Cell Cycle and Cell Division — Class 11 Biology

This chapter explains how cells reproduce through the cell cycle and the two kinds of division, mitosis and meiosis. The cell cycle is the orderly sequence by which a cell duplicates its genome, synthesises other constituents and divides; in a typical human cell it lasts about 24 hours. It has two basic phases: interphase, which occupies over 95 per cent of the cycle and is subdivided into G1 (growth without DNA replication), S (DNA replication, when DNA content doubles though chromosome number stays the same) and G2 (preparation for mitosis), and the M phase (mitosis). Cells that stop dividing enter a quiescent G0 stage. Mitosis, the equational division, keeps the chromosome number constant and proceeds through prophase (chromosome condensation, spindle formation), metaphase (chromosomes align on the metaphase plate), anaphase (centromeres split and chromatids move to poles) and telophase (nuclei reform), followed by cytokinesis (a furrow in animal cells, a cell plate in plant cells). Meiosis, the reductional division, occurs in diploid cells that form gametes; it has two divisions but one round of DNA replication. Meiosis I includes a long prophase I (leptotene, zygotene, pachytene with crossing over, diplotene and diakinesis), and meiosis II resembles mitosis, yielding four haploid cells. Mitosis enables growth and repair; meiosis conserves chromosome number across generations and generates variation.