D2.1 Cell and nuclear division

In every living organism, cell division generates new cells when one parent cell divides to produce two daughter cells.

Producing daughter cells allows an organism to grow, replace worn cells, repair damaged tissue or reproduce asexually. The genetic material must be distributed before the cytoplasm separates so both daughters can function.

A skin cell can divide into two daughter cells that replace cells lost from the surface, while a single-celled organism can divide to produce two organisms.

The parent cell is sometimes called a mother cell, but this does not imply sex or fertilization. Cell division means generation of daughter cells, not growth of one cell alone.

Cytokinesis splits the cytoplasm of a parent cell between its daughter cells, but animal and plant cells achieve the split differently.

An animal cell pinches from its outer edge toward the centre, whereas a plant cell builds the separating plate from the centre outward.

Cytokinesis divides cytoplasm; mitosis or meiosis divides the nucleus. The events can overlap in time but are not the same process.

Cytokinesis is usually equal, but unequal cytokinesis can give daughter cells very different amounts of cytoplasm.

Whatever their final sizes, both daughters must receive at least one mitochondrion and any other organelle that can arise only by growth and division of a pre-existing organelle.

Unequal cytoplasm does not mean unequal nuclear DNA: chromosomes can still be segregated correctly before asymmetric cytokinesis.

Eukaryotic cells use mitosis to maintain chromosome number and genome, whereas meiosis halves chromosome number and generates genetic diversity.

Nuclear division must occur before cell division so each daughter receives a nucleus rather than becoming an anucleate cell.

Meiosis is not simply mitosis twice: homologous chromosomes pair and separate in its first division, creating the reduction in chromosome number.

DNA replication occurs before nuclear division so each daughter nucleus can receive a complete chromosome set.

During S phase, each chromosome becomes two sister chromatids joined at a centromere. Division then separates chromatids or homologues according to the process.

Trace: replication; duplicated chromosome; spindle attachment; chromosome separation; daughter nuclei.

If a cell entered mitosis without replicating DNA, one daughter could receive too little genetic material.

Replication doubles DNA amount, not chromosome number in the usual chromosome-counting convention.

Mitosis and meiosis both condense chromosomes and move them accurately between new nuclei.

Histones organize DNA into nucleosomes, and further supercoiling condenses the long chromatin fibres into compact chromosomes that can be moved without tangling.

Spindle microtubules attach to chromosomes, and microtubule motors plus microtubule shortening or growth generate directed movement. Alignment and separation differ between divisions, but the same general machinery organizes chromosome distribution.

In both mitosis and meiosis II, sister chromatids move toward opposite poles; in meiosis I, homologous chromosomes move apart while sister chromatids remain together.

Shared condensation and movement mechanisms do not make the outcomes identical: the chromosome partners separated and the number of divisions differ.

Mitosis proceeds through prophase, metaphase, anaphase and telophase, each solving a different chromosome-distribution problem.

Chromosomes condense and attach to spindle fibers, align at the equator, separate sister chromatids and re-form nuclei at opposite poles. Cytokinesis follows.

Use the sequence: condense; align; separate chromatids; rebuild nuclei.

A metaphase cell has chromosomes aligned at the equator; anaphase begins when sister chromatids move apart.

Interphase is not a mitosis stage, although it prepares the cell by growing and replicating DNA.

Identify a mitosis phase from the position and appearance of chromosomes, using several features rather than cell shape alone.

Two groups of V-shaped chromatids moving away from the equator indicate anaphase, whether seen in a diagram, a prepared root-tip cell or a micrograph.

A dark stain or rounded cell outline is not enough to identify a phase. Confirm chromosome condensation, alignment or separation and nuclear-envelope state.

Meiosis is a reduction division: one diploid nucleus undergoes two nuclear divisions after one DNA replication to produce four haploid nuclei.

Diploid nuclei contain two homologous chromosome sets; haploid nuclei contain one. Homologous chromosomes separate in meiosis I, halving the number of sets, and sister chromatids separate in meiosis II without another round of replication.

Diploid nucleus → DNA replication → homologous pairs segregate in meiosis I → two haploid nuclei with duplicated chromosomes → chromatids segregate in meiosis II → four haploid nuclei.

In a sexual life cycle, haploid gametes produced by meiosis fuse at fertilization, restoring the diploid chromosome number instead of doubling it in every generation.

