Meiosis
Definition : The process in cell division in sexually
reproducing organisms that reduces the number of
chromosomes from diploid to haploid (half the original
number). Meiosis involves two divisions .
The two haploid cells (23 chromosomes, N * each of
the chromosomes consisting of two sister
chromatids) produced in meiosis I to the end
result is production of four haploid cells (23
chromosomes, N in humans) in meisis II .
(leading to gametes animals and spore in plant ) .
Place : 1-In most sexually reproducing organisms
2-it occurs during animal gameto-genesis or
sporo-genesis in plants
1-sporogenesis (in the Plant )
1N – Secondary
spermatocytes
1N
2-gametogenesis
(in the animal)
2N
2N
‫ إيمان الحازمي‬: ‫إعداد‬
Stages of meiosis
• Interphase I:genetic materials are duplicated
due to active DNA replication.
• A. Meiosis I
• Prophase I:Prophase-I is a very long phase,
it is divided into five-sub-phases.
leptonema, zygonema, pachynema, diplonema, and
diakinesis.
Prophase I
• Leptonema:- (leptos = thin; = band / stripe)
The chromosomes are very thin and so can be hardly seen.
Chromosomes duplicated and the two chromatids of each
chromosome are twined around each other and appear to have
small nodules known as "Chromomeres."
Prophase I
• Zygonema:- (zygon = touching another) In this phase the
homologous chromosomes come to each other to form pairs
Synapses. The paired chromosomes are known as "bi-valents".
Prophase I
• Pachynema:- (pachus = thick) The chromosomes become
thick and dense on the completion of pairing in this phase.
As they are paired, there appear to be four chromatids in
all"tetra-valent".
• Crossing over is now exhibited by the inner chromatid of
each homologous pair of chromosome.
Prophase I
• Diplonema:- (diplous = in twofold )It is a longer subphase. The chromosomes in each pair now stay moving
away from each other. The inner chromatids are in a state
of crossing over and remain attached. These local are
referred to as chiasmata.
‫ إيمان الحازمي‬: ‫إعداد‬
Prophase I
• Diakinesis:- (dia = apart; kinein = to move) there is an
increase in the opposite pulling between the homologous
chromosomes. Because of this the chromatids break at
the chiasmata and exchange their parts. The opposite
pulling of the homologous chromosomes here does not
take them away from each other but they remain nearby.
‫إعداد ‪ :‬إيمان الحازمي‬
Metaphase-I
• At this moment the nuclear membrane starts
disappearing. The spindle fibers developing from the
centrosome and get attached to the two chromosomes of
each homologous pair opposite ends. As a result the
chromosomes appear to be arranged in two lines on
equatorial plane of the nuclear spindle.
•
Anaphase-I
Anaphase-I :the chromosomes of each
homologous pair are pulled towards the opposite
ends because of the contraction and shortening
of spindle fibers.
•
Telophase-I
The nuclear spindle disappears on completion of the
contraction of the spindle fibers. A new nuclear
membrane is formed and two daughter nuclei come into
existence. Each daughter nuclei has half the number of
Chromosomes.
B-MeiosisII
• The same like mitosis except in the end we have
4 tetrads not identical
‫ إيمان الحازمي‬: ‫إعداد‬
Meiosis II
Meiosis II is the second part of the meiotic process. Much of the process is similar to
mitosis.
The four main steps of Meiosis II are: Prophase II, Metaphase II, Anaphase II, and
Telophase II .
In prophase II we see the disappearance of the nucleoli and the nuclear envelope again
as well as the shortening and thickening of the chromatids. Centrioles move to the polar
regions and arrange spindle fibers for the second meiotic division .
In metaphase II, the centromeres contain two kinetochores that attach to spindle fibers
from the centrosomes (centrioles) at each pole. The new equatorial metaphase plate is
rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate.
This is followed by anaphase II, where the centromeres are cleaved, allowing
microtubules attached to the kinetochores to pull the sister chromatids apart. The sister
chromatids are now called sister chromosomes as they move toward opposing poles ,
while all chromatids are Chromsome stand-alone
The process ends with telophase II, which is similar to telophase I, and is marked by
uncoiling and lengthening of the chromosomes and the disappearance of the spindle.
Nuclear envelopes reform and cleavage or cell wall formation eventually produces a
total of four daughter cells, each with a haploid set of chromosomes. Meiosis is now
complete and ends up with four new daughter cells.
• http://www.youtube.com/watch?v=D1_-mQS_FZ0
• http://www.youtube.com/watch?v=35ncSrJOwME
Pollen grains squash method
1- add the anthers in to the tube .
2- Fill the tube about 2-3ml with 1N HCL and cover the tube by gauze then tie it
with rubber
3- Place the tube in 60 C water bath and incubate for 10min.
4- Remove the tube from water bath
5- remove the HCL from the test tube and rinse the anther in the water about 3
times
6- cover the root with the aceto carmine stain and incubate the root in the stain for
12 min.
7- Transfer anther to the slide and remove outer cortex .
8-Cover the root tip by a cover slip .