Question

Describe the different phases of cell cycle with diagrams in detail. (5 MARKS QUESTION).

Answer 1

The cell cycle is divided into two basic  phases:

1. Interphase

2.M Phase (Mitosis phase)

The interphase is divided into three further phases:

G1 phase (Gap 1) : G1 phase corresponds to the interval between mitosis and initiation  of DNA replication. During G1 phase the cell is metabolically active and  continuously grows but does not replicate its DNA.

S phase (Synthesis) : S or synthesis phase  marks the period during which DNA synthesis or replication takes place.  During this time the amount of DNA per cell doubles. In animal cells, during the S phase, DNA replication begins in the  nucleus, and the centriole duplicates in the cytoplasm.

G2 phase (Gap 2): During the G2  phase, proteins are synthesised in preparation for mitosis while cell growth  continues.

Karyokinesis involves  following four stages:

1. Prophase:

Chromosomal material condenses to form compact mitotic

chromosomes. Chromosomes are seen to be composed of two

chattached together at the centromere.

Centrosome which had undergone duplication during interphase,

begins to move towards opposite poles of the cell. Each centrosome

radiates out microtubules called asters. The two asters together

with spindle fibres forms mitotic apparatus.

2. Metaphase :

Spindle fibres attach to kinetochores of

chromosomes.

Chromosomes are moved to spindle equator and get

aligned along metaphase plate through spindle fibres

to both poles.

3. Anaphase :

Centromeres split and chromatids separate.

Chromatids move to opposite poles.

4. Telophase:

Chromosomes cluster at opposite spindle poles and their

identity is lost as discrete elements.

Nuclear envelope develops around the chromosome

clusters at each pole forming two daughter nuclei.

Nucleolus, golgi complex and ER reform.

and the final stage:

cytokinesis: Mitosis accomplishes not only the segregation of duplicated

chromosomes into daughter nuclei (karyokinesis), but the

cell itself is divided into two daughter cells by the separation

of cytoplasm called cytokinesis at the end of which cell

division gets completed.