Lec.15 Dr: Buthaina Al-Sabawi
Date:24/12/2016Meiosis
Meiosis is a process where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells – sperm in males, eggs in females.
Meiosis requires two nuclear division and produces four haploid daughter cells.
Occurs in organism reproduce by the fusion of egg & sperm.
Accompanied by a reduction in the number of chromosomes in the daughter cells.
Two of each kind of chromosomes (homologous chromosome) (diploid or 2N).
Diploid chromosomes
egg (1N) sperm (1N)
This reduction in chromosome number is accomplished by two rounds of nuclear & cell division called (meiosis 1 & meiosis 2)
Interphase: The chromosome separated into two chromatids and not visible under Light microscope.
Meiosis 1
Prophase 1
Divided into 5 stages.
Leptotene stage:
Chromosomes start to condense and thicken, becoming visible as discrete structures. Although the chromosomes have duplicated, the sister chromatids of each chromosomes are not visible in the microscope.
Zygotene stage:
Homologous chromosomes become closely associated (synapsis) to form pairs of chomosomes (bivalent) consist of four chromatids .
A zipper like protein structure (synaptonemal complex) forms along
the length of the paired chromosomes. This complex keeps the homologous chromosomes closely associated and aligned with one another through the pachytene stage, which is persist for several days.
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Pachytene stage:
Which is persist for several days. Full synapsis linked at the sites of (crossing over) (chiasmata).
Recombination nodules appear along the synaptonemal complex.
Crossing-over: Is an exchange of genetic material between non-sister chromatids of a bivalent during meiosis, also it is a way to increase genetic variation in the gametes.
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Diplotene stage:
Synaptonemal complex dissolve but remain associated at the chiasmata, which is critical for their correct alignment at metaphase., each chromosome pair (bivalent) consist of 4 chromatids with clearly evident chiasmata. The number of chiasmata is reduced gradually as the two chromosome continue their separation (terminalization).Diakinesis stage:
Chromosomes become fully condensed & increase in thickness, the chiasmata move toward the end of chromosomes, the number of this chiasmata decrease. The nuclear membrane & nucleolus disappear, spindle become to form. Bivalent, paired chromosomes separated in the nuclear space.
Metaphase 1:
Bivalent take its location in the equatorial plate of spindle, and attached to the spindle fibers.
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Anaphase 1:
Is initial by disruption of the chiasmata at which homologous
chromosomes are joined. The homologous chromosomes then
separate & move to the opposite poles, while sister chromatids remain associated at their centromers.
Telophase1:
The homologous chromosomes move to the opposite of the cell. The
spindle disappears, while the nuclear membrane & nucleolus re-form.
At completion of meiosis 1, after cytokinesis, each daughter has therefore acquired one member of each homologous pair, consisting of
two sister chromatids.
Interkinesis
This is similar to interphase with one important exception: No chromosomal duplication
Meiosis 2
Meiosis 2 initiates immediately after cytokinesis, usually before the chromosomes have fully decondensed. In contrast to meiosis, meiosis2, resembles a normal mitosis.
Prophase 2:
The nuclear membrane & nucleolus disappear, centrosome replicates and move to the poles, new spindle reappear. The chromosomes(2 chromatids) attached to the fibers of spindle.
Metaphase 2:
The chromosomes align at the metaphase plate. Sister chromatids attach to spindle fibers from opposite poles.
Anaphase 2
Centromeres divided and sister chromatids move to opposite poles.
Telophase 2
Chromosomes begin to uncoil. The spindle disappear & nuclear
membrane reappear. Cytokinesis then follows, giving rise to haploid daughter cells, each daughter cell has half the number of chromosomes of the diploid parent cell.
Cytokinesis
The cytoplasm divides, forming four new cells.
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