In Meiosis division cell chromosomes are become half. This process takes place in sexual reproductive cells .In this process chromosomes number become half ,so the product we call it as gamete. Here Gamete is haploid , when it fuses with other gamete produces diploid zygote.
Sexually-reproducing organisms follow this pattern of alternation of generations.Thus, the life cycles alternate between Haploid and Diploid stages, and between the processes of meiosis and syngamy. It is because of the way in which genes recombine in meiosis and syngamy that we have the whole study of genetics.
G1,
G2 and S Phase together called as Interphase.
G1 Phase:
the first growth period of the cell cycle,, the cell grows in size and synthesizes mRNA and proteins that are required for DNA
synthesis.
S Phase:Synthesis phase ,replication takes place .S phase a number of events additional to chromosomee replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called G2.
G2 Phase:
the second growth period of the cell cycle, following DNA replication and preceding prophase,It synthesizes the protein that constructs the microtubules, which are responsible for separating the chromosomes in the anaphase of meiosis.
In Meiosis two parts are present.
Meiosis 1 and meiosis 2
Interphase occure only before meiosis 1 not at meiosis 2.
Meiosis 1 :prophase1,Metaphase1,Anaphase1, and Telophase1.
Meiosis2:Prophase 2, Metaphase 2, Anaphase 2, Telophase 2.
Meiosis 2 is exactly simillar to mitosis.
DNA replication only takes one time before meiosis 1.But meiosis 2 cell division takes place but DNA replication does not take place.
In meiosis prophase is long period , Prophase1 is divide in to 1.Leptotene 2. Zygotene 3.Pachytene 4.Deplotene and 5.Diakinesis.
Synapsis ,crossing over and two homologous chromosomes from bivalent or tetrads . Bivalent having four chromatids with two centromers.
Prophase1 :
During prophase, the replicated pairs of chromosomes condense and compact themselves. The pairs of chromosomes that have been replicated.
1.Leptotene: chromosomes at this stage appear as long thread like structure ,which are closely interwoven. on these thread like chromosomes ,bead like structure called chromomeres are found all along the length of chromosomes.
2.Zygotene:zygotene is characterized by pairing of homologous chromosomes called synapsis. the pairing is brought about in a zipper like fashion and may start at centromere, at chromosomes end or at any other position . this paring takes place between homologous segments even if they are present in non homologous chromosomes.
3.Pachytene:chromosomes become shortened and coiled .the chromosomes appears as thickened thread like structure haploid in number.The pair of each chromosomes called bivalent.Each chromosomes in a bivalent at this stage has two chromatids ,as a result of which a bivalent really consists of four chromatids and called tetrad.At this stage crossing over or exchange of segments of chromatids is brought about.
4.Diplotene:at this stage chromosomes further shortening of chromosomes take place.Homologous chromosomes start separating from one another.This separation starts at centromeres, and towards ends.This type of separation is called terminalisation .Homologous chromosomes are held together at that point is called chiasmata.
5.Diakinesis:At this stage chromosomes are more contracted. Diplotene and Diakinesis only stages chromosomes are contract. Nucleolus may not be seen in this stage.
Metaphase1:spindle fibers start to appearing and bivalent become attached to spindle throught centromeres.
Anaphase1:the movement of chromosomes of bivalent from equatorial plate to poles. in mitotic division chromatids divides longitudinal but in case of meiosis anaphase sister chromatids are not separate they move towards poles.
chromosomes are haploid in number, chromosomes are not reductional and chromosomes are move towards poles is equational .so reductional and equational both are take place in Anaphase1.
Telophase1:Nuclear membrane formed around the chromosomes at poles .
After telophase cytokinesis takes place.
second Meiotic division :after meiosis1,meiosis2 take place without intervening interphase. Meiosis2 is essential to mitotic division.
in meiosis prophase chromosomes are already doubled with single centromeres, they are separate.
