Cell structure and cell division

The cell and how it divides

The cell is the basic unit of life. Prokaryotic cells (bacteria) lack a true nucleus; eukaryotic cells (plants, animals, fungi) have a nucleus with a membrane. A plant cell has a cell wall made mainly of cellulose, a cell membrane, cytoplasm, nucleus, mitochondria (respiration, "powerhouse"), chloroplasts (photosynthesis), ribosomes (protein synthesis), endoplasmic reticulum, Golgi bodies and a large central vacuole. Animal cells have no cell wall or chloroplasts.

The cell cycle has interphase (G₁, S phase where DNA is copied, G₂) and division.

Mitosis (equational division) occurs in somatic cells such as root and shoot tips. Stages: prophase (chromosomes condense), metaphase (chromosomes line up at the equator), anaphase (sister chromatids move to opposite poles) and telophase (new nuclei form), followed by cytokinesis. One cell gives two daughter cells with the same chromosome number as the parent (2n → 2n). It allows growth, repair and vegetative propagation.

Meiosis (reductional division) occurs in reproductive cells to form gametes. It has two divisions (meiosis I and II). In prophase I, homologous chromosomes pair (synapsis) and exchange segments by crossing over. One cell gives four haploid cells (2n → n). Meiosis keeps the chromosome number constant across generations and creates variation.

Example: rice has 2n = 24. A root-tip cell after mitosis has 24 chromosomes; a pollen grain nucleus (gamete) has 12.

Key points

• Plant cell wall: cellulose; chloroplast and large vacuole present.
• DNA replication: S phase of interphase.
• Mitosis: 2 identical diploid cells; meiosis: 4 haploid, genetically different cells.
• Crossing over: prophase I of meiosis.
• Rice 2n = 24, maize 2n = 20, bread wheat 2n = 42.

Exam tips

Questions ask where each division happens, the number and ploidy of daughter cells, and stage-wise events. For chromosome-number problems, halve 2n for gametes.

Common mistakes

• Thinking chromosome number halves in anaphase of mitosis; it does not.
• Placing crossing over in meiosis II; it occurs in prophase I.
• Forgetting that DNA is copied before division, in S phase.