Meosis cell division
Meosis i
Meiosis is also called reduction division occurring in the reproductive germ cells at the time of gamete formation. In plants it takes place during spore formation. The somatic or vegetative cells having complete number of chromosomes are called diploid (2n). White gametes containing half of original number of chromosomes are called haploid (n) or monoploid. Meiosis has two successive divisions of a mother cell. First division is reduction division during which the chromosome number (2n) in both the daughter cells is reduced to half (n), the second division is simple a mitotic division resulting in four cells, each having same reduced number (n) of chromosomes. It is divided into first and second meiotic division. First meiotic division:
PROPHASE I
It is longer in duration and has following sub stages:
Leptotene: During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
Zygotene: It occurs with the movement of similar chromosomes brought together by attraction between them. Thus the chromosomes of each homologous pair approach each other and become associated to form a bivalent. The pairing of homologous chromosomes is known as synapsis.
Pachytene: During this stage bivalent chromosomes now clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
Diakinesis: It is characterized by disappearance of nuclear membrane, nucleolus and completion of spindle apparatus. The separation of bivalents is completed by the process of terminallization in which the movement of the chiasmata from centrosome towards the ends of chromosome arms take place like a zipper and at the end of diakinesis two chromotids are held together only at their ends by the centrosomes. Now the bivalents become more thickened, contracted and visible.
Metaphase I:
The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I:
In this stage each chromosome of bivalent of homologues is pulled towards the opposite pole by the contraction of half spindle fibres. The anaphase is completed when two sets of chromosomes reach the opposite poles of the cell.
Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as diad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Meosis ii
Prophase II:
Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.
Metaphase II: The half or discontinuous spindle fibres attach at the chromosomes of the diads and the two chromatids get separated at the centrosome from each other.
Anaphase II:
It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.
Telophase II: The chromosomes uncoil and form separate groups and around each group a nuclear membrane is formed.
Significance of Meiosis:
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
Meosis i
Meiosis is also called reduction division occurring in the reproductive germ cells at the time of gamete formation. In plants it takes place during spore formation. The somatic or vegetative cells having complete number of chromosomes are called diploid (2n). White gametes containing half of original number of chromosomes are called haploid (n) or monoploid. Meiosis has two successive divisions of a mother cell. First division is reduction division during which the chromosome number (2n) in both the daughter cells is reduced to half (n), the second division is simple a mitotic division resulting in four cells, each having same reduced number (n) of chromosomes. It is divided into first and second meiotic division. First meiotic division:
PROPHASE I
It is longer in duration and has following sub stages:
Leptotene: During leptotene stage the chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
Zygotene: It occurs with the movement of similar chromosomes brought together by attraction between them. Thus the chromosomes of each homologous pair approach each other and become associated to form a bivalent. The pairing of homologous chromosomes is known as synapsis.
Pachytene: During this stage bivalent chromosomes now clearly appears as tetrads. This stage is characterised by the appearance of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. Crossing over is also an enzyme-mediated process and the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material on the two chromosomes. Recombination between homologous chromosomes is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
Diakinesis: It is characterized by disappearance of nuclear membrane, nucleolus and completion of spindle apparatus. The separation of bivalents is completed by the process of terminallization in which the movement of the chiasmata from centrosome towards the ends of chromosome arms take place like a zipper and at the end of diakinesis two chromotids are held together only at their ends by the centrosomes. Now the bivalents become more thickened, contracted and visible.
Metaphase I:
The bivalent chromosomes align on the equatorial plate. The microtubules from the opposite poles of the spindle attach to the pair of homologous chromosomes.
Anaphase I:
In this stage each chromosome of bivalent of homologues is pulled towards the opposite pole by the contraction of half spindle fibres. The anaphase is completed when two sets of chromosomes reach the opposite poles of the cell.
Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called as diad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus. The stage between the two meiotic divisions is called interkinesis and is generally short lived. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Meosis ii
Prophase II:
Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.
Metaphase II: The half or discontinuous spindle fibres attach at the chromosomes of the diads and the two chromatids get separated at the centrosome from each other.
Anaphase II:
It begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell.
Telophase II: The chromosomes uncoil and form separate groups and around each group a nuclear membrane is formed.
Significance of Meiosis:
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half. It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
No comments:
Post a Comment