Gametogenesis
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: July 24, 2012
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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C ONCEPT
1
1
Gametogenesis
• Explain why sexual reproduction leads to variation in offspring.
What’s the biggest cell on Earth?
The ostrich egg - unfertilized, of course. Yes, this egg, just like a human ovum, is just one cell. The egg shell
membrane encloses the nucleus containing the genetic material and the cytoplasm.
Gametogenesis
At the end of meiosis, four haploid cells have been produced, but the cells are not yet gametes. The cells need to
develop before they become mature gametes capable of fertilization. The development of haploid cells into gametes
is called gametogenesis.
Gametogenesis may differ between males and females. Male gametes are called sperm. Female gametes are called
eggs. In human males, for example, the process that produces mature sperm cells is called spermatogenesis. During
this process, sperm cells grow a tail and gain the ability to “swim,” like the human sperm cell shown in Figure 1.2.
In human females, the process that produces mature eggs is called oogenesis. Just one egg is produced from the
four haploid cells that result from meiosis. The single egg is a very large cell, as you can see from the human egg in
Figure 1.2.
Concept 1. Gametogenesis
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FIGURE 1.1
How much DNA is in a gamete? The
sperm cell forms by meiosis and spermatogenesis. Because it forms by meiosis, the sperm cell has only half as
much DNA as a body cell. Notice the
three distinct segments: a head piece,
a flagella tail and a midpiece of mostly
mitochondria. What is the role of each
section?
FIGURE 1.2
A human sperm is a tiny cell with a tail.
A human egg is much larger. Both cells
are mature haploid gametes that are
capable of fertilization. What process is
shown in this photograph? Notice the
sperm with the head piece containing
the genetic material, a flagella tail that
propels the sperm, and a midpiece of
mostly mitochondria, supplying ATP.
Spermatogenesis and Oogenesis
During spermatogenesis, primary spermatocytes go through the first cell division of meiosis to produce secondary
spermatocytes. These are haploid cells. Secondary spermatocytes then quickly complete the meiotic division to
become spermatids, which are also haploid cells. The four haploid cells produced from meiosis develop a flagellum
tail and compact head piece to become mature sperm cells, capable of swimming and fertilizing an egg. The compact
head, which has lost most of its cytoplasm, is key in the formation of a streamlined shape. The middle piece of the
sperm, connecting the head to the tail, contains many mitochondria, providing energy to the cell. The sperm cell
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3
essentially contributes only DNA to the zygote.
On the other hand, the egg provides the other half of the DNA, but also organelles, building blocks for compounds
such as proteins and nucleic acids, and other necessary materials. The egg, being much larger than a sperm cell,
contains almost all of the cytoplasm a developing embryo will have during its first few days of life. Therefore,
oogenesis is a much more complicated process than spermatogenesis.
Oogenesis begins before birth and is not completed until after fertilization. Oogenesis begins when oogonia (singular, oogonium), which are the immature eggs that form in the ovaries before birth and have the diploid number
of chromosomes, undergo mitosis to form primary oocytes, also with the diploid number. Oogenesis proceeds as a
primary oocyte undergoes the first cell division of meiosis to form secondary oocytes with the haploid number of
chromosomes. A secondary oocyte only undergoes the second meiotic cell division to form a haploid ovum if it
is fertilized by a sperm. The one egg cell that results from meiosis contains most of the cytoplasm, nutrients, and
organelles. This unequal distribution of materials produces one large cell, and one cell with little more than DNA.
This other cell, known as a polar body, eventually breaks down. The larger cell undergoes meiosis II, once again
producing a large cell and a polar body. The large cell develops into the mature gamete, called an ovum (Figure
1.3).
FIGURE 1.3
Maturation of the ovum.
Notice only
1 mature ovum, or egg, forms during
meiosis from the primary oocyte. Three
polar bodies may form during oogenesis. These polar bodies will not form mature gametes. Conversely, four haploid
spermatids form during meiosis from the
primary spermatocyte.
Vocabulary
•
•
•
•
•
•
•
egg: Female gamete.
gametogenesis: Development of haploid cells into gametes, such as sperm and egg.
oocyte: Cell formed during meiosis in the female prior to the formation of the ovum.
oogenesis: The process of producing mature eggs.
oogonia: Immature eggs that form in the ovaries before birth; have the diploid number of chromosomes.
ovum: A mature female gamete.
polar body: The cell that results from the unequal distribution of cytoplasm during meiosis in the female.
Concept 1. Gametogenesis
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•
•
•
•
sperm: Male gamete.
spermatid: Cell formed during meiosis in the male prior to the formation of the mature sperm cell.
spermatocyte: Cell formed during meiosis in the male prior to the formation of the spermatid.
spermatogenesis: The process that produces mature sperm cells.
Summary
• Meiosis is a step during spermatogenesis and oogenesis.
• Spermatogenesis produces four haploid sperm cells, while oogenesis produces one mature ovum.
Practice
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Biology for AP* → Search: Gamete Production.
1.
2.
3.
4.
5.
6.
Describe briefly how gametes are produced. What is a germ cell?
Distinguish between a spermatogonium and an oogonium.
Distinguish between a spermatocyte and a spermatid.
What is a follicle?
Describe the corpus luteum.
Compare and contrast oogenesis and spermatogenesis.
Review
1. What is gametogenesis, and when does it occur?
2. What are the main differences between oogenesis and spermatogenesis?