Division of Labour

Division of Labour 🧫

You grew from a single cell into trillions — and you replace worn-out cells every day. All of it runs on one process: **cell division by mitosis**. Learn what gets copied, the order it happens in, and the remarkable **stem cells** that can still become anything.

Chromosomes 🧵

Inside the nucleus, DNA is packaged into structures called **chromosomes**. Each chromosome carries a large number of **genes**, and they normally come in **pairs**. A normal human **body cell** has **23 pairs** — that is **46** chromosomes in total.

Count them

How many chromosomes are there in a normal human body cell?

• 46 (23 pairs)
• 23
• 92
• 2

The cell cycle 🔄

Before a cell divides it goes through the **cell cycle**: 1. The cell **grows** and makes more sub-cellular structures (mitochondria, ribosomes). 2. The **DNA replicates**, forming two copies of each chromosome. 3. **Mitosis** — the chromosomes separate to opposite ends and the nucleus divides. 4. The **cytoplasm and cell membranes divide** to form two identical daughter cells.

Order the cycle

An interactive activity.

What comes out?

Mitosis divides one cell. What does it produce?

• Two genetically identical diploid cells
• Four genetically different haploid cells
• Two genetically different cells
• One larger cell

What mitosis is for 🛠️

Because it makes **identical** copies, mitosis is used for: • **Growth** — building more cells as an organism develops. • **Repair** — replacing damaged or worn-out cells. • **Asexual reproduction** — producing genetically identical offspring.

Stem cells 🌱

A **stem cell** is an **undifferentiated** cell that can keep dividing and can **differentiate** into specialised cells. There are three kinds to know: • **Embryonic** (from embryos) — can become **any** type of cell. • **Adult** (e.g. from bone marrow) — form only a **limited** range, such as blood cells. • **Meristem** (in plant root and shoot tips) — can make **any plant cell**, throughout the plant's life.

Match the stem cell

Therapeutic cloning ⚕️

In **therapeutic cloning**, an embryo is made with the **same genes** as a patient. Stem cells taken from it are **not rejected** by the patient's body, so they could treat conditions like **diabetes** or **paralysis**. But there are objections: some people have **ethical or religious** concerns about using embryos, and there is a risk of transferring **viral infection**. A good answer gives **both** sides.

Weigh it up

Pick the TWO true statements about using stem cells — one benefit and one concern.

• They could treat conditions such as diabetes or paralysis
• Some people object to using embryonic stem cells
• Therapeutically cloned stem cells are always rejected
• Using stem cells raises no ethical concerns at all

In the exam 🎓

Divided and conquered. Grade-9 habits for cell division: • Sequence the **cell cycle**: grow → DNA replicates → **mitosis** → cytoplasm divides. • **Mitosis** makes **two genetically identical diploid cells** — for growth, repair and asexual reproduction. • Stem-cell answers need **both** a benefit (treating disease) **and** a risk/objection (embryo ethics, infection).