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Cell Cycle and Cell Division

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Class 10 Class 12
Describe Meiosis II or Homotypic division.

Meiosis II is very similar to mitosis. It has the following stages which are -:

1. Prophase II. It occurs immediately after cytokinesis of meiosis I. The nuclear membrane disaapars and chromosomes become compact.

2. Metaphase II. The chromosomes arrange themselves on the equator. Each chromosome is attached to the spindle fibres of opposite poles by its centromeres. The spindle microtubules from the opposite poles attach to the kinetochores of sister chromatids.

3. Anaphase II. The centromere of each chromosome divides into two sister chromatids. Thus two chromatids of each chromosome move towards opposite poles. 

4. Telophase II. The group of young chromosomes at each pole arranges itself into a daughter nucleus. The nuclear membrane is formed. Nucleoli appear and spindles disappear. takes place resulting in the formation of four daughter cells. 



What is the significance of meiosis?

Meiosis is the reduction division the reduction in the amount of genetic material.

Significance of meiosis

1. Meiosis maintains the chromosome number from generation to generation. It reduces the chromosome number to half so that the process of fertilisation can restore the original number in the zygote.

2. It has an evolutionary importance as it allows variations to occur. The process of cross-over and the random distribution of homologous chromosomes between daughter cells result in variation in the genetic make-up of the cells. Variations play an important role in evolution.

3. Chromosomal mutations are brought about by the introduction of certain inheritable changes which may be advantageous for an individual.


List the main differences between meiosis and mitosis.

Mitosis Meiosis
1.  Equational division. Chromosome number remains the same in the daughter cells.  

1. Reduction division. Chromosome number is resuced to half in the daughter cell.

2. Occurs in somatic cells

2. Occurs in germ cells. 

3. Occurs in both sexually and asexually reproducing organisms.

3. Occurs only in sexually reproducing organisms.
4. No crossing over takes place.

4. Crossing over takes place during pachytene stage.

5. Prohase is simple and not divided into sub stages. 5. Prohase I of meiosis I is complex and has 5 stages.

What is the average cell cycle span for a mammalian cell ?

24 hours is an average duration. The cell division phase lasts only for one hour while interphase lasts for 95% of the duration of the cycle. 

How does cytokinesis in plant cells differ from that in animal cells?

Cytokinesis in animal cell

Cytokinesis in plant cell

1. It occurs by furrow method.

2. The constriction in plasmalemma is formed by contraction of microfilaments.

3.Cleavage starts at the periphery and then moves inward, dividing the cell into two parts.

1. It occurs by cell plate method.

2. No constriction is formed.

3. Cell plate formation starts at the centre of the cell and grows outward, toward the lateral walls.



Describe the events taking place during interphase. 

Interphase is the resting stage of the cell cycle. It covers 95% of the total duration of a cell cycle. It involves a series of changes that prepare a cell for division. It is the period during which the cell experiences growth and DNA replication in an orderly manner. Interphase is divided into three phases.

(i) Gphase

(ii) S phase

(iii) Gphase

Gphase - It is the interval between mitosis and initiation of DNA replication. It is the stage during which the cell grows and prepares its DNA for replication. In this phase, the cell is metabolically active but there is no replication of the DNA.

S phase - It is known as the synthesis phase as it is the stage during which DNA synthesis occurs. In this phase, the amount of DNA (per cell) doubles, but the chromosome number remains the same.

Gphase - In this phase, the cell continues to grow and prepares itself for the division. The proteins and RNA required for mitosis are synthesised during this stage.