Cell Division
SOL: BIO 6 a-c
Meiosis: Important Vocabulary
 Diploid- contains 2 sets
of chromosomes
 (2n = 46)
 Example: human somatic
cells
 Haploid- contains 1 set
of chromosomes
 (n = 23)
 Example: human gametes
Meiosis: Important Vocabulary
 Fertilization- fusion of two haploid gametes
forms a diploid zygote
 Zygote- fertilized egg cell
 The first cell of a new individual
 What would happen if gametes were diploid?
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: cell division in two parts
Sister
chromatids
separate
Chromatids
separate
Meiosis I
(reduction
division)
Meiosis II
(equational
division)
Diploid 2n
Diploid 2n
Haploid n
Result: one copy of each chromosome in a gamete.
Meiosis I : the reduction division
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 prophase
Late prophase
Chromatids pair.
Crossing over occurs.
Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Metaphase I
Chromatid pairs align
along the equator of the cell.
Anaphase I
Chromosomes separate and
move to opposite poles.
Sister chromatids remain
Attached at their centromeres.
Telophase I
Nuclear envelopes reassemble.
Spindle disappears.
Cytokinesis divides cell into two.
Meiosis II
Gene X
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and thus one copy of
each gene.
Meiosis II : the equational division
Prophase II
(diploid)
Metaphase II
Anaphase II
Telophase II
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
“Putting It All Together” - Fertilization
What Meiosis is About
Meiosis allows the creation of unique individuals
through sexual reproduction.
Mitosis
Meiosis
Number of divisions
1
2
Number of daughter cells
2
4
Yes
No
Same as parent (2n)
Half of parent (n)
Where
Somatic cells
Sex cells in testes and ovaries
When
Throughout life
At sexual maturity
Growth and repair
Make gametes (sperm, egg,
pollen)
Genetically identical?
Chromosome #
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 4 haploid
cells
(n) sex cells