Explain the stages of mitosis (PMAT) — Biology notes

Before we start: what's actually being moved?

A chromosome is counted by its number of centromeres. So a duplicated chromosome (2 chromatids, 1 centromere) is still one chromosome until the centromeres split.

Figure — Explain the stages of mitosis (PMAT)

P — Prophase ("Prepare")

WHAT happens: Loose chromatin coils up tight → visible X-shaped chromosomes (each = 2 sister chromatids).
WHY condense? Long DNA threads would tangle and snap if dragged; coiling makes them compact, portable "packages."
HOW the machinery sets up: Two centrosomes move to opposite ends of the cell. They sprout protein ropes called spindle fibres (microtubules). Nuclear envelope dissolves so the fibres can reach the chromosomes.

M — Metaphase ("Middle / Meet")

WHAT happens: All chromosomes align at the cell's middle line.
WHY align? This is a quality-control checkpoint. If each sister chromatid is held by a fibre from the opposite pole, then when they split, one copy is guaranteed to go each way. Lining up = ensuring fair separation.
HOW: A kinetochore (protein disc on each centromere) acts as a handle; spindle microtubules grip it. Tension from both sides parks the chromosome at the centre.

A — Anaphase ("Apart / Away")

WHAT happens: The "staple" breaks → sister chromatids are dragged to opposite ends.
WHY now? Only after every chromosome is correctly aligned and attached (the metaphase checkpoint passes) does the cell let the centromeres split — so no daughter is short-changed.
HOW: Kinetochore microtubules depolymerise (get shorter), reeling chromatids poleward. The cell also slightly elongates.

T — Telophase ("Two nuclei")

WHAT happens: Basically prophase in reverse — repackaging two nuclei.
WHY decondense? Tightly coiled DNA can't be read/transcribed; the cell must unwind it to resume normal function.
HOW: New nuclear membranes assemble around each chromosome cluster.

Formula Chromosome bookkeeping (derive it, don't memorise)

Start: cell with $2n$ chromosomes, each 1 chromatid. After S phase: still $2n$ chromosomes, now each 2 chromatids → total chromatids $= 2 \times 2n = 4n$ . $\text{DNA content} \xrightarrow{\text{S phase}} \text{doubles},\quad \text{chromosome number} = 2n \text{ (unchanged)}$ After Anaphase (centromeres split): chromatids per pole $= \dfrac{4n}{2} = 2n$ → each pole now has $2n$ chromosomes of 1 chromatid. $\text{Each daughter} = 2n \text{ chromosomes} = \text{same as parent } \checkmark$ WHY this works: doubling the material (S) and then halving it once (anaphase) returns to the original per-cell amount — that's why mitosis keeps chromosome number constant.

Worked Examples

Recall Feynman: explain to a 12-year-old (click to reveal)

Imagine you have one box of LEGO instructions and you need to give two friends a full, identical copy each. First you photocopy every page (that's before mitosis). Now each page is stuck to its copy with tape. Mitosis is the part where you: P — tidy the messy papers into neat stacks; M — line all the taped pairs up in the middle of the table; A — rip the tape so one copy slides to each friend; T — put each friend's pages into their own folder. Now both friends have the exact same complete instructions!

Flashcards

What does each letter of PMAT stand for, in order?

Prophase, Metaphase, Anaphase, Telophase.

In which phase is DNA actually replicated (NOT part of PMAT)?

S phase of interphase, before mitosis begins.

What structure joins two sister chromatids?

The centromere.

Defining event of prophase?

Chromatin condenses into visible chromosomes; nuclear envelope breaks down; spindle forms.

Defining event of metaphase?

Chromosomes line up single-file at the metaphase plate (equator), attached to spindle fibres from opposite poles.

Defining event of anaphase?

Centromeres split; sister chromatids separate and move to opposite poles.

Defining event of telophase?

Chromosomes decondense; two new nuclear envelopes form; spindle disappears.

Why must chromosomes condense before being moved?

So the long DNA doesn't tangle/break while being dragged by spindle fibres.

What attaches spindle microtubules to a chromosome?

The kinetochore (a protein structure on the centromere).

If a parent cell is 2n=46, how many chromatids exist at metaphase?

92 (46 chromosomes × 2 chromatids).

How many chromosomes does each mitotic daughter cell have if parent is 2n=46?

46 — identical to the parent.

Difference between cytokinesis in plants vs animals?

Plants form a cell plate outward; animals form a cleavage furrow that pinches inward.

Why doesn't anaphase change total chromosome number per daughter?

Each duplicated chromosome (2 chromatids) becomes 2 single-chromatid chromosomes, one per pole — no new DNA is made.

Is cytokinesis part of PMAT?

No — PMAT divides the nucleus; cytokinesis separately divides the cytoplasm.

Connections

Cell Cycle — PMAT is the M phase; interphase (G1, S, G2) precedes it.
DNA Replication (S phase) — supplies the sister chromatids mitosis sorts.
Meiosis — compare: meiosis halves chromosome number; mitosis keeps it constant.
Cytokinesis — cytoplasmic division following telophase.
Spindle Apparatus / Centromere and Kinetochore — the machinery of movement.
Cancer and the Cell Cycle — checkpoint failure at metaphase/anaphase.

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, mitosis ka kaam DNA copy karna nahi hai — woh kaam pehle hi S phase mein ho chuka hota hai. Mitosis ka asli kaam hai sorting: duplicate ho chuke chromosomes ko theek se baant kar do exactly identical daughter cells banana. Yaad rakhne ke liye order hai PMAT — "Please Make Another Two".

Prophase mein chromatin condense hoke chromosome dikhne lagte hain (har ek mein 2 sister chromatids, centromere pe jude hue), nuclear envelope toot jaata hai, aur spindle fibres banne lagti hain. Metaphase mein saare chromosomes beech mein (metaphase plate / equator) line mein lag jaate hain, dono poles se fibres pakdi hoti hain — yeh ek checkpoint hai taaki baatwaara fair ho. Anaphase mein centromere split hota hai aur sister chromatids opposite poles ki taraf khinch jaati hain. Telophase mein chromosomes phir se decondense hote hain aur do naye nuclear envelopes ban jaate hain — basically prophase ka ulta.

Ek important baat: anaphase mein chromosome number "double" nahi hota — bas 2-chromatid wala chromosome 2 alag single-chromatid chromosomes ban jaata hai, ek-ek har pole pe. Isliye agar parent 2n=46 hai, to dono daughters bhi 46 hi rahengi. Last mein cytokinesis aata hai (PMAT ka part nahi) jo cytoplasm ko baant deta hai — plants mein cell plate bahar ki taraf banti hai, animals mein cleavage furrow andar ki taraf pinch karta hai. Exam mein zyada tar yahi 80/20 wala high-yield content poocha jaata hai: stage ka defining event + chromosome counting.

Test yourself — Cell Division

Connections