Overview

· Cell division produces new cells from a parent cell; in all living organisms this forms two daughter cells.
· Cytokinesis = division of the cytoplasm; nuclear division must happen first in eukaryotes so daughter cells are not anucleate.
· Mitosis maintains chromosome number and produces genetically identical daughter cells.
· Meiosis halves chromosome number and creates genetic variation for sexual reproduction.
· DNA replication must occur before both mitosis and meiosis; after replication, each chromosome consists of two sister chromatids joined until anaphase.

Cytokinesis

· Cytokinesis = splitting of the cytoplasm between daughter cells.
· In animal cells, a ring of actin and myosin pinches the cell membrane inward to divide the cell.
· In plant cells, vesicles assemble a new cell plate, membrane, and cell wall between daughter nuclei.
· Cytokinesis is usually equal, but can be unequal.
· Unequal cytokinesis examples: oogenesis in humans and budding in yeast.
· Each daughter cell must receive at least one mitochondrion and any organelle that can only arise by division of a pre-existing organelle.

Cell cycle and cell proliferation

· Cell proliferation supports growth, cell replacement, and tissue repair.
· Examples: growth in plant meristems, early animal embryos, routine replacement of skin cells, and wound healing.
· The cell cycle sequence is: G1 → S → G2 → mitosis → cytokinesis.
· Interphase = G1, S, G2 and is a metabolically active period.
· During interphase, cells grow and carry out biosynthesis of proteins, DNA, and other cell components.
· Numbers of mitochondria and chloroplasts can increase during interphase by growth and division of these organelles.
· Cyclins control progression through the cell cycle; a threshold concentration of a specific cyclin is needed to pass each checkpoint.

This diagram shows the cell cycle as G1, S, G2, mitosis, and cytokinesis. It is useful for remembering that interphase takes up most of the cycle and that mitosis + cytokinesis are only part of overall cell division. Source

Mitosis

· Mitosis produces two genetically identical daughter nuclei.
· It preserves the chromosome number and the genome.
· Shared features of mitosis and meiosis: chromosome condensation and chromosome movement by microtubules and microtubule motors.
· Histones help DNA condense by supercoiling so chromosomes become short and visible.
· Know the phases of mitosis in order: prophase → metaphase → anaphase → telophase.
· Prophase: chromosomes condense and become visible.
· Metaphase: chromosomes line up at the equator.
· Anaphase: sister chromatids separate and move to opposite poles.
· Telophase: chromosomes arrive at poles and nuclei reform; followed by cytokinesis.
· Exam link: the whole process gives two daughter cells with identical genetic information.

This diagram shows the sequence of mitosis and the chromosome movements that define each stage. It is ideal for revising how to identify prophase, metaphase, anaphase, and telophase from diagrams. Source

Identifying phases of mitosis

· You must be able to identify mitotic stages in diagrams, micrographs, and under a microscope.
· Focus on chromosome arrangement: condensed but scattered = prophase, lined up centrally = metaphase, separating chromatids = anaphase, two groups at opposite poles = telophase.
· Many cells in a sample may be in interphase because it is the longest part of the cycle.
· In plant cells, look for a cell plate during late telophase/cytokinesis rather than membrane pinching.

This bright-field micrograph shows onion root meristem cells in prophase, metaphase, anaphase, and telophase. It is helpful for practising how mitotic stages look in a real specimen rather than a simplified textbook diagram. Source

Meiosis

· Meiosis is a reduction division: one diploid nucleus produces four haploid nuclei through two divisions.
· Diploid (2n) = two sets of chromosomes; haploid (n) = one set.
· Meiosis is essential in a sexual life cycle so fertilization can restore the diploid number without chromosome doubling every generation.
· There are two rounds of segregation:
· Meiosis I separates homologous chromosomes.
· Meiosis II separates sister chromatids.
· Unlike mitosis, meiosis does not produce identical cells.
· Final outcome = four haploid cells that can develop into gametes or gamete-producing cells.

This image shows how one diploid cell becomes four haploid cells through two nuclear divisions. It is especially useful for visualizing where homologous chromosomes separate first and sister chromatids separate second. Source

Meiosis as a source of variation

· Meiosis generates genetic diversity in two key ways.
· Random orientation of bivalents at metaphase I causes independent assortment of maternal and paternal homologues.
· Crossing over during meiosis exchanges DNA between homologous chromosomes.
· Result: gametes are genetically different from each other and from the parent cell.
· Exam focus: explain why sexual reproduction increases variation and therefore supports evolution by natural selection.

Non-disjunction and Down syndrome

· Non-disjunction = failure of chromosomes to separate properly during meiosis.
· This produces gametes with an abnormal chromosome number.
· Down syndrome is the required example of an error in meiosis.
· It is caused by an extra copy of chromosome 21 in the zygote: trisomy 21.
· Be ready to link the error to failed separation in meiosis I or meiosis II.

This diagram shows non-disjunction during meiosis leading to a gamete with an extra chromosome and, after fertilization, trisomy 21. It is useful for linking a meiotic error to Down syndrome in exam explanations. Source

Cancer and loss of cell-cycle control (HL only)

· HL only — cell-cycle control and cancer
· Mutations in genes controlling the cell cycle can cause uncontrolled cell division.
· Proto-oncogenes can mutate into oncogenes, promoting excessive cell division.
· Mutations in tumour suppressor genes remove normal restraints on division.
· Tumours differ in rate of division, growth, and ability to invade tissue or metastasize.
· Benign tumours do not invade neighbouring tissue or metastasize.
· Malignant tumours are cancerous and can invade nearby tissue and spread.
· Primary tumour = original tumour site.
· Secondary tumour = tumour formed after metastasis to another part of the body.

Exam traps to avoid

· Do not say mitosis produces haploid cells — it maintains chromosome number.
· Do not confuse homologous chromosomes with sister chromatids.
· Do not forget that DNA replication happens before meiosis, but there are two divisions afterwards.
· Do not describe all tumours as cancer — only malignant tumours are cancerous.
· Do not mix up cytokinesis with mitosis: mitosis divides the nucleus, cytokinesis divides the cytoplasm.