Question

Differentiate between S phase and G2 phase.

Answer 1

Hint: A complete cell cycle consists of four stages; and these are - G1, S, G2 and M phases. Amongst these four stages, G1, S & G2 phases constitute the interphase after the completion of which the cell enters the mitotic phase i.e., the division phase.

Complete answer:
Generally, every cell has two major periods in its life cycle – the interphase and the division phase. The synthetic activities taking place during interphase mainly include the synthesis of DNA (replication), RNA (transcription) and proteins (translation). The synthesis of DNA, RNA and proteins continues throughout the interphase i.e. throughout the G1 (Gap 1) and G2 (Gap 2) phases as well as the S (Synthesis) phase. The main differences between S phase and G2 phase are as follows:

Sl.No	S phase	G2 phase
1.	S phase or synthesis phase is the second sub-phase of interphase.	G2 or Gap2 phase is the third and last sub-phase of interphase that directly leads to divisional phase.
2.	It occurs in between the G1 (Gap 1) phase and G2 (Gap 2) phase.	It occurs in between the S phase and divisional phase or mitosis.
3.	It is the phase of the cell cycle in which DNA replication takes place.	It is the phase of rapid cell growth and protein synthesis.
4.	For successful cell division, perfect duplication of genome occurring in the S phase is necessary, that is why the processes taking place during this phase are strictly regulated.	In this phase the cell prepares itself for mitosis. Repairing of DNA double strand breaks (arisen after replication in S phase), takes place in G2 phase.
5.	Synthesis of Histone protein takes place in this phase.	The G2 phase acts as a checkpoint for DNA damage.
6.	Cyclin B1 levels remain suppressed throughout the S phase.	Transcription of mRNA for Cyclin B1 starts with the beginning of the G2 phase and thus, increased levels of Cyclin B1 are found in this phase. This ultimately leads to the formation of a fully active form of Cyclin B1-CDK1 complex (MPF or Maturation Promoting Factor) that is necessary for the cell to enter into the mitotic phase.

Note: In G2 phase, the cell makes sure that all the structures needed for the cell division are ready and it can enter into the divisional phase. Along with other things, assembly of microtubules is also an important process that occurs during the G2 phase. These microtubules form spindle fibres during cell division.