(2)

Some cells in the adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit $G_1$ phase to enter an inactive stage called quiescent stage ($G_0$) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.

(1)

(1)

In S-phase (synthesis phase) of cell cycle, the chromosomes replicate. For this, their DNA molecules function as templates and form carbon copies. The DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C for diploid cells. As a result, duplicate sets of genes are formed. Along with replication of DNA, new chromatin fibres are formed which, however, remain attached in pairs and the number of chromosomes does not increase. As chromatin fibres are elongated chromosomes, each chromosome comes to have two chromatin threads or sister chromatids which remain attached at a common point called centromere."

(2)

If cell has stalled DNA replication fork, it implies that it has crossed C $G_1$or $G_1$ cyclin cell cycle check point and has entered S-phase of cell cycle, where it is preparing for chromosome replication. Afterwards it will enter $G_2$ phase and will soon approach second check point called mitotic cyclin (CM) which lies between $G_2$ and M-phase.

(1)

In diploid cells (somatic cells) during $G_1$ phase, DNA content is 2C and chromosome number is 2n whereas in haploid cells (gamete) during $G_1$ phase, DNA content is 1C and chromosome number is n. S phase is marked by replication of DNA and the amount of DNA per cell is doubled, i.e., it becomes 4C in somatic cells, whereas chromosome number remains same, i.e., 2n. Thus, a somatic cell which has just completed S phase, will have 4C DNA content but 2n chromosome number, while the gamete cell has 1C DNA content and n chromosome number.

(3)

A cell cycle is divided into $G_1$, S, $G_2$, and M phases. $G_1$ or first growth phase is followed by S phase or synthetic phase. DNA replication occurs in S phase and DNA amount doubles up i.e., a cell with 2C DNA in $G_1$ phase will now have 4C DNA. $G_2$ phase is second growth phase where DNA content remains 4C. M phase is the phase of division where DNA content either regains 2C level (mitosis) or becomes halved i.e., 1C (in meiosis). $G_0$ phase is the phase of differentiation where cell contains DNA as in the same amount as its parent cell and does not divide further.

(1)

In S-phase (synthesis phase) of cell cycle, the chromosomes replicate. For this, their DNA molecules function as templates and form carbon copies. The DNA content doubles, i.e., 1C to 2C for haploid cells and 2C to 4C for diploid cells. As a result, duplicate sets of genes are formed. Along with replication of DNA, new chromatin fibres are formed which, however, remain attached in pairs and the number of chromosomes does not increase. As chromatin fibres are elongated chromosomes, each chromosome comes to have two chromatin threads or sister chromatids which remain attached at a common point called centromere."