Sunday, March 13, 2016
MEIOSIS AND REDUCTION DIVISION - TEXT
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.