Can there be mitosis without DNA replication in ‘S’ phase?

(i) Rudolf Virchow ($1858$) gave cell lineage theory stating ‘Omnis cellula e cellula’ which means that cells arise from pre-existing cells.

(ii) All living organisms are composed of cells, and all cells arise from other cells.

(iii) The process by which cells multiply is called cell division.

(iv) Any sexually reproducing organism starts its life cycle from a diploid zygote.

(v) Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.

2. Cell Cycle:

(i) The stages through which a cell passes from one cell division to the next constitute the cell cycle.

(ii) The cell cycle can be divided in two major phases: interphase and M phase (Mitotic phase).

(iii) Interphase is also called ‘resting stage’ because no visible changes occur in the nucleus.

(iv) Interphase is typically much longer than M phase, it lasts more than $95\%$ of the duration of the cell cycle.

(v) $G1$ phase is the period when the cell grows and carries out normal metabolism.

(vi) S phase marks the phase of DNA replication, chromosome duplication, histone synthesis and duplication of centrioles.

(vii) $G2$ phase is the period of cytoplasmic growth.

(viii) The cell cycle is controlled by regulatory molecules called cyclins and cyclin-dependent kinases (CDKs).

(ix) Amitosis is the direct method of cell division.

3. Mitosis:

(i) Mitosis consists of four stages namely prophase, metaphase, anaphase and telophase.

(ii) Chromatin of the nucleus coils to form chromosomes during prophase.

(iii) Simultaneously, the centrioles move to the opposite poles.

(iv) The nuclear envelope and the nucleolus disappear and the spindle fibres start appearing.

(v) Metaphase is marked by the alignment of chromosomes at the equatorial plate.

(vi) During anaphase the centromeres divide and the chromatids start moving towards the two opposite poles.

(vii) Once the chromatids reach the two poles, the chromosomal elongation starts, nucleolus and the nuclear membrane reappear during telophase.

(viii) Nuclear division is then followed by the cytoplasmic division and is called cytokinesis.

(ix) Mitosis thus, is the equational division in which the chromosome number of the parent is conserved in the daughter cell.

4. Meiosis:

(i) In contrast to mitosis, meiosis occurs in the diploid cells, which are destined to form gametes.

(ii) Meiosis is called the reduction division since it reduces the chromosome number by half while making the gametes.

(iii) In sexual reproduction, when the two gametes fuse, the chromosome number is restored to the value in the parent.

(iv) Meiosis involves two divisions of nucleus and one division of chromosomes.

(v) Meiosis is divided into two phases - meiosis $\mathrm{I}$,and meiosis $\mathrm{II}$.

(vi) In the first meiotic division, the homologous chromosomes pair to form bivalents, and undergo crossing over.

(vii) Prophase $\mathrm{I}$ is the longest stage in the first division of meiosis and is divided into a number of sub stages.

(viii) The chronological sequence is leptotene, zygotene, pachytene, diplotene and diakinesis.

(ix) During metaphase $\mathrm{I}$, the bivalents are arranged on the spindle fibre forming the equatorial plate.

(x) Metaphase $\mathrm{I}$ is followed by anaphase I in which homologous chromosomes move to the opposite poles.

(xi) Each pole receives half the chromosome number of the parental cell.

(xii) In telophase-$\mathrm{I}$, the nuclear membrane and nucleolus reappear.

(xiii) Meiosis-$\mathrm{II}$ is an equational division like mitosis and consists of four stages prophase- $\mathrm{II}$, metaphase-$\mathrm{II}$, anaphase-$\mathrm{II}$ and telophase-$\mathrm{II}$.