D2.1 Cell and nuclear division

Cell division produces new cells for growth, repair, replacement or reproduction.

A parent cell first duplicates essential genetic material, then separates copies into daughter cells and divides its cytoplasm. The outcome depends on whether genetic identity or variation is needed.

Trace: growth signal; DNA preparation; nuclear division; cytokinesis; daughter-cell function.

A skin cell divides to replace a damaged cell while preserving the same genetic instructions.

Cell division is not just splitting cytoplasm; accurate genome distribution is required.

Cytokinesis is the physical division of a cell’s cytoplasm after nuclear division, producing separate daughter cells.

Animal cells form a contractile furrow, while plant cells build a cell plate that becomes a new wall. Cytokinesis completes cell division but does not copy DNA.

Identify the sequence: nuclei separated; furrow or cell plate forms; membranes and cytoplasm divide.

A plant cell builds a cell plate between daughter nuclei, whereas an animal cell pinches inward at the equator.

Nuclear division and cytokinesis are linked but distinct events. Cytokinesis can overlap late nuclear events, but it does not replace chromosome segregation.

Cytokinesis may distribute cytoplasm and organelles equally or unequally, depending on the developmental job of the daughter cells.

Equal division supports two similar cells; unequal division gives one daughter more cytoplasm, organelles or determinants, allowing different fates after division.

Compare daughters by checking: size; organelle content; inherited determinants; later function.

An early embryo can divide asymmetrically so one daughter retains more factors that maintain stem-cell properties.

Unequal size does not automatically mean unequal DNA; nuclear division can still distribute chromosomes accurately.

Mitosis produces genetically similar diploid cells for growth and repair, while meiosis produces haploid gametes and increases genetic variation.

Mitosis has one division after replication. Meiosis has two divisions, pairing and recombination, reducing chromosome number so fertilization can restore diploidy.

Choose the process by checking: purpose; number of divisions; chromosome number; variation.

A muscle precursor uses mitosis for growth, while a germ cell uses meiosis to produce haploid sperm or eggs.

Mitosis is not always identical if mutations occur, and meiosis is not simply ‘mitosis twice’.

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.

Both mitosis and meiosis use spindle fibers, chromosome condensation, alignment and movement to distribute DNA accurately.

Condensation makes chromosomes manageable, spindle attachment provides direction and checkpoints reduce segregation errors. Meiosis adds homologous pairing and a second division.

Compare a division by checking: replicated chromosomes; spindle; alignment; separation target; number of rounds.

In both processes spindle fibers pull chromosomes toward opposite poles, but meiosis I separates homologous chromosomes rather than sister chromatids.

Shared machinery does not make the outcomes identical; chromosome partners and division count change the result.

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.

Mitosis stages can be identified from chromosome condensation, nuclear-envelope state, spindle arrangement and chromatid position.

The visible structures change predictably: intact nucleus before division, condensed chromosomes during alignment, separated chromatids in anaphase and two nuclei in telophase.

Identify a micrograph by checking: envelope; chromosome shape; equator alignment; pole separation.

A cell with two groups of V-shaped chromatids moving toward opposite poles is in anaphase.

Cell shape or stain intensity alone is not enough; use several chromosome features together.

Meiosis produces haploid cells by separating homologous chromosomes in meiosis I and sister chromatids in meiosis II.

DNA replicates once but the nucleus divides twice. Homologues pair and separate first, reducing ploidy; chromatids separate later without another replication.

Track: one replication; homologues apart; no second replication; chromatids apart; four haploid products.

A diploid cell with two homologous chromosome pairs can produce four haploid cells after meiosis.

Meiosis II is not a second DNA-copying event; chromosome number was reduced in meiosis I.

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 increases genetic variation through independent assortment of homologues and crossing over between homologous chromosomes.

Homologous pairs align in different orientations, and exchange of DNA creates new allele combinations. Fertilization then combines gametes randomly. Crossing over exchanges DNA between homologues, while independent assortment distributes each parental chromosome combination differently to gametes. Fertilization adds another random combination.

Separate sources: assortment changes whole-chromosome combinations; crossing over changes linked segments; fertilization combines gametes.

A gamete may receive a maternal chromosome with a paternal segment after crossing over, producing a new combination. Two siblings can inherit different mixtures of parental chromosomes because each meiotic division and fertilization samples a different combination.

Meiosis creates variation but does not direct which combination will be advantageous.

Cell proliferation is repeated cell growth and division that increases the number of cells in a tissue or organism.

Signals regulate entry into the cell cycle, DNA replication and division. Balanced proliferation supports development and repair; excess or insufficient proliferation disrupts tissue function.

Assess proliferation by checking: cell-cycle entry; division rate; tissue demand; control signals.

After a wound, local signals stimulate nearby cells to proliferate and replace lost tissue. A tissue grows when signals raise the number of cycling cells faster than cells are lost, increasing total cell number over time.

More proliferation is not always better; uncontrolled division can damage tissue organization.

The cell cycle includes growth phases, DNA replication and mitosis, with checkpoints coordinating readiness before the next stage.

G1 supports growth, S copies DNA, G2 checks preparation and M separates chromosomes. Checkpoints pause the cycle when DNA or spindle conditions are unsafe.

Place an event in sequence: growth; replication; preparation; nuclear division; cytokinesis.

A cell pauses in G2 if replication is incomplete, preventing chromosome segregation until the problem is addressed.

Interphase is active preparation, not a resting gap with no molecular work.

During interphase a cell grows, makes organelles and proteins, and duplicates DNA before nuclear division.

G1 builds cell material, S copies the genome and G2 prepares spindle and division machinery. These resources ensure daughter cells inherit enough cytoplasm and DNA.

Distinguish phases by asking: growth and organelles; DNA copying; final checks and division preparation.

A cell increases protein and organelle content in G1, copies DNA in S, then checks readiness in G2.

Interphase does not include nuclear division, but it is essential to make division possible.

Cyclins are regulatory proteins whose changing concentrations activate cyclin-dependent kinases and help move cells through checkpoints.

Cyclin production and degradation create pulses of kinase activity. These signals trigger events such as DNA replication or mitotic entry only when conditions are suitable.

Explain a cyclin effect by linking: cyclin level; kinase activation; target process; checkpoint decision.

A rise in a mitotic cyclin can activate proteins that prepare the cell for chromosome condensation and spindle formation.

Cyclins do not act as permanent on-switches; their levels are controlled and different cyclins act at different stages.

Mutations in genes controlling checkpoints, DNA repair or apoptosis can allow cells to divide despite damage.

A loss of checkpoint function permits replication or mitosis with errors, while altered growth signals can keep cyclin pathways active. Clonal expansion then increases the chance of further changes.

Trace: gene mutation; control failure; abnormal division or survival; clone expansion.

A cell lacking a functional checkpoint protein can continue through S phase after DNA damage instead of pausing for repair.

One mutation does not guarantee cancer; multiple controls and tissue context influence tumor development.

Tumours are abnormal cell populations whose growth can become uncontrolled; malignant tumours invade tissues and may spread.

Benign tumors remain localized, while malignant cells can breach tissue boundaries, induce blood-vessel growth and enter circulation. These behaviors reflect accumulated changes in regulation and cell interactions.

Classify a tumor by checking: local expansion; invasion; angiogenesis; metastasis; tissue effects.

A malignant cell that enters a blood vessel and establishes a colony elsewhere has metastasized.

A tumour is not defined only by size; invasion and spread distinguish dangerous behavior.