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Meiosis
Review Mitosis
• What is the purpose of Mitosis?
– cell division
• 2 daughter cells produced that are exact copies of the parent
cell
– What are the parts of the cell cycle?
• Interphase
– G1, S, G2
• M-Phase
– Mitosis
– Cytokinesis
– What were the phases of Mitosis?
• PMAT – Prophase, Metaphase, Anaphase, & Telophase
Meiosis
Yes, it is different from Mitosis!!
• Mitosis occurs in any cell in the body that was NOT a
reproductive cell
– Body cells are called somatic cells
• MEIOSIS is cell division by which gamete cells
(reproductive cells---sperm and egg) are produced
Remember!!!
• Chromosomes = tightly coiled
DNA (sister chromatids)
– Contain genes which determine
hereditary traits
TOTAL NUMBERS
• We have a total of 46 chromosomes in EVERY cell of our
body
– We get…
•23 from Mom
•23 from Dad
*During the S phase of Interphase,
Those chromosomes are duplicated
Dad
Mom
So each chromosome duplicates to make an
identical one; these structures are now called
SISTER CHROMATIDS!!!!!
So each pair of chromosomes will have 2 “X”
(sister chromatids) shapes
SO……
• We call the pair of matching sister chromatids Homologous
Chromosomes
• Homologous
chromosomes
contain the
same genes for
the same traits,
but not
necessarily the
same alleles
– Genes are your traits
• Eye color, hair color, height, foot length, etc…
– Alleles are alternate forms your traits can take
• Usually one is dominant (brown eyes) vs. recessive (blue eyes)
*Somatic cells and Gametes have
different numbers of chromosomes
• DIPLOID (2N) = Somatic cells (Body Cells)
– describes a cell that has two copies of each
chromosome: 1 from mom and 1 from dad
– Contains 2 sets of alleles for each gene
– Chromosome # is 46
• HAPLOID (N) = Gametes (Sex Cells)
– describes a cell that has only one set of chromosomes
– Contains 1 set of alleles for each gene
– Each egg or sperm has 22 autosomes and 1 sex
chromosome.
• Autosomes – pairs 1-22
• Sex chromosome - pair 23
Chromosome Number
Example: Humans
Chromosome # (N) = 23
Diploid Cells = 2N =
2x23 = 46
Haploid Cells = N = 23
Sexual Reproduction
• During sexual reproduction, an egg is fertilized
by a sperm to create a zygote
• The zygote should be diploid; that is, it should
have the same number of chromosomes as all
normal human body cells
• Sperm + Egg = 46
• (23) + (23)
Sexual Reproduction
• What if gametes (sex cells) went through
Mitosis?
– Mitosis produces 2 identical daughter cells
so…
– Diploid produces diploid
– If sperm and egg were diploid they would
create a polyploid (“many sets”) zygote when
they came together
– Obviously we do not have “many sets” of
chromosomes do we?
SO WHAT’S THE SOLUTION?
MEIOSIS!
• Where a Diploid cell makes 4 genetically
different Haploid daughter cells
• Meiosis is a process of reduction division in
which the number of
chromosomes per cell is cut
in half through the
separation of homologous
chromosomes in a diploid cell.
Genetic Recombination
• So, when a haploid sperm fertilizes a
haploid egg, a diploid zygote is the result
• Meiosis helps create Genetic
recombination, where 2 different sets of
chromosomes come together. This keeps
you from being an exact copy of one of
your parents!
• It’s what creates genetic diversity in our
world!!
Stages of Meiosis
• Meiosis I
• Meiosis II
Meiosis I
• Prophase I
• Metaphase I
• Anaphase I
• Telophase I
• Cytokinesis
Interphase
Same as in Mitosis
– G1: cell growth, duplication of
organelles
– S: DNA synthesis/replication of
DNA
– G2: preparing for division
Prophase I
• Nuclear membrane and nucleolus disappear
• Centrioles begin to separate and spindles begins to
form
• The homologous chromosomes pair up creating a
tetrad
– a tetrad = 2 sets of sister chromatids = 4 chromatids
• The chromosomes pair
up precisely, gene for
gene, down their entire
length.
