Question

1) what are the main events of mitosis ?

2) what are the main events of meiosis?

3) whay are the main events of the cell cycle ?

Answer 1

1),Mitosis is the process the partitions newly replicated chromosomes equally into two daughter cells.

Main events of mitosis are follows:

• Prophase MetaphaseAnaphaseTelophase- then cytokinesis.  

Prophase

• The coiling and condensation of chromatin filaments to form short and thick chromosomes.
• polarisation or outward movement of chromosomes towards the pheriphery of the nucleus.
• the longitudinal splitting of each chromosome and the formation of the kinetochore.
• the cytoplasmic events include the formation of the mitotic apparatus.
• Prometaphase- marked by dissolution and disappearance of nucleolous and disintegration of nuclear envelope, and the condensed chromosomes moves randomly makes disorder.

Metaphase-

• the centripetal metaphasic chromosomes movement is called metakinesis.
• the chromosomes get attached to the chromosomal spindle fibers with the help of kinetochore  and arranged in a single plane or row in the equator of the cell - equitorial plate.
• The sister centromeres of each chromosomes attche to the spindle from opposite poles.

Anaphase

• the sister chromatids separate from each other and are pulled towards opposite ends of the cells.

Telophase

• the cell finishes the nuclear division and re-establishes its normal structures and cytokinesis takes place.

2), Meiosis is the reduction division where the diploid number of chromosomes are halfed to haploid number of chromosomes.

Meiosis is divided into two parts: meiosis 1 and meiosis 2. As a result, there are four daughter cells rather than two produced at the end of the mitotic process.

Meiosis 1

Prophase 1 of meiosis 1 is divided into five sub stages:

• Leptotene: coiling and condensation of chromatin filaments
• Zygotene: pairing (synapsis) of homologous chromosomes and the formation of bivalents
• Pachytene: condensation ,crossing over and recombinaton of bivalents.
• Diplonema: separation of homologous and segregation of the sister chromatids of each homologue.
• Diakinesis: tight coiling ,condensation and contraction of chromosomes.

Metaphase 1: the homologous pairs-line up at the metaphase plate for separation.

Anaphase 1: the homologous chromosomes are pulled apart and move apart to opposite ends of the cell. and the sister chromatids of each chromosomes never separate apart.

Telophase1: the chromosomes arrive at opposite poles of the cell and uncoil partially revert to their interphasic condition. In some animals ,nuclear evelope may be formed.

Meiosis 2.

Prophase 2: brief stage, appearance of mitotic spindle and if the nuclear membrane appeares in the telophase 1 then it degenerates and disappeares.uncoiling of chromosomes .

Metaphase 2: chromosomes move to the centre of the cell, attaches to spindle fibres and forms equitorial plate. In vertebrates oocytes meiosis gets suspended at this stage and gets resumed only after fertilization.

Anaphase2: sister centromeres separate out ie, the sister chromatids segregate from each other and migrate to opposite poles by spindle activity. now each pole consist of the haploid set of single stranded chromosomes.

Telophase 2: last stage of meiosis , nuclear envelop appeares around the polar group of chromasomes and nucleolus appears inside it. soon chromosomes undergoes uncoiling and decondensation and form chromosome reticulum. Spindle fibers disappears, followed by cytokinesis.

3),The main events of the cell cycle are as follows:

Interphase

• G1 phase: called first gap phase, the cell grows larger , copies its organelles , builds molecular blocks.
• S phase:the cell synthesis a complete copy of the DNA takea place.
• G2 phase: calles second gap phase, the cell grows more, make proteins ,organelles,and prepare for mitosis(M phase).

Mitosis phase

• prophase,metaphase, anaphase, and telophase.

cytokinesis: division of the cell cytoplasm .