Cell and nuclear division

The **repeating sequence of events** a cell goes through from when it is formed to when it divides into two.

A long **interphase** (the cell grows and copies its DNA) and a short **mitotic phase (M)** where it divides.

The part of the cell cycle when the cell is **not dividing** — it **grows** and **replicates its DNA**. It is made of G1, S and G2.

**G1, then S, then G2.**

The cell **grows larger** and makes new proteins and organelles.

The **DNA is replicated** (copied) — so the amount of DNA in the cell **doubles**.

The cell keeps **growing** and **prepares to divide**, checking the copied DNA is ready.

The **nucleus divides** (mitosis) and the **cell splits in two** (cytokinesis).

**No** — interphase is only G1, S and G2. The M phase is the separate dividing part.

A cell at **G2 has twice as much DNA** as a cell at G1, because DNA is copied in S phase (in between).

**Interphase** — the cell spends most of its time growing and copying DNA; the M phase (division) is short.

The cell is in **S phase**, copying (replicating) its DNA.

The cell is **dividing (mitosis)** and sharing its DNA equally between two daughter cells.

The division of a nucleus into **two genetically identical** daughter nuclei, each with the **same number of chromosomes** as the parent.

**Prophase, Metaphase, Anaphase, Telophase** (remember **PMAT**).

Chromosomes **condense** (coil up) and become visible; the spindle starts to form and the nuclear membrane breaks down.

Chromosomes line up **single file along the middle (equator)** of the cell, attached to spindle fibres.

The centromeres split and the **sister chromatids are pulled apart** to opposite poles.

**Two** new nuclear membranes form around the two groups of chromosomes, giving **two nuclei**.

The **two identical copies** of a chromosome, made by DNA replication and joined at the **centromere** until anaphase.

The DNA was **replicated once** into identical sister chromatids, which **separate** in anaphase so each cell gets one complete, identical set.

**Yes** — mitosis does not change the chromosome number; both daughters have the same (diploid) set as the parent.

**Growth, repair** of tissue, and **asexual reproduction** — making more identical cells.

**DNA replication** beforehand (also chromosome condensation and spindle formation).

Mitosis = **one** division → **two identical diploid** cells; meiosis = **two** divisions → **four different haploid** gametes.

The division of the **cytoplasm** to form two separate daughter cells, after mitosis has divided the nucleus.

**Mitosis** divides the **nucleus** (chromosomes); **cytokinesis** divides the **cytoplasm** into two cells.

A **contractile ring** contracts and pulls the membrane inwards, forming a **cleavage furrow** that deepens until the cell is pinched in two.

**Vesicles** of wall material fuse to form a **cell plate**, which grows **outwards** to the existing walls and divides the cell.

Plant cells have a rigid **cell wall** that cannot pinch in, so a new wall (cell plate) must be built across the middle.

A groove formed in an **animal** cell's membrane that deepens until the cell is pinched into two daughter cells.

A new wall built across the middle of a dividing **plant** cell, growing outwards until it separates the two daughter cells.

**Yes** — in most divisions the cytoplasm is split roughly equally.

**Egg (gamete) formation** — almost all the cytoplasm goes to one large egg, leaving tiny polar bodies.

The proportion of cells that are dividing: **cells in mitosis ÷ total cells counted**.

Divide the number of cells **in mitosis** by the **total** number of cells counted (e.g. 30 ÷ 200 = 0.15).

A **large proportion of cells are dividing** → rapidly growing tissue (a meristem), or uncontrolled division in a tumour.

The division that makes **gametes**: one **diploid (2n)** cell divides **twice** into **four haploid (n)** cells that are genetically different.

Because it **halves** the chromosome number — a **diploid (2n)** cell becomes **haploid (n)** gametes.

A cell with **two copies of each chromosome** (one set from each parent); in humans, 46 chromosomes = 23 pairs.

A cell with **one copy of each chromosome** — half the diploid number; in humans, 23 chromosomes. Gametes are haploid.

A **matching pair** of chromosomes — same size, carrying the same genes — one inherited from each parent.

The **homologous chromosomes** (the matching pairs) — this is where the chromosome number **halves**.

The **sister chromatids** — finishing the division to give **four** haploid cells.

In **prophase I**, homologous chromosomes **pair up and swap matching sections**, mixing the alleles on each chromosome.

In **metaphase I**, each homologous **pair** is sorted to the poles **at random**, shuffling maternal and paternal chromosomes.

**Crossing over** (prophase I) and **independent assortment** (metaphase I).

So that **fertilisation** (two gametes joining) restores the **diploid** number without doubling it each generation.

In the **anthers** (making pollen / male gametes) and the **ovules** (making the female gametes / egg cells).

**Four** haploid cells, all **genetically different** from each other and from the parent cell.

Mitosis = one division → **two identical** diploid cells; meiosis = two divisions → **four different** haploid gametes.

The **number and appearance** (size and shape) of all the chromosomes in a cell.

A processed **photograph** of a cell's chromosomes, cut out and arranged in **homologous pairs** by size and centromere position.

The number of complete **chromosome sets** in a cell — haploid (n), diploid (2n) or polyploid (3n, 4n…).

A **matching pair** — same size, same centromere position, carrying the same genes (one from each parent).

**Two** sets — found in **body (somatic) cells** (human 2n = 46).

**One** set — found in **gametes** (egg, sperm) (human n = 23).

A cell with **three or more** chromosome sets (3n, 4n…), common in plants.

**Size (length)**, **centromere position**, and **banding pattern**.

A chromosome whose **centromere is near one end** rather than in the middle.

**Single** chromosomes (one set) = haploid **gamete**; chromosomes in **pairs** (two sets) = diploid **somatic** cell.

When a chromosome pair (or sister chromatids) **fails to separate** during meiosis, giving a gamete an **extra or missing** chromosome.

Having **three copies** of one chromosome instead of a pair — e.g. **trisomy 21 (Down syndrome)** or **trisomy 18 (Edward's syndrome)**.

**Stain** the chromosomes, **photograph** them, **cut out** each one, **pair up the homologues**, and **arrange** the pairs largest to smallest.

Because then they are **condensed (short and thick)** and clearly **visible** under a microscope.