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.