NEET Practice Questions (MCQs) with Answers & Solutions

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.

Mitosis is an equational division in which the chromosome number of the parent is conserved in the daughter cell. The genome is duplicated during S phase, and then precisely distributed to the two daughter cells, ensuring they contain intact and identical genomes.

The NCERT text defines health as 'a state of complete physical, mental and social well-being.' While economic prosperity can be a result of health ('When people are healthy, they are more efficient at work. This increases productivity and brings economic prosperity.'), it is not part of the definition of health itself.

The NCERT text states, 'The discovery of blood circulation by William Harvey using experimental method and the demonstration of normal body temperature in persons with blackbile using thermometer disproved the 'good humor' hypothesis of health.' This contrasts with early Greek and Indian Ayurveda systems that asserted a balance of certain humors determined health.