Transcript PPT

11-4 Meiosis
Describe
the process of meiosis.
Compare meiosis and mitosis.
Introduction
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Meiosis –
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A Process by which the number of chromosomes per
cell is cut in HALF through the separation of
homologous chromosomes in a diploid cell
Meiosis has TWO Phases: Meiosis I and Meiosis II
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Starts: with ONE DIPLOID (2n) CELL
Ends: with FOUR HAPLOID (n) CELLS – that are NOT
identical to each other or to the parent cell
Why do Cells Undergo Meiosis?
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Gametes (aka sex cells: egg and sperm) need
to be HAPLOID (n) so that when they
combine, the new organism is DIPLOID (2n)
instead of TETRAPLOID(4n)
Chromosome
Number
1.
Each parent has a full
number of chromosomes,
46 in humans
2.
During meiosis this
number has to be cut in
half or the human
offspring would have 92
chromosomes
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When the sperm (male sex
cell) fertilizes the egg
(female sex cell) the
result is a zygote that
contains the diploid
number of chromosomes
3.
Haploid: (1n) a cell with half the
number of chromosomes
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4.
Diploid: (2n) a cell with the full
number of chromosomes
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5.
23 chromosomes in human sex cells
aka gametes
46 in humans in somatic human cells
Homologous chromosomes: two
chromosomes, one from mom and
one from dad. Each one has
genes for the same trait.
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Ex. B and b could be the dominant
and recessive forms of hair color.
One allele on each chromosomes
located in the gene. One
chromosome from each parent.
This offspring is heterozygous for
hair color Bb and heterozygous Aa.
The Phases of Meiosis
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Consists of two separate cell divisions named Meiosis I and
Meiosis II
Starts with one diploid cell and ends with four haploid cells
called gametes (sex cells)
Meiosis I is known as REDUCTION
Meiosis I: REDUCTION: homologous
chromosomes are separated
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Interphase I: DNA is duplicated,
chromatin & centrioles appear
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Prep for cell division
DNA duplicates S phase
Cell grows
Organelle duplication
Very similar to interphase of mitosis
Meiosis I - Prophase I
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Prophase I: Nuclear envelope and
nucleolus disappears, chromatin
condenses into chromosomes (sister
chromatids), homologous
chromosomes pair up (TETRADS),
crossing over may occur
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Synapsis - formation of tetrads when homologous pairs find one
another (this is the main difference
between meiosis and mitosis)
Tetrad – two homologous
chromosomes grouped together
(total of FOUR chromatids)
Crossing over – process by which
homologous chromosomes exchange
pieces of chromatids forming new
combinations
Meiosis I - Prophase I
Crossing Over - Chromatids of
homologues exchange equal
portions (usually)
 Chiasma – cross-over exchange
point
 New chromatids are called
recombinants
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Meiosis I – Metaphase I
and Anaphase I
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Metaphase I: Tetrads Homologous chromosomes line up in
the middle (the metaphase plate)
Anaphase I: Homologous
chromosomes separate, are pulled
apart towards the poles
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Tetrads split – sister chromatids are
still together, attached at centromere
Meiosis I: Telophase I and
Cytokinesis
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Telophase I: Homologous
chromosomes gather at opposite
poles
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Nuclear membrane and nucleolus
begin to reform
Chromosomes begin to unwind 
chromatin
Followed by cytokinesis I
Cytokinesis: Cytoplasm divides
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TWO new cells – NON
IDENTICALS (crossing over)
Sister chromatids are still together
Meiosis II: Sister chromatids
are separated (just like mitosis)
Just like mitosis – may be preceded by brief
period of rest (Interkinesis or Interphase II)
 NO DNA REPLICATION OCCURS
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Meiosis II – SAME AS MITOSIS
 Prophase II: same as mitosis (nuclear membrane and nculeolus
disappear, centrioles move to opposite poles, spindle fibers stretch
across cell , chromatin condenses  chromosomes
 Metaphase II: sister chromatids line up in the middle/equator
 Anaphase II: Centromeres divide, sister chromatids separate and
move apart to the poles
 Telophase II: reverse of prophase - nuclear envelope and nucleolus
reform, chromosomes uncoil  chromatin, cytokinesis begins
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Result: 4 haploid cells, genetically different
Meiosis – End Result
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Start  1 diploid (2n)
parent cell
End  4 haploid (n)
daughter cells
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Each cell contains HALF
as much DNA as the
original parent cell
Each cell is genetically
different (they are
NOT all identical)
Meiosis and Genetics
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Genetic Reassortment – leads to greater genetic
variation (why you have characteristics from both
your parents)
Sources
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Crossing Over (synapsis) – new combinations of genes on
chromosomes created by crossing over during prophase I
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Results in creation of chromosomes with genes from both
parents on them “hybrid” mom-dad chromosome
Humans average crossing over 2-3X per homologous pair
Independent Assortment – new assortment of chromosomes,
when chromosomes line up at the equator during metaphase I
– it is completely random and lead to LOTS of different
combinations
Fertilization – random meeting of sperm and egg
Gamete Formation
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Males =
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Spermatogenesis – “birth” of sperm
4 equal size sperm produces for every meiotic division
Females =
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Oogenesis – “birth” of egg (largest cell in female body)
1 egg and 3 polar bodies produced for every meiotic division
Comparing Mitosis and Meiosis
1
1
1
2
2
4
2n
n
Sister
chromatids
identical
Production
of somatic
cells
Homologous then
sister chromatids
different
Production
of gametes
Check Mitosis, Meiosis or Both
No pairing of homologous chromosomes
Two divisions
Four daughter cells produced
Associated with growth and asexual reproduction
Associated with sexual reproduction
One division
Two daughter cells produced
Involves duplication of chromosomes
Chromosome # maintained
Chromosome # is halved
Crossing over between homologous may occur
Daughter cells are identical to parent cell
Daughter cells are not identical to parent cell
Produces gametes
Synapsis occurs in prophase
Mitosis
Meiosis
Both