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Cell Cycle and Cell Division

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Class 10 Class 12
Describe cell cycle.

The cycle of cell, growth and division is called cell cycle. The cell cycle is divided into four phases.

1. First gap phase (G1 phase) or presynthetic phase.

2. Synthetic phase (S phase)

3. Second gap phase (G2 phase) or Mitotic phase (M phase)

4. Cell division phase or Mitotic phase (M phase).

The G1, S and G2 phases together constitute the Interphase. This phase takes about 95% of the duration of cell cycle. The mitotic or M phase only lasts for about an hour.

1. G1 phase. This stage starts from the birth of the daughter cell and ends upto the time when the cell begins to synthesis of DNA. It is the phase of initial growth in which RNA and proteins are synthesised. Its duration varies from 6 to 8 hours.

2. S phase. It is the period of synthesis of DNA. It lasts for 5-6 hours.

Fig. Cell cycle

3. Gphase. During this phase the centrioles, mitochondria, golgi bodies and other cytoplasmic organelles are doubled. Proteins are synthesised for the formation of astral rays and spindle and energy is stored for the cell division or M phase. The duration of this stage starts from the end of S phase upto the beginning of M phase. It lasts for 4-8 hours.

4. M phase or mitosis. It is the phase when the cell division takes place and this process gives rise to two daughter cells.  collectively form the cell cycle.

Name an important event that occurs during meiosis and not during mitosis.

Crossing over takes place during meiosis but not during mitosis.

Namethe five phases of of Prophase I.

The five phases of prophase 1 are
i. Leptotene
ii. Zygotene
iii. Pachytene
iv. Diplotene
v.  Diakinesis.

What are two types of cells on the basis of presence or absence of nucleus ?

On the basis of the presence and absence of nucleus the cells are divided into two types Eukaryotic and prokaryotic cells.

Name the process in which there is an exchange of genetic material between non-sister chromatids of homologous chromsomes during meiosis.

Crossing over.

What is polyteny ?

Polytene chromosomes are giant chromosomes which are formed due to repeated DNA replication without cell division. They are found in some insects.

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