Meiosis occurs in what
type of cells?
Reproductive cells (the
ones that divide to form
sperm and eggs)
What is “reduction
division?”
-Cell division that reduces the
amount of DNA from parent cell to
daughter cells
-In meiosis, the chromosome
number is cut in half
What do the terms
“haploid” and “diploid”
mean?
•Diploid refers to a cell that has two
copies of every chromosome: 1
from mom and 1 from dad
•Haploid refers to a cell that has
only 1 copy of every chromosome
What are the diploid
and haploid
chromosome
numbers in humans?
Diploid (2N) cells = 46
chromosomes
Haploid (N) cells = 23
chromosomes
What are the only
cells in a human that
are haploid?
Sex cells (gametes):
sperm in males,
eggs in females
Why do gametes
(sperm and eggs)
need to be haploid?
•During sexual reproduction, a
sperm cell fertilizes an egg cell and
the two form a zygote. The zygote
must be diploid.
•If the sperm and egg were diploid,
they would create a polyploid
zygote that could not survive.
Does interphase
occur before meiosis?
Yes, all cells go through G1, S,
and G2 no matter what type of
division they use
What is the result of
meiosis?
1 diploid (2N) parent cell
produces 4 genetically
different haploid (N)
daughter cells
What are the two major
phases of meiosis
called?
Meiosis I
and
Meiosis II
What are the phases of
meiosis I?
Prophase I,
Metaphase I,
Anaphase I, and
Telophase I
What are homologous
chromosomes?
2 chromosomes that contain the
same genes for the same
traits, but not necessarily the
same alleles
When homologous
chromosomes pair up,
they form a structure
called a what?
tetrad
“Tetra” means 4. What
does this have to do with
a tetrad?
Why do cells have
homologous
chromosomes?
What happens during
Prophase I?
A tetrad is made up of 4 sister
chromatids (2 chromosomes
each containing 2 chromatids)
Every cell gets 1 copy
from mom and 1 copy
from dad for every
chromosome pair
•The nucleolus and nuclear membrane
“break up” and temporarily disappear.
•Centrosomes and centrioles migrate to
opposite sides of the cell
•Spindle fibers grow from the centrioles
and radiate toward the center of the cell.
•Homologous chromosomes pair up to form
tetrads and crossing-over can occur
What is crossing-over?
The process where homologous
chromosomes exchange genetic
information by crossing arms,
breaking parts off of each
chromosome, and switching the
pieces
Why doesn’t crossingover occur in prophase
of mitosis?
Homologous chromosomes do
not pair up to form tetrads in
mitosis.
Why is crossing-over
beneficial?
It increases genetic
diversity
What happens during
metaphase I?
Spindle fibers attach to the tetrads
and help line the tetrads up across
the center of the cell on an
imaginary line called the
metaphase plate or cell equator
What is the only difference
between metaphase I of
meiosis and metaphase of
mitosis?
In metaphase I of meiosis the
tetrads line up, but in
metaphase of mitosis, individual
chromosomes line up
What happens during
anaphase I?
Spindle fibers shorten,
separating the homologous
chromosomes. The
chromosomes are pulled to
opposite ends of the cell
What is the only difference
between anaphase I of meiosis
and anaphase of mitosis?
What happens during
telophase I?
What is the difference
between the results of
mitosis and the results of
meiosis I?
In anaphase I of meiosis,
homologous chromosomes are
separated, but the sister
chromatids stay together. In
anaphase of mitosis, sister
chromatids are separated.
-The nuclear envelopes reappear
-The spindle disappears
-Cytokinesis occurs
Mitosis produces 2 genetically
identical, diploid daughter cells.
Meiosis I produces 2 genetically
different, haploid daughter cells.
What are the phases of
Meiosis II?
Prophase II,
Metaphase II,
Anaphase II, and
Telophase II
Does interphase occur
before meiosis II?
No, interphase only occurs
before meiosis I. The two
cells made from meiosis I
go directly into meiosis II
What happens during
Prophase II?
•The nucleolus and nuclear membrane
“break up” and temporarily disappear.
•Centrosomes and Centrioles migrate to
opposite sides of the cell
•Spindle fibers grow from the centrioles
and radiate toward the center of the cell.
What happens during
Metaphase II?
Spindle fibers attach to the
chromosomes and help line the
chromosomes up across the center
of the cell on an imaginary line
called the metaphase plate or cell
equator
What happens during
Anaphase II?
Spindle fibers shorten, breaking
sister chromatids apart to form
separate chromosomes. The
chromosomes are pulled to
opposite ends of the cell
What happens during
Telophase II?
-Chromosomes begin to unwind back
to chromatin.
-The nuclear envelope and nucleolus
reappear in each daughter cell
-The spindle disappears
-Cytokinesis occurs
How does meiosis II
compare to mitosis?
They are the same except the final
result: mitosis produces 2
genetically identical, diploid
daughter cells, but meiosis II
produces 4 genetically different,
haploid daughter cells.
What is
spermatogenesis?
Meiosis that occurs in male
reproductive cells to form
sperm.
What is oogenesis?
Meiosis that occurs in
female reproductive cells
to form eggs (ova)
Why is only 1 egg
produced in oogenesis?
When cytokinesis occurs, the
cytoplasm is divided unevenly.
Only the cell that gets the most
cytoplasm forms an egg. The
other 3 cells are called polar
bodies and they do not
participate in reproduction.
What is this
phase?
Prophase I
What is this
phase?
Metaphase I
What is this
phase?
Anaphase I
What is this phase?
Telophase I
What is this phase?
Prophase II
What is this phase?
Metaphase II
What is this phase?
Anaphase II
What is this phase?
Telophase II