NCERT illustrates the eukaryotic cell cycle using human cells in culture, which divide approximately once every 24 hours.
The average cell cycle span for a typical mammalian cell is about 24 hours.
During M phase, duplicated chromosomes first segregate into daughter nuclei by karyokinesis. The cell then divides its cytoplasm into two daughter cells by cytokinesis.
Karyokinesis is division of the nucleus; cytokinesis is division of the cytoplasm.
In G1 phase, the cell is metabolically active, grows and does not replicate DNA. In S phase, DNA synthesis occurs and DNA content doubles, though chromosome number remains the same; in animal cells the centriole also duplicates. In G2 phase, cell growth continues and proteins required for mitosis are synthesised.
Interphase consists of G1, S and G2 phases and prepares the cell for division.
Cells in G0 remain metabolically active but do not proliferate unless required by the organism. Examples include many adult animal cells that divide only occasionally or not at all.
G0 or quiescent phase is an inactive stage entered by cells that stop dividing after G1.
Sister chromatids separate and are distributed equally to daughter nuclei. Thus the chromosome number and genetic complement are conserved in daughter cells.
Mitosis is called equational division because daughter cells receive the same chromosome number as the parent cell.
In metaphase, chromosomes align at the spindle equator. In anaphase of mitosis or anaphase II, centromeres split and chromatids separate. Synapsis occurs in zygotene. Crossing over occurs in pachytene between non-sister chromatids of homologous chromosomes.
(i) Metaphase. (ii) Anaphase. (iii) Zygotene of prophase I. (iv) Pachytene of prophase I.
In animal cells, a furrow appears in the plasma membrane and deepens inward until the cytoplasm separates. In plant cells, wall formation begins at the centre with a cell plate that grows outward and joins the existing lateral walls, forming the middle lamella and new cell walls.
Animal cells divide by a cleavage furrow; plant cells divide by cell-plate formation.
In male gamete formation, meiosis produces four functional spermatids of approximately equal size. In female gamete formation, meiosis produces one large ovum and small polar bodies, so the products are unequal.
Equal-sized meiotic products occur in spermatogenesis; unequal-sized products occur in oogenesis.
Mitotic anaphase involves splitting of centromeres and movement of daughter chromatids to opposite poles. In anaphase I, reductional segregation occurs: homologous chromosomes move to opposite poles, but centromeres do not split and sister chromatids remain associated.
In mitotic anaphase, sister chromatids separate; in anaphase I of meiosis, homologous chromosomes separate while sister chromatids remain joined.
Mitosis occurs mainly in somatic cells, has one nuclear division, no synapsis or crossing over, conserves chromosome number and produces two genetically identical daughter cells. Meiosis occurs in germ/spore mother cells, has two divisions after one DNA replication, includes synapsis and crossing over in prophase I, halves chromosome number and produces four genetically different haploid cells.
Mitosis is equational and produces two identical daughter cells; meiosis is reductional/equational in two divisions and produces four haploid, genetically varied cells.
Meiosis produces haploid gametes or spores by reducing the chromosome number by half. Fertilisation then restores diploidy. Crossing over and independent segregation generate variation, which is important for evolution.
Meiosis maintains species chromosome number across generations and creates genetic variation.
DNA replication produces sister chromatids so that each daughter cell receives a complete genome. Without S-phase replication, chromosomes could not be equally distributed as complete copies to daughter cells.
No, normal mitosis cannot occur without DNA replication in S phase.
During G1: chromosomes = 2N, DNA = 2C. During S phase: chromosome number remains 2N, DNA doubles from 2C to 4C. During G2 and early M phase: chromosomes = 2N, DNA = 4C. At anaphase, sister chromatids separate and may be counted as daughter chromosomes within one cell. After cytokinesis, each daughter cell has 2N chromosomes and 2C DNA.
In a diploid mitotic cell, chromosome number remains 2N through G1, S, G2 and metaphase, while DNA content changes from 2C in G1 to 4C after S and remains 4C until division restores 2C in each daughter cell.