Meioooosis
Meiosis animation
Meiosis
Meiosis
• Form of cell division where there are two
successive rounds of cell division following DNA
replication
• Produces haploid cells (n)
• Start with 46 double stranded chromosomes (2n)
– After 1st division - 23 double stranded
chromosomes (n)
– After 2nd division - 23 single stranded
chromosomes (n)
• Occurs in our germ cells
– cells that produce our gametes
• egg and sperm
Why do we need meiosis?
- It is the fundamental basis of sex
What is the purpose of sex?
-To bring two haploid gametes
together to form a diploid zygote.
-n (mom) + n (dad) = 2n (offspring)
Replication of chromosomes
• Replication is the
process of duplicating
a chromosome
• Occurs prior to
division
• Replicated copies are
called sister
chromatids
• Held together at
centromere
Meiosis
a cell division forming gametes
Goal: reduce genetic material by half
Why?
from mom
from dad
child
too
much!
meiosis reduces
genetic content
Meiosis: cell division in two
parts
Sister
chromatids
separate
Homologs
separate
Meiosis I
Meiosis II
Diploid
Haploid
Haploid
Result: one copy of each chromosome in a gamete.
Meiosis II : the equational
division
Prophase II
(haploid)
Metaphase II
(haploid)
Anaphase II
(haploid)
Four
Telophase II
nonidentical
(haploid)
haploid
daughter cells
Called a tetrad
A replicated chromosome
Gene X
sister chromatids
homologs
same genes
maybe different alleles
same genes
same alleles
Homologs separate in meiosis
I and therefore different
alleles separate.
Spermatogenesis: sperm
formation
Spermatogeneis
Stem cells in testes divide
mitotically to create a pool
of spermatocytes.
Meiosis produces four spermatids.
Oogenesis
First polar body
may divide
(haploid)
a
a
X
a
X
a
Meiosis I
A
Oogonium
(diploid)
X
X
Mitosis
X
Primary
oocyte
(diploid)
Polar
bodies
die
Meiosis II
(if fertilization
occurs)
A
X
A
X
Secondary
oocyte
(haploid)
Ovum (egg)
A
X
Second
polar body
(haploid)
Mature
egg
Oogenesis: ovum formation
• One of four meiotic products becomes an
ovum.
• The three remaining meiotic products are
polar bodies.
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
Homologs pair.
Crossing over occurs.
Chromosomes condense.
Spindle forms.
Nuclear envelope fragments.
Metaphase I
Homolog pairs align
along the equator of the cell.
Anaphase I
Homologs 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.
Have 2 cells, each with 46
chromosomes
Meiosis II
Gene X
Only one homolog
of each chromosome
is present in the cell.
Sister chromatids carry
identical genetic
information.
Meiosis II produces gametes with
one copy of each chromosome and thus one
copy of each gene.
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 decondense.
Spindle disappears.
Cytokinesis divides cell into
two.
Results of meiosis
Gametes
Four haploid cells
One copy of each
chromosome
One allele of each gene
Different combinations
of alleles for different
genes along the
chromosome
Fig 2.19
Mitosis
Meiosis
2
Number of
divisions
1
Number of
daughter cells
2
4
Yes
No
Same as parent
Half of parent
Somatic cells
Germline cells
Genetically
identical?
Chromosome #
Where
When Throughout life
Role
Growth and
repair
At sexual maturity
Sexual reproduction
Mitosis vs. Meiosis
NOVA Online | 18 Ways to
Make a Baby | How Cells
Divide: Mitosis vs. Meiosis
(Flash)
Why do we have meiosis?
- to generate haploid gametes
- to make new combinations of genes
-How?
random (independent)
assortment
recombination
Independent assortment
The homolog of one chromosome can be inherited
with either homolog of a second chromosome.
Fig 2.20
Random assortment
Since the combination of maternal and parental
chromosomes received by a gamete is random.
And we have 23 pairs of chromosomes
The possible combinations in an egg or a
sperm are 223 = 8,388,608
combinations in an offspring
223 X 223 = 70,368,744,177,664
Result: Generates new combinations of genes
(alleles) when the genes are located on different
chromosomes.
Recombination
Crossing-over
- the physical exchange of chromosomal
material between chromatids of
homologous chromosomes.
- Result: Generation of new combinations
of genes (alleles) if the genes are
located on the same chromosome.
Recombination (crossing over)
• Occurs in prophase of
meiosis I
A
A
B
B
C
• Generates diversity
b
C
D D
E
F
E
F
a
a
b
c
c
d
d
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
e
f
Recombination (crossing over)
• Occurs in prophase of
meiosis I
A
a
B
b
C
• Generates diversity
Letters denote genes
Case denotes alleles
C
D D
E
F
c
c
d
E
F
d
e
f
•Creates chromosomes with new combinations
of alleles for genes A to F.
e
f
Recombination (crossing over)
• Occurs in prophase of
meiosis I
a
A
B
b
C
• Generates diversity
Letters denote genes
Case denotes alleles
D
E
F
A
a
B
b
C
D
E
F
c
c
d
d
e
f
•Creates chromosomes with new
combinations of alleles for genes A to F.
e
f
Gametogenesis
- spermatogenesis
- oogenesis
Fertilization
• Fertilization is the joining of sperm and ovum.
• Meiosis II in the ovum is completed at the time of
fertilization forming one ovum and one polar body.
• Following fertilization, chemical reactions occur
preventing additional sperm from entering the ovum.