Question

(a) What is the sequence of different phases in cell cycle ?
(b) When does the synthesis of DNA takes place ?
(c) Describe briefly the various stages of cell cycle during the interphase preceding mitosis.

Answer 1

(a) The cell cycle is a four-stage process in which the cell increases in size (gap 1, or G1, stage), copies its DNA (synthesis, or S, stage), prepares to divide (gap 2, or G2, stage), and divides (mitosis, or M, stage).

(b) During Mitosis, DNA is replicated during the S phase (Synthesis phase) of Interphase.

(c) There are three stages of cellular interphase, with each phase ending when a cellular checkpoint checks the accuracy of the stage's completion before proceeding to the next. The stages of interphase are:

• G1 (Gap 1), in which the cell grows and functions normally. During this time, a high amount of protein synthesis occurs and the cell grows (to about double its original size) – more organelles are produced and the volume of the cytoplasm increases. If the cell is not to divide again, it will enter G0.[3]

• Synthesis (S), in which the cell synthesizes its DNA and chromosome number is doubled (via semi-conservative replication).

• G2 (Gap 2), in which the cell resumes its growth in preparation for division. The mitochondria divide and the cell continues to grow until mitosis begins. In plants, chloroplasts also divide during G2.

• In addition, some cells that do not divide often or ever, enter a stage called G0 (Gap zero), which is either a stage separate from interphase or an extended G1.