Download Meiosis PPT

Cell Division –
Asexual reproduction
• Mitosis
– produce cells with same information
Meiosis
• identical daughter cells
– exact copies
& Sexual Reproduction
• clones
– same amount of DNA
• same number of chromosomes
• same genetic information
Asexual reproduction
How about the rest of us?
• Single-celled eukaryotes and
simple multicellular eukaryotes
reproduce asexually
• What if a complex multicellular organism
(like us) wants to reproduce?
– joining of egg + sperm
• Do we make egg & sperm by mitosis?
Budding in Yeast
Budding in
Hydra
46
egg
+
46
92
sperm
zygote
Binary fission in Amoeba
1
How do we make sperm & eggs?
Review: Mitosis
•
•
•
•
• reduce 46 chromosomes → 23 chromosomes
– half the number of chromosomes
23
46
meiosis
23
egg
Division of the NUCLEUS
Purpose: growth & repair
Four Stages
Can you identify them?
46
23
46
23
fertilization
sperm
Mitosis
Four Mitotic Stages
•Pro
Prophase
Prophase
•Meta
Metaphase
Metaphase
Anaphase
•Ana
Anaphase
•Telo
Telophase
Telophase
• Produces two new cells
identical to the original cell
• Cells are diploid (2n)
Chromosomes during
Metaphase of mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
2
Facts About Meiosis
• Preceded by interphase which includes
chromosome replication
• Two meiotic divisions --Meiosis I and Meiosis II
• Called reduction-division
• Original cell is diploid (2n)
• Four daughter cells produced that are
haploid (1n)
Meiosis = reduction division
- 2 copies
- diploid
- 2n
Facts About Meiosis
• Produces gametes (eggs & sperm)
• Occurs in the testes in males
= Spermatogenesis
• Occurs in the ovaries in females
= Oogenesis
Homologous chromosomes
• Paired chromosomes – one from each parent
– both chromosomes of a pair carry genes
• control same inherited characters
• homologous = same information
diploid
2n
-
1 copy
haploid
1n
homologous
chromosomes
Replicated
homologous chromosomes
3
Meiosis: Two Part Cell Division
Preparing for meiosis
• Preceded by Interphase
– Duplication of DNA
– S phase of interphase
of MEIOSIS
(just like in mitosis)
Meiosis II
Meiosis I
2n = 6
single
stranded
Homologs
separate
Sister
chromatids
separate
Diploid
2n = 6
double
stranded
M1 prophase
Homologous Chromosomes
separate in meiosis I
Homologs
same genes,
different alleles
Sister
Chromatids
same genes,
same alleles
Haploid
Haploid
Meiosis I: Reduction Division
Spindle
Late
Nuclear
Prophase I fibers
Metaphase I
envelope
Early Prophase I
Anaphase I
(Chromosome
number doubled)
Telophase I
Nucleus
4
Prophase I
Tetrads Form in Prophase I
Homologous chromosomes
(each with sister chromatids)
Early prophase
•Nuclear
Nuclear envelope fragments
Sister
chromatids
from
dad
Called Synapsis
tetrad
Metaphase I
- homologous pairs swap
pieces of chromosome
- Creates new combinations of traits
from
mom
TETRAD
Late prophase
•Homologs
Homologs pair
•Crossing
Crossing over occurs
•Chromosomes
Chromosomes condense
•Spindle
Spindle forms
Crossing over
Join to form a
synapsis
Homologous pairs
of chromosomes
align along the
equator of the cell
homologs
5
Independent Assortment
- chromosomes line up randomly
- independent of one another
Independent Assortment
- in humans produces 223 (8,388,608)
different combinations
Before meiosis
Maternal
Paternal
After meiosis
- How many possibilities???
- 2n (n=number of chromosomes)
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.
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Review: Meiosis I
• 1st division of meiosis
separates
homologous pairs
prophase1
Meiosis 2
2n = 4
single
stranded
2n = 4
double
stranded
• 2nd division of
meiosis separates
sister chromatids
1n = 2
double
stranded
prophase2
metaphase1
synapsis
telophase1
tetrad
1n = 2
double
stranded
2n = 4
double
stranded
1n = 2
double
stranded
Meiosis II
Only one homolog of each
chromosome is present in the cell.
cell
metaphase2
1n = 2
single
stranded
telophase2
Meiosis II: Reducing Chromosome
Number
Sister chromatids
are still attached
Meiosis II produces gametes with
one copy of each chromosome and thus
one copy of each gene.
Prophase II Metaphase II
Anaphase II
Telophase II
4 haploid cells
7
Prophase II
Nuclear envelope
fragments.
Spindle forms.
Anaphase II
Metaphase II
Chromosomes align
along equator of cell.
Telophase II
Equator
Pole
Sister chromatids
separate and move
to opposite poles.
Nuclear envelope
assembles
Chromosomes
decondense
Spindle disappears
Cytokinesis divides cell
into two
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Results of Meiosis
Meiosis 1
Gametes (egg & sperm) form
Four haploid cells with one
copy of each chromosome
One allele of each gene
Different combinations of
alleles for different genes
along the chromosome
Meiosis 2
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The value of meiosis
• Meiosis introduces genetic variation
– gametes of offspring do not have same genes
as gametes from parents
– This is the raw material for EVOLUTION
Sources of genetic variability
• Genetic variability in sexual reproduction
– independent assortment
• homologous chromosomes in Meiosis 1
– crossing over
• between homologous chromosomes in prophase 1
– random fertilization
• random ovum fertilized by a random sperm
from Mom
new gametes
made by offspring
from Dad
Any 2 parents will produce a
zygote with over 70 trillion
(223 x 223) diploid combinations
Sexual reproduction creates variability
Sexual reproduction allows us to maintain both
genetic similarity & differences.
Michael & Kirk
Douglas
Baldwin brothers
Martin & Charlie Sheen, Emilio Estevez
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