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 that carry the instructions for building the cell.

Count them

A normal human body cell contains 23 pairs of chromosomes. How many chromosomes is that in total?

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

Put the stages of the cell cycle in the order they happen.

• The cell grows and copies its sub-cellular structures
• The DNA replicates. Two copies of each chromosome
• Mitosis: the chromosomes separate and the nucleus divides
• The cytoplasm and membranes divide into two cells

What comes out?

A skin cell containing 46 chromosomes divides by mitosis. What are the two new cells like?

• Two cells, each with 46 chromosomes, genetically identical to the original
• Two cells, each with 23 chromosomes, genetically different from each other
• Four cells, each with 23 chromosomes, all genetically different
• Two cells, each with 46 chromosomes, but genetically different from the original

What mitosis is for

Because mitosis makes genetically identical cells, organisms use it for _____ as they develop, for _____ of damaged or worn-out cells, and for _____, which produces 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, and what separates them is how much they can become.

Match the stem cell

Which stem cell?

A gardener wants to grow many identical copies of a rare plant. Which stem cells make that possible, and why?

• Meristem cells from root and shoot tips, which can make any plant cell throughout the plant's life
• Embryonic stem cells from the plant's seeds, which can become any cell in any organism
• Adult stem cells from the mature stem, which form only a limited range of cell types
• No stem cells are involved, because plants cannot be cloned from cuttings

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 in medicine: one benefit and one concern.

• They could be used to treat conditions such as diabetes or paralysis
• Some people have ethical or religious objections to using embryonic stem cells
• Stem cells from therapeutic cloning are always rejected by the patient
• Using stem cells raises no ethical concerns of any kind
• There is no risk at all of transferring a viral infection with the cells
• They are already a proven cure for every condition they have been trialled on

Treat the patient

A patient has been paralysed by damage to their nerve cells. Work through whether stem-cell treatment could help, and what has to be weighed against it.

• Which property of stem cells makes them a candidate for repairing damaged nerve cells?
• Donor stem cells risk being rejected by the patient. How does therapeutic cloning get around that?
• A hospital ethics panel reviews the plan. Which objection do they have to take seriously?
• The panel also asks about a medical risk of the procedure. Which one genuinely applies?

Both sides

Evaluate the use of stem cells in medicine. Give arguments for and against, and reach a conclusion.

• Start with what stem cells can do: differentiate into specialised cells.
• Give at least one concrete benefit, naming a condition they could treat.
• Explain how therapeutic cloning avoids rejection, using the word genes.
• Give at least one objection: the embryo ethics, or the risk of viral infection.
• An evaluation needs BOTH sides and a conclusion. Marks are lost for giving only one.