Cell Division
Meiosis
 Meiosis occurs in sexual reproduction when a
diploid germ cell produces four haploid
daughter cells that can mature to become
gametes (sperm or egg).
Meiosis
Goal: reduce genetic material by half
Why? n (mom) + n (dad) = 2n (offspring)
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Just
right!
Meiosis: 2 part cell division
Sister
chromatids
separate
Homologous Chromosomes
pairs separate
Meiosis I
Diploid
Meiosis II
2n
Haploid n
Haploid
n
Result: one copy of each numbered chromosome in a gamete
Meiosis I
Spindle
fibers
Nucleus
Nuclear
envelope
Prophase I
(early)
(diploid)
Prophase I
(late)
(diploid)
Metaphase I
(diploid)
Anaphase I
(diploid)
Telophase I
(diploid)
Prophase I Early
Early prophase
.
Chromosomes condense
Spindle forms
Nuclear envelope breaks apart
Prophase I Late
Late prophase
Chromatids pair (synapse)
Crossing over occurs between chromatids
of the homologous chromosome pair
Metaphase I
Homologous chromosome pairs
line up on the cell’s equator
Anaphase I
Chromosomes separate and
move to opposite poles.
Sister chromatids remain
Attached at their centromeres.
Telophase I
Nuclear envelopes reforms
Spindle disappears.
Cytokinesis divides cell into two
Meiosis II
Sister chromatids have carry
different genetic information
due to crossing over
End product of meiosis is Each Sex cell /gamete has :
1 copy of each chromosome
1 copy of each gene segment
Meiosis II
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Telophase II
(haploid)
Four
Non-identical
haploid
daughter cells
Prophase II
Nuclear envelope fragments.
Spindle forms.
Metaphase II
Chromosomes align
along equator of cell.
Anaphase II
Sister chromatids separate
and move to opposite poles.
Telophase II
Nuclear envelope
assembles.
Chromosomes unravel.
Spindle disappears.
Cytokinesis divides cell into
two.
Results of meiosis
• Four haploid cells
• One copy of each
chromosome
•Not the same genes
on each chromosome
“Putting It All Together” - Fertilization
What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
Meiosis Video Link
 Meiosis Video Link #1
 Meiosis Video Link #2
 Meiosis Video Link#3
 2 diploid 4 haploid
 Crossing over between nonsister




chromatids
Reduction divisiion
Increases genetic variability vs. clones
Meiosis 1 Homologous chrom separate
Meiosis 2 Sister chromatids separate
Mitosis
Meiosis
Number of divisions
1
2
Number of daughter
cells made through
process
2
4
Are the new cells
Genetically identical?
Yes
No
# of Chromosomes
compared to parent
cell
Same as parent
Half of parent
Somatic cells
Sex cells
Throughout life
At sexual maturity
Growth and repair
Sexual reproduction
What cell go thru this
division process
When do cells got
thru this division
process
Role
MITOSIS
MEIOSIS
Occurs in somatic (body) cells
Occurs only in gonads (sex organs:
ovary/testes)
Produces cells for repair,
maintenance, growth, asexual
reproduction
Only produces gametes (sex cells:
egg/sperm)
Results in identical diploid (2n) daughter Reduction division results in haploid (n)
cells
cells
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of
each gene.