Sexually-reproducing organisms follow this pattern of alternation of generations.Thus, the life cycles alternate between Haploid and Diploid stages, and between the processes of meiosis and syngamy. It is because of the way in which genes recombine in meiosis and syngamy that we have the whole study of genetics.
Cell CyclesChromosomes and chromatin structureCell DivisionMitosisEukaryotic Gene and Gene Expression |
G1 Phase:
the first growth period of the cell cycle,, the cell grows in size and synthesizes mRNA and proteins that are required for DNA
synthesis.
S Phase:Synthesis phase ,replication takes place .S phase a number of events additional to chromosomee replication take place. Cell growth continues through S phase, as does the rate of synthesis of a number of proteins and enzymes that are involved in DNA synthesis. Once DNA replication is complete the cell contains twice its normal number of chromosomes and becomes ready to enter the phase called G2.
G2 Phase:
the second growth period of the cell cycle, following DNA replication and preceding prophase,It synthesizes the protein that constructs the microtubules, which are responsible for separating the chromosomes in the anaphase of meiosis.
In Meiosis two parts are present.
Meiosis 1 and meiosis 2
Interphase occure only before meiosis 1 not at meiosis 2.
Meiosis 1 :prophase1,Metaphase1,Anaphase1, and Telophase1.
Meiosis2:Prophase 2, Metaphase 2, Anaphase 2, Telophase 2.
Meiosis 2 is exactly simillar to mitosis.
DNA replication only takes one time before meiosis 1.But meiosis 2 cell division takes place but DNA replication does not take place.
In meiosis prophase is long period , Prophase1 is divide in to 1.Leptotene 2. Zygotene 3.Pachytene 4.Deplotene and 5.Diakinesis.
Synapsis ,crossing over and two homologous chromosomes from bivalent or tetrads . Bivalent having four chromatids with two centromers.
Prophase1 :
During prophase, the replicated pairs of chromosomes condense and compact themselves. The pairs of chromosomes that have been replicated.
1.Leptotene: chromosomes at this stage appear as long thread like structure ,which are closely interwoven. on these thread like chromosomes ,bead like structure called chromomeres are found all along the length of chromosomes.
2.Zygotene:zygotene is characterized by pairing of homologous chromosomes called synapsis. the pairing is brought about in a zipper like fashion and may start at centromere, at chromosomes end or at any other position . this paring takes place between homologous segments even if they are present in non homologous chromosomes.
3.Pachytene:chromosomes become shortened and coiled .the chromosomes appears as thickened thread like structure haploid in number.The pair of each chromosomes called bivalent.Each chromosomes in a bivalent at this stage has two chromatids ,as a result of which a bivalent really consists of four chromatids and called tetrad.At this stage crossing over or exchange of segments of chromatids is brought about.
4.Diplotene:at this stage chromosomes further shortening of chromosomes take place.Homologous chromosomes start separating from one another.This separation starts at centromeres, and towards ends.This type of separation is called terminalisation .Homologous chromosomes are held together at that point is called chiasmata.
5.Diakinesis:At this stage chromosomes are more contracted. Diplotene and Diakinesis only stages chromosomes are contract. Nucleolus may not be seen in this stage.
Metaphase1:spindle fibers start to appearing and bivalent become attached to spindle throught centromeres.
Anaphase1:the movement of chromosomes of bivalent from equatorial plate to poles. in mitotic division chromatids divides longitudinal but in case of meiosis anaphase sister chromatids are not separate they move towards poles.
chromosomes are haploid in number, chromosomes are not reductional and chromosomes are move towards poles is equational .so reductional and equational both are take place in Anaphase1.
Telophase1:Nuclear membrane formed around the chromosomes at poles .
After telophase cytokinesis takes place.
second Meiotic division :after meiosis1,meiosis2 take place without intervening interphase. Meiosis2 is essential to mitotic division.
in meiosis prophase chromosomes are already doubled with single centromeres, they are separate.