During Prophase I
• Crossing-over can occur
– Involves the exchange of genetic
material between homologous
chromosomes
– Breaks arms of each chromosome and
switches the material
Metaphase I
• Spindle fibers attach to the tetrads
• The tetrads line up along the metaphase
plate
• Each side of the equator has
chromosomes from
both parents
Anaphase I
• The paired homologous chromosomes are pulled
apart and move toward opposite ends of the cell
• Sister chromatids remain attached
Telophase I and Cytokinesis
• The nuclear membrane forms again in
some species, the spindle fibers
disassemble and the cell undergoes
cytokinesis.
• 2 daughter cells are produced
– A “reduction division” has occurred
because each daughter cell has half of
the number of chromosomes the original
parent cell had
Important Note:
Meiosis II will occur in both of
the daughter cells created in
Meiosis I
Review
• Meiosis I – Homologous Chromosomes separate
– Begins w/ 1 diploid cell
– Prophase I, Metaphase I, Anaphase I,
Telophase I
– Ends with 2 haploid cells
Meiosis I
Interphase
Prophase I
Metaphase I
Cells undergo a round of
DNA replication, forming
duplicate Chromosomes.
Each chromosome pairs with its Spindle fibers attach to the
corresponding homologous
chromosomes.
chromosome to form a tetrad.
Anaphase I
The fibers pull the homologous
chromosomes toward the
opposite ends of the cell.
Meiosis II
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II
• Cytokinesis
Prophase II
• The DNA DOES NOT
duplicate
– The nuclear membrane
disappears
– Centrioles move to opposite
sides of the cell
– Spindle fibers start to
assemble
Metaphase II
• Spindle fibers attach to
the chromosomes at the
centromere
• The 23 chromosomes line
up along the metaphase
plate
• Each chromosome still has
two sister chromatids
Anaphase II
• Centromeres break,
spindle fibers shorten,
and the sister
chromatids (now
chromosomes) are pulled
apart from each other
and move to opposite
ends of the cell
Telophase II and Cytokinesis
• Spindle fibers break down
• Nuclear envelopes and nucleolus
reform
• Cell membrane pinches in and
cytoplasm is divided
• 4 haploid daughter cells are
produced
– A “reduction division” has
occurred because each daughter
cell now has half of the number
of chromosomes the original
parent had
Results of Meiosis
• The 4 haploid daughter
cells…
– Have only 1 set of genes
– Are not considered to be
in any phase because
they will not grow and
divide
– Now referred to as
gametes (sex cells)
Review
• Meiosis II – Sister chromatids separate
– NO DNA replication
– Prophase II, Metaphase II, Anaphase II, Telophase II
– Ends with 4 haploid cells
Meiosis II
Prophase II
Metaphase II
Anaphase II
Meiosis I results in two haploid (N)
daughter cells, each with half the
number of chromosomes as the
original.
The chromosomes line up in a similar The sister chromatids separate and
way to the metaphase stage of
move toward opposite ends of the
mitosis.
cell.
Telophase II
Review Questions
• What type of cells are produced from meiosis?
– Gametes (sex cells), haploid
• What happens to the chromosome number in the
cells that are produced from meiosis?
– Goes through reduction division
– Cell chromosome number is cut in half - Start with 2N
and end with 1N
– 4 haploid gametes are produced, each genetically
different from the parents
• What do genes code for?
– traits
Spermatogenesis
• Meiosis in males to
produce sperm
• Thousands of
reproductive cells
undergo meiosis each
day to produce large
numbers of sperm
Oogenesis
• Meiosis in females to produce
ova (eggs)
• Only occurs in one cell once a
month
• The cytokinesis at end of
meiosis I and meiosis II is
uneven and cytoplasm is divided
unequally
• One cell gets the majority of
the cytoplasm and becomes the
egg
• The other cells are called polar
bodies and do not participate in
reproduction
Comparing Mitosis
and Meiosis
Comparison Animation
http://www.pbs.org/wgbh/nova/body/how-cells-divide.html
Mitosis vs. Meiosis
Mitosis
Meiosis
Starts as one cell, ends as two
cells
Starts as one cell, ends as four
cells
Starts 2N (diploid), ends 2N
Starts 2N, ends 1N (haploid)
New cells are identical to each
other AND to parent
New cells NOT identical to
each other OR parent
Used to produce body (somatic)
cells
Used to produce gametes (sex
cells)
Mitosis results in the
production of two genetically
identical diploid cells, whereas
meiosis produces four
genetically different haploid
cells