Lab. 3
Cell Division
2. Meiosis division
Main topics:
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What’s Meiosis?
Meiosis I phases
Meiosis II phases
Cytokinesis division
What is meiosis?
In biology, meiosis is the process by which one diploid eukaryotic cell divides to
generate four haploid cells often called gametes. Meiosis is essential for sexual
reproduction and therefore occurs in all eukaryotes (including single-celled organisms)
that reproduce sexually. Meiosis does not occur in archaea or bacteria, which reproduce
via asexual processes such as mitosis or binary fission.
During meiosis, the genome of a diploid germ cell, which is composed of long segments
of DNA packaged into chromosomes, undergoes DNA replication followed by two
rounds of division, resulting in haploid cells called Gametes. Each gamete contains one
complete set of chromosomes, or half of the genetic content of the original cell. These
resultant haploid cells can fuse with other haploid cells of the opposite sex or mating
type during fertilization to create a new diploid cell, or zygote. Thus, the division
mechanism of meiosis is a reciprocal process to the joining of two genomes that occurs
at fertilization. Because the chromosomes of each parent undergo genetic
recombination during meiosis, each gamete, and thus each zygote, will have a unique
genetic blueprint encoded in its DNA.
Meiosis uses many of the same biochemical mechanisms employed during mitosis to
accomplish the redistribution of chromosomes. There are several features unique to
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meiosis, most importantly the pairing and genetic recombination between homologous
chromosomes.
The phases of meiosis I & II:
Meiosis I:
Figure (4): Meiosis I Phase.
a. Prophase I:
The prophase I is relatively long and can be subdivided into five stages.
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Leptotene
Zygotene
Pachytene
Diplotene
Diakinesis
1. leptotene
The chromosomes become gradually visible under the
light microscope. The compaction of chromosomes
continues throughout leptotene.
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2. zygotene
During this stage chromosomes start pairing together
and this process of association is called synapsis. Such
paired
chromosomes
are
called
homologous
chromosomes. Electron micrographs of this stage
indicate that chromosome synapsis is accompanied by
the formation of complex structure called synaptonemal
complex. The complex formed by a pair of synapsed
homologous chromosomes is called a bivalent or a
tetrad. However, these are more clearly visible at the
next stage.
3. Pachytene
During this stage bivalent chromosomes now clearly
appears as tetrads. This stage is characterized by the
appearance of recombination nodules, the sites at
which
crossing
over
occurs
between
non-sisters
chromatids of the homologous chromosomes. Crossing
over is the exchange of genetic material between two
homologous chromosomes. Crossing over leads to
recombination
of
genetic
material
on
the
two
chromosomes. Recombination between homologous
chromosomes is completed by the end of Pachytene,
leaving the chromosomes linked at the sites of crossing
over.
4. Diplotene
It is recognized by the dissolution of the synaptonemal
complex and the tendency of the recombined homologous
chromosomes of the bivalents to separate from each other
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except at the sites of crossovers. These X-shaped structures,
are called chiasmata
5. Diakinesis
During
this
phase
the
chromosomes
are
fully
condensed and the meiotic spindle is assembled to
prepare the homologous chromosomes for separation.
By the end of diakinesis, the nucleolus disappears and
the nuclear envelope also breaks down.
b. Metaphase I:
The bivalent chromosomes align on the equatorial plate. The microtubules from the
opposite poles of the spindle attach to the pair of homologous chromosomes
c. Anaphase I:
The homologous chromosomes separate, while sister chromatids remain associated at
their centromeres
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d. Telophase I:
Nuclear envelopes may reform, or the cell may quickly start meiosis II.
Cytokinesis:
It is may happen:
Legumes
Not Happen
Cereal
Happen and this is called as Dyad
Meiosis II:
Is similar to mitosis. However, there is no "S" phase. The chromatids of each
chromosome are no longer identical because of recombination. Meiosis II separates the
chromatids producing two daughter cells each, and each chromosome has only one
chromatid.
Figure (5): Meiosis II Phase.
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a. Prophase II
The nuclear membrane disappears by the end of prophase II. The chromosomes
again become compact.
Legumes
Cereal
b. Metaphase II
At this stage the chromosomes align at the equator and the microtubules from
opposite poles of the spindle get attached to the kinetochores (centromere
region) of sister chromatids.
Legumes
Cereal
c. Anaphase II
It begins with the simultaneous splitting of the centromere of each
chromosome (which was holding the sister chromatids together), allowing
them to move toward opposite poles of the cell.
Legumes
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d. Telophaes II
Meiosis ends with telophase II, in which the two groups of chromosomes once
again get enclosed by a nuclear envelope; cytokinesis follows resulting in the
formation of tetrad of cells i.e., four haploid daughter cells.
Legumes
Cereal
Cytokinesis:
Legumes
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Cereal “Tetrad”
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Comparing Meiosis and Mitosis:
Differences
Meiosis
Mitosis
Chromosome
Homologous
Homologous
behavior
chromosomes pair
chromosomes
forming bivalents until
independent
anaphase I
Chromosome
daughter cells haploid
identical daughter cells
numberreduction in
meiosis
Genetic identity Daughter cells have new identical daughter cells
of progeny
assortment of parental
chromosomes.
chromatids not identical,
crossing over
References:
http://www.vcbio.science.ru.nl/en/virtuallessons/meiostage/
https://s10.lite.msu.edu/res/msu/botonl/b_online/e09/4.htm
http://www.usca.edu/biogeo/zelmer/122/celldivision/meiosis/
http://biology4isc.weebly.com/meiosis.html
http://www.ncert.nic.in/html/learning_basket/biology/cc&cd.pdf
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Training
1. Find and draw a cell showing each stage of mitosis
Prophase I
Metaphase I
Anaphase I
Telophase I
Prophase II
Metaphase II
Anaphase II
Telophase II
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Exercise 3
Choose the correct answer for each question:
1. Meiosis results in _____
a) 2 haploid daughter cells
b) 4 haploid daughter cells
c) 2 diploid daughter cells
d) 4 diploid daughter cells
2. Which of the following cells undergo meiosis?
a) sperm cells
b) liver cells
c) unicellular organisms
d) all of these
3. Synapsis is the side-by-side alignment of
a) Sister Chromatids of a Chromosome.
b) Homologous Chromosomes.
c) Non-Homologous Chromosomes.
d) Spindle Fibers.
e) Nuclei prior to the division of the cytoplasm.
4. Crossing-over occurs in metaphase I.
a) True
b) False
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