Question

In the eukaryotic cell cycle, a cell fusion experiment shows that?
(a) When S- phase cell is fused with a ${G }_{ 1 }$ phase cell, ${G }_{ 1 }$ phase cell is stimulated to synthesize the DNA
(b) When an S- phase cell is fused with a ${ G }_{ 2 }$ phase cell, DNA synthesis is induced in the ${ G }_{ 2 }$ phase.
(c) When a ${G }_{ 1 }$ phase cell is fused with a ${ G }_{ 2 }$ phase cell, DNA synthesis is induced in both ${G }_{ 1 }$ and ${ G }_{ 2 }$ phase cell
(d) When a ${G }_{ 1 }$ phase cell is fused with an M-phase cell both ${G }_{ 1 }$ and M phase cells are stimulated to synthesize DNA.

Answer 1

Hint: Fusion between the cells in the phase where it is metabolically active and the cell in the phase where DNA replication occurs and no increase in chromosome number.

Complete answer:
The cell cycle in a eukaryotic cell has two phases: Interphase and M-phase. The interface is further divided into three phases namely ${G }_{ 1 }$phase, S phase, ${ G }_{ 2 }$phase. The period in which the DNA synthesis or replication takes place is the S phase or Synthesis phase. When a cell in the S phase fuses with the cell in ${G }_{ 1 }$phase, synthesis or replication of DNA occurs in the cell in the ${G }_{ 1 }$phase.

Additional Information: - Cell division in eukaryotes i.e., human beings takes place approximately every 24 hours.
- It consists of two phases namely Interphase and M-phase or Mitotic phase.
- The phase between two successive M Phases is called interphase and the M phase is the phase where actual cell division occurs. It is also called apparent division.
- It is divided into three phases namely the ${G }_{ 1 }$phase, S phase, ${ G }_{ 2 }$ phase.
- ${G }_{ 1 }$phase or Gap1 phase is the interval between Mitosis and initiation of DNA replication. In this phase, the cell is metabolically active and it grows continuously but DNA replication is absent.
- S phase or synthesis phase is the second phase of interphase where replication or synthesis of DNA occurs. The amount of DNA per cell doubles in this phase. When the cell in S-phase is fused with the cell ${G }_{ 1 }$phase, DNA replication occurs in the cell in the ${G }_{ 1 }$phase.
So, the correct answer is “when an S-phase cell is fused with a ${G }_{ 1 }$phase cell, ${G }_{ 1 }$phase is stimulated to synthesize DNA”.

Note: - In the S-phase only replication of DNA takes place but the number of chromosomes remains the same.
- Some cells like a nerve cell, cardiac cells do not replicate or divide further. They exit the ${G }_{ 1 }$phase and enter into the ${G }_{ 0 }$ phase of the cell cycle.