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.
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.
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.
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.
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.
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.