Chromosome number is reduced in meiosis I, even though each chromosome still has two chromatids until meiosis II.

Nondisjunction is failure of homologous chromosomes or sister chromatids to separate, producing gametes with abnormal chromosome numbers.

If an extra chromosome enters a gamete, fertilization can create a zygote with three copies of one chromosome. The phenotype depends on chromosome and gene dosage.

Trace: division error; abnormal gamete; fertilization; chromosome count; developmental consequence.

A gamete containing two copies of chromosome 21 can combine with a normal gamete to produce trisomy 21.

Nondisjunction is a chromosome-segregation error, not a point mutation in one gene.

Meiosis generates genetic diversity through random orientation of bivalents and crossing over between non-sister chromatids.

At metaphase I, each bivalent can face either pole independently. The maternal and paternal homologues therefore segregate into many possible whole-chromosome combinations.

During prophase I, non-sister chromatids of homologous chromosomes exchange corresponding DNA at chiasmata, producing recombinant chromatids with new combinations of linked alleles.

One gamete can receive a maternal chromosome carrying a short paternal segment after crossing over, plus a different random mixture of the remaining maternal and paternal homologues.

Random fertilization adds further variation but is not a meiotic process. Meiosis does not direct combinations toward future advantage.

Cell proliferation is repeated passage through growth and mitosis that increases cell number for growth, routine replacement or tissue repair.

Proliferation increases cell number only when new cells are produced faster than cells are lost. Signals regulate which cells enter and continue through the cell cycle.

Cell growth increases the size or contents of one cell; cell proliferation increases the number of cells. Uncontrolled proliferation can disrupt tissue organization.

The cell cycle is the ordered sequence G1 → S → G2 → mitosis → cytokinesis that enables cell proliferation.

G1, S and G2 together form interphase; mitosis follows interphase, and cytokinesis completes production of separate daughter cells.

Interphase is not a stage of mitosis. Checkpoint control belongs to the later cyclin Objective; this card establishes the cycle's sequence.

Interphase is a metabolically active period in which a cell grows and biosynthesizes the components needed for division.

Proteins and other cell materials are synthesized, DNA is replicated in S phase, and cytoplasm increases so the future daughter cells inherit sufficient cellular contents.

Mitochondria—and chloroplasts in photosynthetic cells—increase in number by growth and division of pre-existing organelles. Their increase is therefore part of cell growth, not manufacture from nothing.

Before a plant cell divides, it can synthesize proteins, replicate nuclear DNA and increase its chloroplast and mitochondrial populations during interphase.

Interphase is not a resting period and does not include nuclear division; active biosynthesis and preparation make division possible.

Different cyclins rise and fall in concentration during the cell cycle, and a cell passes a checkpoint only when the relevant cyclin reaches its threshold level.

Cyclin synthesis raises concentration before a transition; cyclin breakdown lowers it afterwards. This repeating pattern coordinates each checkpoint with the correct stage of the cycle.

Cyclin below threshold → checkpoint is not passed. Cyclin reaches threshold → transition can be triggered. Cyclin is degraded → its signal falls before the next cycle stage.

A graph may show one cyclin accumulating before mitosis, crossing a threshold as mitosis begins, then falling rapidly after it is degraded.

The syllabus requires changing cyclin concentrations and threshold control, not names or detailed roles of specific cyclins.

Mutations can cause uncontrolled cell division by converting proto-oncogenes into oncogenes or by disabling tumour-suppressor genes.

A clone carrying one control mutation can continue dividing and acquire further mutations, progressively weakening the systems that restrain proliferation.

An activated oncogene can keep a growth pathway switched on while loss of a tumour suppressor allows damaged cells to pass a checkpoint; together they strongly favour tumour growth.

One mutation does not guarantee cancer. Several independent controls normally limit division, so tumour development often involves accumulated mutations.

Tumours differ in division and growth rate, invasion of neighbouring tissue and capacity for metastasis; these features determine whether they cause cancer.

Metastasis occurs when cells leave a primary tumour, travel through the body and establish one or more secondary tumours.

If 24 of 200 observed tumour cells are in mitosis, the mitotic index is 24 ÷ 200 = 0.12, or 12%. Under comparable sampling conditions, a higher index indicates a larger proportion actively dividing.

Tumour size alone does not establish malignancy. Invasion and metastasis distinguish malignant cancer, while mitotic index estimates division activity only for the sampled population and time.