Meiosis
Biology
Review: Mitosis


Results in 2 identical
daughter cells
Occurs in somatic cells




Body cells
Ex: skin, stomach, muscle
# of chromosomes in
parent cell = # of
chromosomes in daughter
cells
Resulting cells are diploid
(2N)
What is meiosis?




Process of making
gametes (sperm/egg)
Chromosomes number
is reduced in half
(haploid)
When gametes
combine, offspring has
correct # of
chromosomes
Haploid + haploid =
diploid
Example: Humans
Because of meiosis…
For the remaining slides…


Pay attention to what is happening to the
DNA in each phase – not the details of what
goes on in each
Red text in the remaining slides = important
information – focus on this!
Meiosis

Occurs in 2 phases:



Meiosis I
Meiosis II
Understand homologous
chromosomes!


Pair of identical chromosomes
containing copies of genes
One from each parent
Meiosis I

Interphase


DNA is copied
Prophase I

Crossing over occurs


Pieces of chromosomes break off and switch places on sister
chromatids
Leads to genetic variation

Metaphase I


Anaphase I



Homologous chromosome line up double file at equator
Homologous chromosomes separate
Telophase I
Cytokinesis I

Results in 2 haploid daughter cells containing one set
of replicated chromosomes
Meiosis II



NO interphase (why??)
Prophase II
Metaphase II


Anaphase II



Chromosomes line up single file at equator
Sister chromatids separate
Telophase II
Cytokinesis II

Results in 4 haploid daughter cells containing ONE
copy of each homologous chromosome
Meiosis
Parent cell – chromosome pair
Chromosomes copied
1st division – homologous pairs split
2nd division – sister chromatids
split; produces 4 gamete cells
with ½ the original number of
chromosomes
Think you are 1 in 1,000,000?





Don’t sell yourself short!
Humans with 23 pairs of chromosomes, a gamete (egg or
sperm) could have 223 or 8,388,604 possible
combinations of chromosomes from that parent.
Any couple could have 223 × 223 or 70,368,744,177,644
(70 trillion) different possible children, based just on the
number of chromosomes, not considering the actual
genes on those chromosomes.
Thus, the chance of two siblings being exactly identical
would be 1 in 70 trillion.
In addition, crossing over can add further variation to an
individual’s genetic make-up.


Meiosis Animation
Meiosis Cartoon