CHAPTER 17
Section 17.3-2
Gametogenesis
Gametogenesis
• Gametogenesis – the formation of gametes or sex cells
in animals.
• Gametes are all created by the general process of
meiosis.
• However, there are some differences in the formation of
sperm and egg cells.
Spermatogenesis
• Formation of sperm.
• Very basic meiosis.
• The first meiotic division is the splitting of a spermatocyte
(spermacytes = plural) (46 chromosomes) into two
spermatids (23 chromosomes).
• The second meiotic division is the splitting of the
spermatids into sperm cells. This split does not change
the number of chromosomes.
Spermatogenesis
spermatid
Oogenesis
• Formation of eggs.
• The difference in oogenesis is that the cytoplasm of the
female gametes does not split evenly.
• In the first meiotic division, the primary oocyte (46
chromosomes) divides to form the secondary oocyte (23
chromosomes) and the first polar body.
• In humans, the first polar body does not undergo a
second meiotic division because it dies off due to lack of
cytoplasm.
Oogenesis Cont’d
• The second meiotic division of the secondary oocyte is
quite different. It occurs in two steps.
• As we have learned, in order for the secondary oocyte to
divide into the ovum and the second polar body, it must
become fertilized.
• In the first step of meiosis II in oogenesis, the secondary
oocyte pauses its division at metaphase II.
• While it is sitting in metaphase II, half-way through the
second division, it is known as an ootid.
Oogenesis Cont’d
• An ootid is formally known as an unfertilized ovum,
because it is halfway between the secondary oocyte and
the fertilized ovum.
• Once sperm becomes present and the ootid is fertilized,
the meiotic division will continue on to produce the
fertilized ovum and the second polar body.
• Meiosis II is now complete.
Oogenesis
Primary Oocyte
46
23
Secondary Oocyte
Ootid
Fertilized Ovum
23
46
23
First Polar Body
pause at metaphase II
23
Second Polar Body
INCORRECT TEXTBOOK DIAGRAM
Can you see what is wrong with the diagram?
Comparing Plant and Human Life Cycles
• Certain plants that reproduce sexually have similar life
cycles to humans.
• Both sexually producing plants and humans, have haploid
and diploid stages of their life cycle.
Lodgepole Pine Life Cycle
Human Life Cycle