Sexual reproduction
takes place in animals and plants by forming gametes. The gametes fuse to form
zygote. The zygote forms the organism. The number of chromosomes remain
constant in the successive generations. How the number of chromosomes remain
constant from generations to generations? August Weismann proposed the
hypothesis that “'there must be a kind of cell division in which the chromosome
number is halved”. There are two types of cell. (I) somatic cells which make up
the body, (ii) germline cells which produce the gametes. Both germline cells
and somatic cells are diploid (2n). When a germline cell undergoes division it
produces cells with half of the diploid number of chromosomes i.e. haploid (n)
and the process of division is called reduction division or meiosis. Sperms and
eggs are produced by meiosis in animals. Spores are produced by meiosis in
plants.
Definition
Meiosis is a process of
nuclear division in which the number of chromosomes in a cell is halved during
cell division.
The Stages of Meiosis
Meiosis is a continuous
process. It can be described most easily by dividing it into two arbitrary
stages. The two stages of meiosis are called meiosis I and meiosis II. These
are further subdivided into prophase, metaphase, anaphase and telophase.
The First Meiotic Division
Interphase I: The DNA
duplicates by its replicating process. The fine chromosomes are double stranded
i.e. chromosome replicate. The interphase lack G2 stage.
Prophase
I:
It is a lengthy process of meiosis. Each chromosome has two chromatids. It is
further divided into five stages:
(1) Leptotene (2)
Zygotene (3) Pachytene (4) Diplotene (5) Diakinesis.
Leptotene: (Thin Thread
Stage)
Chromosomes look like
interweaving threads. Due to condensation of chromatin material; chromosomes
become more apparent and distinct, nucleus increases in size. Initially thin
chromosomes become shorter and thicker. The chromosome number of the cells is
seen. Homologous chromosomes start getting closer to each other. In the cell
there are two of each type of chromosome. The identical chromosomes are called
homologous chromosomes (Greek: homologue; agreement). The morphology and
position of the centromere of the homologous chromosome is same. For example in
man, the number of chromosome is 46 and the homologous pairs of chromosome is
23.
Zygotene: (Pairing Stage)
The homologous
chromosomes begin to pair length wise with its homologue. The process of
pairing is called synapsis (Greek: union). The synapsis may start from any
point with the corresponding complementary DNA strand of the other.
 |
| Prophase: 1 |
Pachytene: (Thickening
Stage)
After the completion of
synapsis the chromosomes shorten and thicken due to condensation of the
chromatin material and their double nature is evident. Each pair of synapsed chromosome
consists of four chromatids, two centromeres and is called a tetrad or
bivalent.
Crossing Over: The
chromatids of the homologues may cross each other and the point of crossing is
X shaped figure under the light microscope. It is called chiasma (Greek:
chiasma; cross, plural: chiasmata). Chromosome segment is exchanged between the
two homologous chromosomes at the chiasma and is called crossing over.
Diplotene: (Duplication
Stage)
The paired homologous
chromosomes begin to separate by repelling. (The bivalent is still attached at
chiasmata or chiasma). Diplotene can last for months or years. The chromatids
are clearly visible due to the progress in condensation cycle.
Diakinesis: (Moving Apart
Stage)
The tetrad are more
evenly distributed in the nucleus against the nucleoli envelope. The number of
chiasmata begins to reduce as the diakinesis reaches to its end, but the
terminal chiasmata are still present. The nucleoli disappear. The nuclear
membrane is still present. The diad (the two chromatids attached to a single
centromere is called diad) behaves as a single unit because they are held
together by a common centromere, throughout meiosis.
Metaphase I
The nuclear membrane
disintegrate. The microtubules form the spindle. The chromosome line up in
double row i.e. pair on the equator. Microtubules bind only to one kinetochore
of each centromere. The centromere of one homologue becomes attached to microtubules
extending to one pole, whereas the centromere of the other homologue becomes
attached to microtubules extending to other pole.
Anaphase I
The attachment of
chromosomes to the spindle is complete. The microtubules that are attached to
the homologous chromosomes begin to slide over one another, as a result the
spindle poles move apart. The spindle fibers become shorter as the spindle
fibers are dragged to the poles. They are broken by the action of enzymes. With
the shortening of spindle fibers, the chromosomes that are attached to the
spindles are also pulled towards each pole. When the shortening of spindle
fiber is completed, each pole has single set of chromosomes, consisting of one
member of each homologous pairs.
 |
| Crossing Over |
Telophase I
In telophase I, each set
of chromosomes is present at their respective poles. By cytokinesis two cells
are formed, which are haploid.
The Second Mitotic
Division: Meiosis II is simply a mitotic division. It occurs to separate the
sister chromatids as in mitosis.
Interphase
II:
DNA does not duplicate in interphase II. Interphase II is very brief. Each of
the two cells resulting from meiosis I progress into meiosis II very quickly.
Prophase
n:
The complicated nuclear events of prophase do not take place.
Metaphase II and Anaphase n
The chromosomes lineup
in the same fashion as they did in metaphase I. Here again a random
distribution of chromosomes take place. The centromere divides, as a result the
two chromatids are separated. As each chromatid is now a separate structure
they are called chromosomes. Chromosomes are distributed in equal number, to
each pole. DNA does not replicate but the centromere divides
Telophase
II:
The nuclei are reconstructed in the typical manner. Each nucleus now contains
haploid set of chromosomes, because DNA has duplicated only once during the
cell division.
 |
Meiosis
in Animal Cell
|
Fate
of haploid cells: Each of the four haploid products of
meiosis contains a basic set of chromosomes. These haploid cells may function
directly as gametes, as they do in animals or may continue to divide by
mitosis, as they do in plants, fungi and many protists.
#sexualreproduction #gametes #zygote #chromosomes #generations #AugustWeismann #celldivision #somaticcells #germlinecells #diploid #haploid #reductiondivision #meiosis #animals #plants #nucleardivision #prophase #metaphase #anaphase #telophase #leptotene #zygotene #pachytene #diplotene #diakinesis #synapsis #crossingover #chiasma #tetrad #bivalent #diad #nucleoli #kinetochore #microtubules #spindle #enzyme #cytokinesis #mitosis #nuclearmembrane #homologouschromosomes #chromatinmaterial #condensation #chromatids #centromere #meiosisI #meiosisII #spores #sperms #eggs #haploidcells
