You have learnt
that meiosis is the reduction division and occurs in the germ cells that
produce gamets. Majot difference between mitosis and meiosis is in the number
of chromosomes in daughter cells.
In mitosis, daughter cells have exactly the
same number of chromosomes as the mother cell while, in meiosis the daughter
cells have half the number of chromosomes of mother cell.
Daughter cells formed by meiotic division lead to
the formation of male of female gametes. Each gamete has half the number of
chrosomes of mother cell. During fertilization, the gametes fuse to form
zygote.
In zygote has two sets of chromosomes – one from the male parent
(sperm) and other from female parent
(ovum). Zygote develops into an organism by repeated mitotic divisions.
A CELL with only one set of chromosomes is called a
haploid cell. As the number of chromosomes in the daughter cell are reduced by
half, meiosis is also called reduction dividsion. In general, the male and
female gametes are haploid cells.
In majority of organisms, each cell has two sets of
chromosomes are passed in to the two daughter cells. However, in meisosis only
one of the two sets of chromosomes is passed on to each of the daughter.
During meiosis nucleus divides twice- these division
are called meiosis-I and meiosis-II. In meiosis-I the daughter cells receive
only one set of chromosomes that is reduction in the number of chromosomes
occurs in meiosis-I.
MEIOSIS-II is a simple mitotic division which occurs
immediately after meiosis-I. daughter cells of meiosis-I divide into a second
generation of daughter cells- there are total four daughter cells each of with
only one set chromosomes of four hploid cells are formed at the end of meiosis.
Meiosis –I This occurs in 5 stages .1. Prophase 2. Metaphase 3. Anaphase-I 4. Telophase-I and cytokinesis-I.
1. Prophase -
number of changes occur in the cell during prophase-I. to und prophase-I is
divided into five sub-stages. They are leptotene, zygotene, pacheytene,
diplotene,and diakinesis. Chromatin in the nucleus of the mother cell condenses
and forms chromosomes.
Following this, chromosomes start pairing. Each pair has
two identical chromosomes. One chromosome of the pair is derived from the
father and the ofther from the mother. In this stage, appear like letter’X’.
this is called crossing over the chromosomes move towards the centre of the
cell. Centrioles also move to the opposite ends over to the cell and spindle fibre formation begins. Nucler
envelope and nuleous disappear.
Chromosomeal movement is completed. Chromosomes lie
in the centre of the cell in the form pf plate- called quatorial plate. Some of
the spindle fibres are attached to the centromere of the chromosome and rest of
the spindle fibres attach to the centrosome of the opposite end of the cell.
Length of spindle fibres is reduced. As a result
chromosome are pulled apart. Each chromosomes splits into two sister chromosomes. Each sister chromosomes has a
segment of chromatid derived from mother cell and the father cell. Chromosome
move towards opposite end of the cell.
Chromosomal movement towards opposite poles of the
cells completed. Spindle fibres disappear.
Nuclear envelope reappears around chromosomes and the nucleus is also
formed again. With these meiosis-I is completed and the diploid parental cells
has given rise to daughters cells with haploid
numbers of chromosomes.
This is the second division nuceus in meiosis.
Haploid daughter cells formed in meiosis-I participate in this division and as
parental cells. They divide to produce two haploid daughter cells each. Before
dividing, chromosomal number of the haploid cell doubles. As the end of
meiosis-II four haploid daughter cells are formed. meiosis-II also proceeds in
stages – prophase-II 2.
Metaphas--II 3.
Anaphase-II 4. Telophase-II and cytokinesis-II .
This division
is similar to mitotic division.. you may refer the chapter on mitosis for the
change in the cell that occur in this division and for the process of cell
division.
Points to
remember-
During meiosis, only one set of chromosomes are
passed on to the daughter cells. Hence daughter cells have hald the number of
the chromosomes of the mother cell.
In meiosis , karyokinesi and cytokinesis occur two
times.
Chromosomes number is not doubled during meiosis-I .
during this division, the chromosomes number is reduced by half. The diploid
mother cell gives to two haploid daughter cells.
Prophase-I of meiosis is divided into five
sub-stages.
Before meiosis-II, chromosomes number is doubled and
the haploid mother cell gives rise to two haploid daughter cells.
The events in meiosis-II are similar to mitotic
division.
Crossing over of chromosomes occur during meiosis.
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