of “haploid” chro-
mosomes. The number has been reduced from 46 to 23 at
each pole. However, each chromosome still consists of two
sister chromatids attached by a common centro-mere. This
“duplication condition” will be corrected in the second
meiotic division. Now the spindle disappears, the
chromosomes uncoil and become long and thin, and a new
nuclear membrane forms around each cluster of
chromosomes at the opposite poles. Cytokinesis occurs and
we have two new cells formed at the end of the first meiotic
division. The second meiotic division closely re-sembles
the occurrences in mitosis.

Prophase II
In each of the two daughter cells produced in the first
meiotic division, a spindle forms, and the chromosomes
shorten, coil, and thicken. The nuclear membrane
disappears but no duplication of DNA occurs.

Metaphase II
In each of the two daughter cells, the chromosomes line up
on the equatorial plate. Spindle fibers bind to both sides of
the centromere. Each chromosome consists of two sister
chromatids and one centromere.

Anaphase II
The centromeres of the chromosomes divide. The spindle
fibers contract, pulling the sister chromatids apart and
moving each one to an opposite pole. Now each chro-
mosome is truly haploid, consisting of one chromatid and
one centromere.

Telophase II
New nuclear membranes form around the separated
chromatids, the spindle disappears, and the chromo-somes
uncoil and decondense. The result is four -haploid daughter
cells each containing one-half the genetic ma-terial of the
original parent cell, or, in our case, each cell having 23
chromosomes instead of 46.

Chapter 4

Gametogenesis: The Formation of the Sex Cells.

of the Sex Cells

The four haploid cells produced by meiosis are not yet

mature sex cells. Further differentiation must now occur.

This is known as gametogenesis (gam-eh-toh-JEN-eh-

sis). The process occurring in the seminiferous tubules of

the testes is called spermatogenesis (sper-mat-oh-JEN-

eh-sis) (Figure 4-15). The cytoplasm of each of the four

cells produced, called spermatids, becomes modified into a

tail-like flagellum. A concen-tration of mitochondria

collects in the middle piece or collar. The mitochondria will

produce the ATP neces-sary to propel the flagellum, which

causes the sperm to swim. The nucleus of each cell

becomes the head of the sperm. The genetic material is

concentrated in the head of the sperm. The sperm cell will

penetrate an egg and fuse with the genetic material of the

egg in the process called fertilization, producing a fertilized

egg or zygote.

The formation of the female egg, called oogenesis

(oh-oh-JEN-eh-sis), occurs in the ovary (Figure 4-16).

However, only one functional egg is produced. In the first

meiotic division, there is an unequal distribution of the

cytoplasm so that one cell is larger than the other. The

larger cell in the second meiotic division also has un-equal

distribution of the cytoplasm. The three smaller cells

produced are called polar bodies and eventually die.

They have contributed cytoplasm to the single larger cell

that will become the functional egg. The union of sperm

and egg is called fertilization and restores the diploid

number of chromosomes to 46.

A Comparison of Mitosis

and Meiosis

The two types of cellular division consisting of mitosis and

meiosis are easy to confuse. They have similari-ties but

they also have differences. In both mitosis and meiosis, the

chromosomes duplicate or replicate in the phase of the cell

cycle called interphase. However, in mitosis the end result

is two daughter cells each with exactly the same number of

chromosomes as the parent cell, whereas in meiosis the end

result is four daughter cells each with only half the number

of chromosomes as the parent cell. Mitosis consists of one

division, whereas meiosis consists of two divisions.

In mitosis, when the genetic material duplicates, the

homologous chromosomes are scattered in the -nucleus and

do not seek one another out. In meiosis, -after duplication,

the homologous pairs of chromosomes