Question

Draw a labelled diagram and write an explanation.

With the help of a suitable diagram, describe the cell cycle.

Answer 1

A series of events occurring in the life of a cell is called the cell cycle. Interphase and M-phase are the two phases of the cell cycle.

1. Interphase: It is the stage between two successive cell divisions. It is the longest phase of a cell cycle during which the cell is highly active and prepares itself for cell division.
   The interphase is subdivided into three sub-phases as G₁-phase, S-phase and G₂-phase.
2. M-phase: 'M' stands for mitosis or meiosis. M-phase involves karyokinesis and cytokinesis. Karyokinesis is the division of the nucleus into two daughter nuclei, whereas cytokinesis is the division of cytoplasm resulting in two daughter cells.

1. G₁ – phase (First gap period/First Gap Phase):
   It begins immediately after cell division. RNA (mRNA, rRNA and t-RNA) synthesis, protein synthesis and synthesis of membranes take place during this phase.
2. S – phase (Synthesis phase):
   In this phase DNA is synthesized (replicated), so that amount of DNA per cell doubles. Synthesis of histone proteins takes place in this phase.
3. G₂ – phase (Second growth phase/Second Gap Phase):
   Metabolic activities essential for cell division occur during this phase. Various proteins which are necessary for cell division are also synthesized in this phase. Apart from this, RNA synthesis also occurs during this phase. In animal cells, a daughter pair of centrioles appears near the pre-existing pair.