Meiosis
By
Tibor Cemicky
What is Meiosis?
• Meiosis is a two-part cell division process in organisms
that reproduce sexually
• Occurs in Humans, animals and plants. Never in
prokaryotic organisms such as bacteria.
• Meiosis is used in sexual reproduction of organisms to
combine male and female genes, to create a new,
biological organism.
• There are two stages of meiosis: meiosis I and meiosis II
• At the end of the meiotic process, four haploid daughter
cells are produced. Each contains halve the
chromosomes of the parent cell.
Diploid vs. Haploid Cells
• Diploid: In a diploid cell, there are 2 copies of
each chromosome, making for a total of 40+
chromosomes. These chromosomes are not
identical, they have the same genes arranged in
the same sequence however they usually do not
have the same number of alleles. Therefore they
are not identical but homologous.
• Haploid: In a haploid cell, there's only one copy
of each chromosome, therefore there are only
20+ chromosomes.
Stages of Meiosis
• Meiosis 1:
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–
–
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Prophase 1
Metaphase 1
Anaphase 1
Telophase 1
• Meiosis 2:
–
–
–
–
Prophase 2
Metaphase 2
Anaphase 2
Telophase 2
Process of
Meiosis
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
Prophase 1
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
•
Nuclear envelope breaks up and disappears
•
Chromosomes become visible as they shorten, coil,
and thicken
•
Each chromosome is composed of two sister chromatids
(attached at centronome) containing identical genetic information
•
Homologous chromosomes (chromosomes containing sequences for the same
genes) become associated along their entire lengths (they synapse), to form a
structure known as a tetrad or homologous pair (contains 2 chromosomes and 4
chromatids)
•
During synapsis, the two chromosomes in the homolog pair exchange segments of
DNA in a process called crossing over. As a result, the gene combinations on a
chromosome can be changed.
Example: If one chromatid of a chromosome initially contained genes for brown eyes and brown
hair, after crossing over, it could contain genes for blue eyes and brown hair, by taking the gene
for blue eyes from one of the chromatids of the other chromosome.
Metaphase 1
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
• Nuclear membrane dissapears completely
• Tetrads (homologous pairs)
move to a plane called the
"metaphase plate“ halfway between
the two poles of the cell
• Spindle fibers attach to different chromosomes
in each homologous pair at the centromeres
Anaphase 1
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
• The cell elongates
• Two chromosomes 1 from each
chromosome pair (tetrad) are pulled
toward opposite poles as the
microtubules of the spindle apparatus
shorten
Telophase 1
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
•
The chromosomes reach the poles
•
At each pole, now, there is a
complete haploid set of
chromosomes consisting of
one member of each homologous
pair (tetrad)
•
Nuclear membrane reforms around the daugher nuclei
•
Cytokinesis occurs and the cell splits
into 2 daughter cells
Prophase 2
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
•
Meiosis 2 begins. It is very similar to mitosis.
•
Chromatids are no longer identical because
they mixed during Prophase 1 (crossing over)
•
Nuclear envelope breaks down
•
Spindle apparatus begins growing outward
from the centrosomes
Metaphase 2
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
•
Chromosomes line up at the metaphase II plate at the cell's center
•
Spindle fibers bind to both sides of the centromeres (region of
found near the middle of a chromosome where two sister
chromatids come in contact)
Anaphase 2
Prophase 1 => Metaphase 1 => Anaphase 1 => Telophase 1 => Prophase
2 => Metaphase 2 => Anaphase 2 => Telophase 2
• Spindle fibers shorten and
separate the sister chromatids
which move toward the
opposite cell poles.
Telophase 2
• Distinct nuclei form at the opposite poles
• Cytokinesis occurs, the two cells divide to
form 4 haploid daughter cells
• Nuclear envelopes form in each
new daughter cell
• Now there are four daughter cells each
with one half the number of chromosomes
of the original parent cell.
• Meiosis is now complete
Mitosis vs. Meiosis
Purpose of Meiosis and
Mitosis in eukaryotic
organisms:
•Both Meiosis and Mitosis
are found in complex
organisms which
reproduce sexually
including Humans.
•Identical versions of cells
can be created through
Mitosis. Mitosis may
therefore be used for
growth and reparation of
damaged organs and
tissues.
•Meiosis is a special
process reserved only for
the creation of the egg
and sperm cells.