Meiosis
Process of
making
gametes.
(Sperm
and egg)
• Gametes (gametic cells) contain ½ the
genetic material as somatic cells and are
genetically unique.
• They are called haploid. Examples are
sperm and egg. (Not somatic or body
cells)
• They are used in sexual reproduction to
form the new offspring. Sperm + egg =
zygote
• The goals of meiosis are two-fold
1. to create diversity within the chromosomes
and
2. to reduce the number of total chromosomes
in the cell to ½ that of the parent cell.
2 goals = 2 rounds of phases
Which goal is achieved in each
phase?
• Interphase ONLY occurs before
Meiosis I
• Both Meiosis I and Meiosis 2 have
a cytokinesis
• Between Meiosis 1 and Meiosis 2
there is a process called
interkinesis
• In every phase but
prophase 1, things
occur just like in
mitosis.
• However, in P1, we
allow homologous
chromosomes to come
in contact with each
other. They form a
structure called a
tetrad.
Crossing Over
• Allowing 2 pairs of
chromosomes to
physically touch
allows the process
of crossing over to
occur.
• Crossing over is the
actual exchange of
genes (pieces of
DNA) from one
chromosome pair to
the other.
• This allows for DIVERSITY to occur
in the chromosomes of the cells
made during meiosis.
Q: Why didn’t we do this in mitosis?
• After Prophase 1 all other phases of meiosis are
the same as mitosis, followed by cytokinesis.
• BUT, we are not done at the end of meiosis 1
• So, we have to have a meiosis 2. Second round
of the same steps as mitosis!
Think back to MITOSIS! What are the steps?
Share with a partner!
Interphase
Prophase 1
Metaphase 1
Anatphase 1
Telophase 1
Cytokinesis 1
Interkinesis
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Cytokinesis 2
Polar Bodies
• In males, all 4 haploid cells become sperm
• In females, only 1 of the haploid cells become an
egg. That one egg takes all of the cytoplasm and
organelles from the other 3 cells
• The other 3 cells produced are
waste that is re-absorbed. These
cells are known as polar bodies.
Compare Mitosis and Meiosis
SIMILARITIES: Both involve similar phases, replicate
cells, occur in eukaryotic organisms.
DIFFERENCES:
Mitosis
Meiosis
1
# of DNA replications:
# of times through the stages:
1
# of daughter cells produced:
2
Chromosome number in
46
daughter cells:
Chromosome number in parent 46
cell:
Growth and
Purpose:
repair
1
2
4
23
46
Gamete
production
(sexual
reproduction)