Cell cycle and Cell Divisions MCQs for NEET — Botany Questions with Answers

During the S phase, DNA replication occurs, so the DNA content doubles from 2C to 4C, but the chromosome number remains 2n because sister chromatids are still joined. In M phase (mitosis), the sister chromatids separate and are distributed to daughter cells. Therefore, each daughter cell (and thus the cell at the end of M phase) will have 2n chromosomes and 2C DNA content, identical to the parent cell in G1. Mitosis is an equational division where the chromosome number in parent and progeny cells remains the same.

If the DNA content after the M phase (i.e., in a daughter cell in G1) is 2C, then during the S phase, DNA replication doubles the content. Therefore, at G2 phase, the DNA content will be 4C. The question provided in the NCERT text states: 'Also, what will be the DNA content of the cells at G1, after S and at G2, if the content after M phase is 2C?' This implies that the '2C' after M phase refers to the baseline content for a diploid cell in G1. Thus, G2 will have 4C.

During G1, the DNA content is C and chromosome number is 2n. After S phase, DNA replication occurs, doubling the DNA content to 2C, but the number of chromosomes remains 2n (as each chromosome now consists of two sister chromatids). Prophase follows the S and G2 phases, so an early prophase cell will have already completed DNA replication. Therefore, the DNA content will be 2C, and the chromosome number will still be 2n.

At the start of mitosis (after S phase), a diploid cell has 2n chromosomes, each with two chromatids, and 4C DNA content. During anaphase, the centromeres split, and sister chromatids separate, moving to opposite poles. Each separated chromatid is now considered an individual chromosome. Therefore, temporarily, the chromosome number doubles to 4n (each pole receives 2n chromosomes), while the total DNA content within the dividing cell is still 4C (before cytokinesis is complete).

Plants can show mitotic divisions in both haploid and diploid cells. If a haploid cell (n chromosomes, C DNA content in G1) undergoes mitosis, it first duplicates its DNA in S phase (n chromosomes, 2C DNA content). In M phase, these chromosomes align and then separate into daughter cells. Mitosis is an equational division, so the daughter cells will also be haploid with n chromosomes and C DNA content (after cytokinesis). The chromosome number and DNA content are conserved relative to the parent cell in G1.

The S phase (synthesis phase) is the period during which DNA replication occurs. If the cell had a diploid or 2n number of chromosomes at G1, even after S phase, the number of chromosomes remains the same, i.e., 2n. However, the DNA content doubles from 2C to 4C.

Some cells in adult animals do not appear to exhibit division and exit G1 phase to enter an inactive stage called the quiescent stage (G0) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so.

In animal cells, during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm. This is an important event for the formation of the spindle poles during mitosis.

In animals, mitotic cell division is only seen in diploid somatic cells, with a few exceptions like male honey bees. Against this, the plants can show mitotic divisions in both haploid and diploid cells. This is a key difference mentioned in the NCERT text.

Mitosis is also called as equational division because the number of chromosomes in the parent and progeny cells is the same. Therefore, if the parent cell has 2n chromosomes, the daughter cells will also have 2n chromosomes.