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

According to the NCERT text, 'if the cell had diploid or 2n number of chromosomes at G1, even after S phase the number of chromosomes remains the same, i.e., 2n.' However, 'S phase marks the phase of DNA replication and chromosome duplication.' DNA replication doubles the amount of DNA. Therefore, if DNA content was 2C at G1, it becomes 4C at the end of S phase, while the chromosome number remains 2n.

The NCERT text states that for an onion root tip cell with 16 chromosomes, we need to determine the chromosome number at G1 and after M phase. Mitosis is an equational division where the chromosome number in the parent and progeny cells is the same. Therefore, if the cell has 16 chromosomes, it will have 16 chromosomes at G1 (before replication) and 16 chromosomes after M phase (in each daughter cell).

The NCERT 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?' After M phase, each daughter cell has 2C DNA content. G1 phase follows M phase, so the DNA content in G1 will be 2C. During S phase, DNA replication occurs, doubling the DNA content from 2C to 4C. G2 phase follows S phase, so the DNA content remains 4C. Therefore, the sequence is 2C, 4C, 4C.

In G1, a cell has 2n chromosomes and 2C DNA. After S phase, it becomes 2n chromosomes and 4C DNA. During anaphase, sister chromatids separate, briefly making the chromosome count effectively 4n (as each chromatid is now considered a chromosome) with 4C DNA content in the cell. However, in Telophase, two distinct nuclei are formed at opposite poles. Each forming nucleus receives 2n chromosomes and 2C DNA content. After cytokinesis, two daughter cells, each with 2n chromosomes and 2CDNA, are formed. The question asks about 'in each forming nucleus' during telophase, which would correspond to 2n chromosomes and 2C DNA.

During the S phase, DNA replication occurs, doubling the DNA content from 2C to 4C, but the chromosome number remains 2n because sister chromatids are still attached at the centromere. The G2 phase follows the S phase, so the chromosome number remains 2n, and the DNA content remains 4C, as no further DNA synthesis has occurred. 'During the G2 phase, proteins are synthesised in preparation for mitosis while cell growth continues.'

In G1 phase, the cell has 2n=10 chromosomes, and each chromosome has one chromatid. During S phase, DNA replication occurs, so each of the 10 chromosomes now consists of two sister chromatids. The chromosome number functionally remains 10 (as they are still counted by centromeres). Therefore, in metaphase, when chromosomes are condensed and aligned at the equator, there will be 10 chromosomes, each with two chromatids, totaling 20 chromatids.

The NCERT text states, 'Since the number of chromosomes in the parent and progeny cells is the same, it is also called as equational division.' This statement refers to mitosis. Mitosis involves 'segregation of duplicated chromosomes into daughter nuclei (karyokinesis)' and results in daughter cells with the same chromosome number as the parent cell.

The NCERT explicitly states, 'In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes. It is called the reduction division since it reduces the chromosome number by half while making the gametes.' This directly answers the question.

The NCERT mentions that 'plants can show mitotic divisions in both haploid and diploid cells.' If a haploid cell (n chromosomes, C DNA in G1) undergoes mitosis, it first undergoes S phase increasing DNA content to 2C while chromosome number remains n. Then, during mitosis (equational division), the sister chromatids separate, and the cell divides, resulting in daughter cells that are also haploid (n chromosomes) and have the original DNA content (C). Therefore, the resulting daughter cells will have n chromosomes and C DNA content.

In G1, the cell has 2n chromosomes and 2C DNA. After S phase and until metaphase, the cell maintains 2n chromosomes (each with two chromatids) and 4C DNA. During anaphase, the centromeres split, and sister chromatids separate, moving to opposite poles. At this point, each chromatid is considered an individual chromosome. Therefore, temporarily, the chromosome number doubles to 4n, and the DNA content remains 4C in the entire cell during anaphase, before cytokinesis fully separates them.