Download Reading: The Cells of Genetic Continuity

Reading: The Cells of Genetic Continuity
Revised by IHS
What, exactly, do we get from our parents? And
how does it get from them to us? The legacy
given to us by our parents is their
chromosomes. Their chromosomes come to us
packaged in their gametes. To understand sexual
reproduction, we need to explore where
gametes originate, how they develop, and what
their purpose is.
During development of an organism, cells take
on special functions in addition to their
“housekeeping” functions of metabolism. Muscle
cells make proteins that enable the cells to
expand and contract. Brain cells make proteins
that enable them to communicate by chemical
and electrical interactions. Similarly, certain cells
in the reproductive organs (ovaries or testes)
become sex cells and develop the capacity to
carry out the reproductive functions. Males
develop sperm cells; females develop egg cells.
Within these cells is all the information needed
to create the next generation.
complete set of unpaired chromosomes. The
cytoplasm of the oocyte is distributed unequally.
This results in one large cell and one smaller
cell. In Meiosis II, the two chromatids of each
chromosome separate and cell division takes
place. This results in three small cells and one
large cell; each cell contains one set of unpaired
chromosomes. The large cell matures to
become the ovum or egg. It contains one copy
of each chromosome of the woman and is ready
to be fertilized by the sperm. Thus, during
maturation of the egg the chromosome number
is reduced from 46 to 23. This is the haploid
number in humans.
What do these sperm and egg cells look like?
How do they form? All human body cells,
except sex cells, contain 46 chromosomes.
Meiosis is the process that produces sex cells.
This process reduces the chromosome number
to half.
The female reproductive cell (ovum or egg)
develops by a process called oogenesis. At
birth, a human female contains about 400,000
primary oocytes in her ovary. These oocytes
contain 46 chromosomes (the diploid or full
chromosome number) or 23 pairs. This is the
same number of chromosomes as every other
cell in the body. Maturation of the primary
oocyte involves several steps as illustrated in
Figure 1. Before meiosis, the cell contains a
complete set of paired or homologous
chromosomes. Then, the chromosomes
replicate; they form two chromatids in each
chromosome. In Meiosis I, the homologous
chromosomes separate and the oocyte divides.
This results in two cells; each cell contains one
At monthly intervals after puberty, one egg is
released from the ovary (ovulation) and
makes its way down the Fallopian tube to
the uterus (see Figure 2). If the egg is
fertilized, it will attach to the uterine wall and
develop. Otherwise, it will travel down the
vaginal canal and be released in the monthly
menses (menstruation). During a woman’s
reproductive life, approximately 400 eggs will
make this journey.
Fig. 3. Male reproductive organs,
viewed in mid-sagittal section. In other
words, we see the left leg only.
Fig. 2. Female reproductive organs,
viewed in mid-sagittal section. In other
words, we see the left leg only, as well
as the left ovary and left Fallopian tube
only.
A similar process of chromosome reduction or
meiosis takes place during sperm development
or spermatogenesis. The primary
spermatocyte also has 46 chromosomes.
Development takes place in the male sex organ,
the testis, and involves several steps as shown
in Figure 1 (on front side of handout). The cell
contains a complete set of paired or
homologous chromosomes. Prior to meiosis,
the chromosomes replicate; they form two
chromatids in each chromosome. In Meiosis I,
the homologous chromosomes separate, and
the spermatocyte divides. This results in two
cells; each cell contains one complete set of
unpaired chromosomes. In Meiosis II, the two
chromatids of each chromosome separate and
cell division takes place. This results in four cells;
each cell contains one set of unpaired
chromosomes. Each of these cells then matures
to become sperm. Each sperm contains a
complete set of unpaired chromosomes (23) of
the man, a compacted head, and a long powerful
flagellum (tail) that enables the sperm to
move.
In the female, only one mature egg is produced
for each oocyte. But a primary spermatocyte
will produce four mature sperm. After
maturation is complete, sperm leave the testes
and travel through a system of ducts or tubules.
These ducts produce fluids to help move the
sperm to the opening of the penis where they
are released (see Figure 3). After puberty, males
continually produce vast numbers of sperm—
approximately 200 million per day.
When egg and sperm meet and fuse in the
process called fertilization, a new kind of cell is
formed. This cell is called a zygote. The zygote
contains a full complement of 46 chromosomes.
It is the starting place for the development of a
complex, multicellular organism. 