CELL DIVISION
II. MEIOSIS
DEFINITION:
Meiosis is a nuclear division process that occurs in germ cells only and leads to
formation of haploid daughter cells.
1. PRINCIPAL CHARACTERISTICS:
Meiosis reduces the diploid number by half for the forthcoming gametes. Each gamete
produced is haploid, with only one of each pair of homologous chromosomes. The union of
two gametes at fertilization restores the diploid number in the new individual.
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There are two divisions, called reduction division (meiosis I) and equation division
(meiosis II).
Meiosis reduced the diploid number by half, to the haploid number (n). This means that
while the normal number of human chromosomes is 46 over cell, the final number in each germ
cell is 23. And not just only half: Each gamete ends up with one pair of homologous
monochromatidian chromosomes.
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2. THE IMPORTANCE OF MEIOSIS
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It links parents with their offspring;
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It ensures a constant number of chromosomes in a species in all generations;
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It permits exchange of genetic information between pair of homologous
chromosomes (crossing-over), which in turn leads to variation in the traits of offspring.
3. STAGES OF MEIOSIS
MEIOSIS I: is the reduction division; in consequence, from the one diploid cell with
46 bichromatidian chromosomes (primary cyte) two haploid cells with 23 bichromatidian
chromosomes (secondary cytes) are formed.
a. PROPHASE I:
1) LEPTOTENE
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The chromatin fibres condense by coiling and folding to form chromosomes,
which are like thin threads, with each thread being two sister, chromatids in close alingment.
2) ZYGOTENE
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Homologous chromosomes begin to pair by attaching (each paternal
chromosome attaches to the homologous maternal chromosome). Such a pair is called bivalent
and this phenomenon is called synapse;
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The contact between chromosomes is so perfect that they become stitched point
by point, gene by gene along their entire length, with little space between them;
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Sex chromosomes are found in the condensed form, they don’t conjugate along
their length but ‘’head by head’’ and form a ‘’sex vesicle’’.
3) PACHYTENE
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The connection between chromosomes is very tight;
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Each bivalent has 4 chromatids, forming a tetrad;
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Non-sister chromatids undergo breakage at one or more sites along their length
and exchange corresponding segments ate the breakage point; this recombination of the genetic
material is called crossing-over. This type of recombination is called intra-chromosomal
recombination.
4) DIPLOTENE
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The separation of the homologous cèromosomes from tetrads begins, they remain
be united only on the level of chiasmata (points in which breakage and re-union occurs).
5) DIAKINESIS
Errors during meiosis
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The chromosomes attain their maximal condensation
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The homologous chromosomes in a bivalent remain connected by one or two
chiasmata, which persist until first meiotic anaphase
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The two sex chromosomes X and Y differentiate from the sex vesicle,
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The nuclear envelope start disappearing
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Formation of spindle.
The stages of meiosis
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Errors during meiosis
b. METAPHASE I:
The bivalents become positioned with the centromeres of the two homologous
chromosomes on opposite sides of the plane through the middle of the spindle
▬ The co-orientation of the undivided centromeres of each bivalent relative to the two
poles of the spindle occurs at random and determines the members of each pair of
chromosomes that will subsequently move to a particular pole.
c. ANAPHASE I:
▬ During this stage the homologous chromosomes, each composed of two chromatids
joined at an individual centromere, separate from one another and move to apposite
poles of the spindle. Each member of the pair moves randomly. This phenomenon is
called inter-chromosomal recombination.
d. TELOPHASE I:
▬ A haploid set of chromosomes consisting of one homologue from each bivalent is
located near each pole of the spindle.
▬ The spindle brakes down
▬ A nuclear envelope forms around each group of chromosomes.
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MEIOSIS II, which resembles a mitotic division, takes place after a short interphase
called interkinese. During prophase II the chromatin condenses, the nuclear envelope brake
down and the second-division spindles form. In metaphase II the centromeres of the
chromosomes align on the central plane of the spindle. During anaphase II the centromeres
separate (longitudinal cleavage) and the chromatids of each chromosome move to opposite poles.
Telophase II is marked by a transition to the interphase condition of the chromosomes in the
four haploid nuclei accompanied by division of the cytoplasm.
A
B
The stages of spermatogenesis (A) and oogenesis (B)
THE ERRORS DURING MEIOSIS
Normally each gamete contains a haploid number of chromosomes and the normal state
of cells is monosomy. But different errors during first or second meiotic divisions may be source
of abnormal sperm or egg cells.
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Errors during meiosis
a. Chromosomal non-disjunction
▬ This error takes place during
anaphase I.
▬ Both chromosomes from pair joined
by chiasmata move to the some pole.
▬ One of the cells will contain a
nullisomy and another – disomy.
▬ After the second division will obtain
two cells with nullisomy (50%) and
two cells with disomy (50%).
▬ The fertilization of a gamete with
nullisomy with a normal one leads to
a monosomic zygote.
▬ The fertilization of a gamete with disomy with a normal one leads to a trisomic
zygote.
b. Anaphase I lag
▬ This is a rare error, which takes
place during anaphase I.
▬ One chromosome is lagging and
will be lost.
▬ One of the cells will contain a
monosomy (normal) and another –
nullisomy.
▬ After the second division will
obtain two cells with nullisomy
(50%) and two cells with
monosomy (50%).
c. Secondary cytes non-separation
▬ This error takes place during
telophase II usually in oogenesis.
▬ The first meiotic division is
normal, but during the second
some
cells
lack
normal
cytokinesis.
▬ Two cells are normal (with
monosomy) and one – contains a
diploid set of chromosomes.
▬ If a diploid egg is fertilized with a
normal sperm a triploid zygote
will be formed.
d. Chromatidian non-disjunction
▬ This error takes place during
anaphase II.
▬ After first division there are
two normal haploid cells.
▬ Some cells may have a normal
second division.
▬ In some cells a chromatidian
non-disjunction take place.
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Errors during meiosis
▬ As result 50% of cells are normal, 25% – nullisomic and 25% - disomic.
e. Anaphase II lag
▬ This error may take place more
often than Anaphase I lag.
▬ After the first division both cells
are normal.
▬ 75% of gametes are normal and
25% are nullisomic.
f. Transversal cleavage of centromere
▬ As result of transversal
cleavage of centromere during
second meiosis some cells may
contain isochromosomes.
▬ 50% of gametes are normal,
25% - with isop and 25% with isoq.
As results of fertilization of an abnormal gamete with a normal one the whole organism
will consists of cells with an abnormal number of chromosomes (the errors during mitosis
usually lead to mosaics).
Define the terms:
Meiosis, leprotene, zygotene, diplotene, pachytene, diakinesis, chromosomal disjunction,
chromosomal non-disjunction, inter-chromosomal recombination, intra-chromosomal
recombination, anaphase lag, isochromosome.
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