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Meiosis
The Production
of
Reproductive
Cells
Remember:
• Mitosis produced 2 genetically identical
cells with 2 copies of every
chromosome (one from mom and one
from dad)
• This is how body (somatic) cells are
produced.
• REPRODUCTIVE cells do NOT go
through MITOSIS because these cells
MUST have HALF the number of
chromosomes.
Mitosis
Meiosis Overview
• Purpose = so that the number of chromosomes
gets reduced by ½ in reproductive cells
• Body cells have 1 complete set of genes from
each parent SO there are 2 sets of genes in
every cell
• Diploid = 2 complete sets of genes
(chromosomes); Diploid in humans = 46
• Gametes = sex (reproductive) cells = sperm/egg.
Gametes only have 1 set of chromosomes in
each cell.
• Haploid = 1 set of genes (chromosomes);
Haploid in humans = 23
Meiosis - Overview
MEIOSIS I BIG IDEA
• Produce 2 Haploid Cells by
SEPARATING HOMOLOGOUS
CHROMOSOMES
Meiosis I – Interphase
• G1-Growth
• S-DNA
replication
takes place
• G2Organelles
replicate
Meiosis I – Prophase I
• Homologous = same gene
(1 from each parent)
• Chromatin condenses into
chromosomes
• Homologous
chromosomes pair up
• Crossing over – homologs
swap genes to increase
genetic diversity
Meiosis I – Metaphase I
• Homologous
chromosomes are
moved to the
middle
Meiosis I – Anaphase I
• Homologs are
pulled apart
• Crossing over
is completed
when the
chromosomes
are separated.
Error in Meiosis
• If the chromosomes fail to separate
after crossing over, Nondisjunction will
occur.
• This can happen during Anaphase I or
Anaphase II.
• This causes one daughter cell to have
both copies of the homologous
chromosome and the other to not have
a copy of it.
• Downs Syndrome (Trisomy 21) is
caused by Nondisjunction.
Meiosis I – Telophase I
•
•
•
•
Nuclear envelopes may reform
Cytokinesis (2 cells)
Cells are NOT identical
Cells are HAPLOID
2 Divisions
• Unlike Mitosis, there are 2 Divisions
in Meiosis.
• There is NO Interphase or DNA
replication between Meiosis I and
Meiosis II
MEIOSIS II BIG IDEA
• Produce 4 Genetically Different Haploid
Cells by SEPARATING SISTER
CHROMATIDS
Meiosis II – Prophase II
• Cells are NOT identical
• Haploid cells prepare to divide
Meiosis II – Metaphase II
• Chromosomes line up in the middle
Meiosis II – Anaphase II
• Sister chromatids are pulled apart
• Nondisjuction can occur
Meiosis II – Telophase II
• Nuclear envelopes reform
• Cytokinesis
• 4 genetically DIFFERENT
haploid cells
Mitosis
•
•
•
•
Meiosis
•
Somatic (body) cells
•
2 cells
•
Diploid (2 sets of
genes; 1 from each •
parent)
Genetically Identical
(to each other & to
•
original cell)
Gametes (sex cells)
4 cells
Haploid (1 set of genes)
Genetically Different
– Crossing Over
– Some have “mom’s”
copy; others have “dad’s”
Nondisjunction may
occur
• 2 divisions
• Watch the following Mitosis
& Meiosis Comparison
Video
http://www.youtube.com/wat
ch?v=Ba9LXKH2ztU
Gametes
• Gametogenesis is the production of
Gametes (aka Meiosis)
• The production of sperm is called
spermatogenesis.
• The production of eggs is called
oogenesis.
• Gametes are haploid.
– 23 chromosomes in human sperm
– 23 chromosomes in human eggs
Sperm
• Spermatogenesis
produces 4 viable
sperm cells through
meiosis.
• Sperm contain either
an X or Y
chromosome. This
determines the gender
of the offspring.
• This occurs every 2-3
days in human males.
Eggs
• In females, only one large egg is
produced through meiosis.
• Cytokinesis is uneven. One cell receives
the majority of the cytoplasm.
• The other 3 cells are called polar bodies
and are not involved in reproduction.
• Eggs always contain one X chromosome.
• Prophase I occurs before birth in human
females. Meiosis is completed during
puberty. No “new” eggs during lifetime.
Oogenesis