Meiosis
Meiosis is referred to as reduction
division because meiosis results in
haploid cells called gametes (sex cells).
Male and female gametes fuse during
fertilization (syngamy) to form a zygote.
The zygote divides by mitosis to form an
embryo.
Human embryo develops into a a fetus.
• Meiosis may be thought of as a “double
mitosis.”
• Characteristics of meiotic phases are
similar to the characteristics of mitotic
phases.
• If you know what happens during
mitosis, then you know most of what
happens during meiosis (with a few
differences).
Meiosis occurs in two parts:
Meiosis I and then Meiosis II.
Meiosis I –(Starts with a diploid cell-2N)
Phases: prophase I, metaphase I, anaphase I,
and telophase I, then cytokinesis which results in
two haploid cells.
PROPHASE I
• During Prophase I pairs of homologous
chromosomes called tetrads align themselves
closely and exchanges pieces.
• When chromosomes of the tetrad are near
enough next to each other, it is called synapsis.
• During Prophase I, as chromosomes of a tetrad
are in synapsis, and exchange of genes can
occur.
• This exchanges of genetic material is called
“crossing-over.”
• The point at which crossing-over occurs is
referred to as a chiasmata
What is the advantage of crossingover?
Crossing-over increases genetic variety
in organisms, by “shuffling” the genes for
different combinations.
• Meiosis I then proceeds through metaphase I,
and then anaphase I.
• All phases are similar to the mitotic phases.
(spindle formation, tetrads move toward center,
tetrads line-up along the center, tetrads separate
and chromosomes move to opposite poles)
• Cytokinesis occurs and the 2N cell has now
become 2 haploid daughter cells.
Two Haploid cells result
These cells go directly into prophase II --the first phase of Meiosis II.
Meiosis I leads to Meiosis II
• Meiosis I leads directly to Meiosis II, with no interphase
in between.
• During meiosis II phases are similar to mitosis phases:
(chromatid pairs move toward the center, sister
chromatids separate and then cytokinesis)
Finally – 4 haploid cells result
Meiosis
Meiosis
FAST MEIOSIS
If the gametes are to be male,
then 4 individual sperm cells form.
The
process of
forming
sperm
cells is
called
spermatogenesis
If the gametes are to be female, then the
result is 1 egg cell and 3 polar bodies.
The process of
producing
the egg cell is called
oogenesis.
The egg cell gets
most of the
cytoplasm and the
much smaller
polar bodies
disintegrate.
Egg cell – Ovum
Egg cells - Ova
Relative Sizes of Gametes