Meiosis
The preparatory steps of meiosis are identical to the interphase of the mitosis. Interphase is divided into the same three phases i.e. G1, S phase, and G2. Interphase is followed by meiosis I and meiosis II.
Meiosis 1
In meiosis I, the homologous chromosomes in a Thomas Hunt Morgan observed diploid cell separate and so two haploid daughter the phenomenon of crossing cells are produced. It is the step in meiosis that over in the fruit fly Drosophila generates genetic diversity.Meiosis I occurs in two melanogaster. main steps i.e. karyokinesis and cytokinesis.
The karyokinesis of Meiosis I is subdivided into prophase I, metaphase I, anaphase I, and telophase I.
Prophase I:
Prophase I is the longest phase in meiosis. During this stage, individual chromosomes begin to condense within the nucleus. Then the homologous chromosomes line up with each other and form pairs. The combined homologous chromosomes are said to be bivalent. They may also be referred to as a tetrad, a reference to the four sister chromatids.The two non-sister chromatids of homologous chromosomes become "zipped" together, forming complexes known as chiasmata, in a process known as synapsis. In the next stage, the non-sister chromatids of homologous chromosomes randomly exchange their segments and the phenomenon is known as crossing over .The exchange of segments results in a recombination of genetic information. After crossing over the homologous chromosomes separate from one another.
However, they remain tightly bound at chiasmata, the regions where crossing over occurred. Chromosomes condense further, the nucleoli disappear, and the nuclear envelope disintegrates. Centrioles, which were duplicated during interphase, migrate to the two poles of the cell.
They give rise to spindle f chromosomes. While the non-kinetochore spindle fibres from botfibres. The kinetochore spindle fibres attach to the kinetochores oh sides interact with each other. There are two kinetochores on each tetrad, one for each kinetochore spindle fibre. chromosomes chromosomes
Metaphase I:
As kinetochore spindle fibers from both centrioles attach to their respective kinetochores, the homologous chromosomes align along an equatorial plane forming the metaphase plate.Anaphase I:
Kinetochore spindle fibers shorten, breaking the chiasmata and pulling homologous chromosomes apart. Since each chromosome only has one kinetochore, one chromosome is pulled toward one pole, forming two diploid sets. Each chromosom Sister chromatids of a chromosome.Nomsister chromatids of a homologous chromosome pair contains a pair of sister chromatids.
Telophase I:
The first meiotic division ends when the chromosomes arrive at the poles. Each pole now has half the number of chromosomes but each chromosome still consists of a pair of chromatids. The spindle network disappears, and a new nuclear envelope surrounds each haploid set.The chromosomes uncoil back into chromatin.
Cytokinesis
The pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells After meiosis I both haploid daughter cells enter a period of rest known as interkinesis or icterphase II. The interphase II is different from the interphase of mitosis and meiosis I.There is no S-phase and so no DNA replication occurs during this stage. During crossing over, genetic material is exchanged between sister/non-sister chromatids of homologous/non-homologous chromosomes.
MEIOSIS
It is the second part of the meiotic process. Much of this part is similar to mitosis.It is subdivided into prophase II, metaphase II, anaphase II, and telophase II. Prophase II It takes much less time compared to prophase I. In this prophase the nucleoli and the nuclear envelope disappear and the chromatin condenses. Centrioles move to the polar regions and make spindle fibres.
In metaphase II, the chromosomes attach with the kinetochore spindle fibers and align at the equator of the cell. This is followed by anaphase II, where the centromeres are cleaved and sister chromatids are pulled apart.
The sister chromatids are now called sister chromosomes, and they are pulled toward opposing poles. The telophase II is marked with uncoiling and disappearance of the chromosomes.
Nuclear envelopes reform; cleavage or cell wall formation eventually produces a total of 4 daughter cells, each with a haploid set of chromosomes .
SIGNIFICANCE OF MEIOSIS
The significance of meiosis for reproduction and inheritance was described in 1890 by German biologist August Weismann, who noted that meiosis was necessary to transform one diploid cell into four haploid cells if the number of chromosomes had to be maintained. Meiosis is essential for reproduction and therefore occurs in all eukaryotes, including single-celled organisms that reproduce sexually.Meiosis does not occur in archaea or prokaryotes, which reproduce asexually by binary fission. Humans, for example, are diploid creatures. The diploid gamete-mother cells undergo meiosis to produce haploid gametes, which are spermatozoa in males and ova in females. These gametes then fertilize, producing a diploid zygote. The zygote undergoes repeated mitosis and develops into the new organism
Significance of meiosis in maintaining chromosome number constant Male parent Female parent Many fungi and many protozoa are haploid. Such organisms produce haploid gametes through mitosis. When two gametes fuse they form diploid zygote, which undergoes meiosis immediately, creating four haploid cells. These cells undergo mitosis to create the haploid organism.
Plants' life cycle shows alternation of generations. The cells of the diploid sporophyte generation undergo meiosis to produce haploid spores, which grow into haploid gametophyte generations. The haploid gametophyte generation produces haploid gametes through mitosis.
The gametes combine to produce the diploid zygote. The zygote undergoes repeated mitosis to become the diploid sporophyte. Because the chromosomes of each parent undergo genetic recombination during meiosis, each gamete, and thus each zygote, will have a unique genetic makeup. In other words, meiosis and sexual reproduction produce genetic variation. Thus meiosis allows a species to bring variations to handle the changes in the environment.
ERRORS IN MEIOSIS
The normal separation of chromosomes or sister chromatids in meiosis is termed as disjunction. When the separation is not normal, it is called nondisjunction. This results in the production of gametes which have either more or less than the usual number of chromosomes i.e. there may be trisomy (2n + 1) or monosomy (2n -1).It causes several medical conditions in humans such as; Down's Syndrome (trisomy of chromosome 21), Klinefelter's Syndrome (an extra X chromosome in males), and Turner's Syndrome (only one X chromosome present in females). Such individuals have 45 or 57 chromosomes.
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