Question

a. Compare the process of mitosis with the process of meiosis. Describe major cell cycle events. (points: 4) What is significance of these two cell division types.

Answer 1

◆ comparision of mitosis and meiosis :-

• refer table :

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◆ major cell events are :-

• cells ( eukaryotic )divide once in every 24 hours approximately , illustrated by human cells in culture.
• Time for division of cells in a cell cycle varied from cell type to cell type and organism to organism.

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◆ cell cycle involves 2 stages :-

• interphase
• Division phase / M phase

● interphase :-

• it is phase between the two successive M phase .
• Cell grow in size to prepare for next division.
• Most active phase
• Metabolism of cell increase in interphase
• Cells not visible under microscope , so called interphase is resting phase but now it is known as most active phase .

​​​​​​● interphase divided into 3 substage :-

• G1 - PHASE
• S - PHASE
• G2 - PHASE

​​​​​​​● G1 - PHASE

• It is also known as pre DNA synthesis phase or 1st gap phase.
• G1 phase is the interval between mitosis and the initation of DNA replication.
• In that phase cell continuously grows and metabolically active.
• Cells rapidly synthesize different type of RNA and protein amd cell organelles increase .
• Cells grow maximum in this phase.

● S - PHASE

• also known as DNA synthesis phase .
• Synthesis of histone proteins and replication of nuclear DNA occours in that phase
• In this phase amount of DNA per cell doubles
• No increase in chromosome number
• Centriole replicate in late S phase and duplicates in the cytoplasm

● G2 - PHASE

• also known as post DNA synthesis phase or pre mitosis phase or 2nd gap phase
• Prepration for division phase occours .
• Tublin protein which is required for spindle fibres are required for division phase are synthesized in G2 phase.

G0 phase :

• some cells in adult animals do not exhibit division e.g heart cells and some divide rarely to replace dead cells in G0 phase .
• Cells are metabolically active in that phase but proliferate only when needed.

◆ division phase :.

• also known as M - phase
• It is the shortest phase in according to time.
• It is the phase where actual cell division occour.
• The division phase start with nuclear division (karyokinesis ) and ends with division of cytoplasm ( cytokinesis).

◆ M phase is divided into 4 stages in meiosis and mitosis :-

• prophase
• Metaphase
• Anaphase
• Telophase

for description of phases refer the table given above

◆ significance of mitosis :-

• Development of organism occour by mitosis .
• Organism start it's life cycle as single i.e zygote and with repeated mitosis in zygote leads to formation of whole body.
• Growth of multicellular organisms is due to mitosis .
• It is essential for cell to divide to restore nucleo- cytoplasmic ratio.
• Main significance of mitosis is cell repair

​​​​​​​◆ significance of meiosis:-

• chromosome number is reduced by half in meiosis.
• Genetic variability is increased from one generation to next generation in the organisms .

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