GCSE/iGCSE Biology Week 6
- Sallyann Clark
- 20 hours ago
- 7 min read
WEEK 6: One Genome, Many Jobs
How Does a Skin Cell End Up Different from a Nerve Cell?
The Big Question This Week
"If every cell in your body has the exact same DNA, how does a skin cell end up different from a nerve cell?"
Write this in your learning journal as a title before you begin. You wrote this question down at the end of Week 5, go back and read what you predicted then, if you wrote anything. Don't change it yet.
Before You Begin — Predict!
Write down whether you think these are true or false, and why:
"Once a cell in your body is 'made,' it stays exactly the same for the rest of your life."
"Stem cells can only be found in very young embryos."
Hold onto your predictions. We will return to them at the end of the week.
Story of the Week
You began life as a single cell. One fertilised egg, smaller than a full stop, carrying one complete copy of your entire genetic instruction manual. That single cell divided into two, then four, then eight, and kept dividing until, today, your body is built from somewhere around 37 trillion cells.
Here's the part that should genuinely stop you in your tracks: nearly every one of those 37 trillion cells carries an identical copy of the same DNA you started with. The DNA in a neuron firing electrical signals in your brain is the same DNA sitting in a skin cell on your elbow, which is the same DNA in a muscle cell in your heart. Same instruction manual, cover to cover, in almost every single cell.
And yet a neuron looks nothing like a skin cell, behaves nothing like a skin cell, and does a completely different job. So does a muscle cell. So does a red blood cell, which is so specialised for its one task, carrying oxygen, that it actually ejects its own nucleus during development and spends the rest of its short life without one.
The trick is not that different cells have different instructions. It's that different cells read different parts of the same instructions. Imagine an enormous cookbook containing every recipe ever written. A skin cell only ever opens the pages for "skin," and never touches the pages for "nerve" or "muscle," even though every recipe is sitting right there the whole time, bound into the same book. This week we find out how a cell decides which pages to open, and meet the remarkable cells that haven't decided yet.
Video Session One
While you watch, look out for:
What cell specialisation actually means, and why a multicellular organism benefits from having different cell types instead of millions of identical general-purpose cells
The idea of surface area to volume ratio, and why it limits how large a single cell can usefully grow
How homeostasis (which you first met back in Week 2) depends on many different specialised cell types working together
Draw a quick sketch in your journal of three specialised cells you already know something about (for example: a red blood cell, a nerve cell, a muscle cell) and note one feature of each shape that helps it do its job. useful
A good place to stop is once the video ends, if you need to.
Video Session Two
While you watch, look out for:
What a stem cell is, and what makes it different from a specialised (differentiated) cell
The difference between a cell having a gene and a cell actually using, or expressing, that gene
Where stem cells are found in the body after the embryo stage — they don't disappear once you're born
Draw a simple table in your journal with two columns headed Stem Cell and Specialised Cell. Note down at least two differences between them.
⚠️ Common Misconception Alert
Many students assume stem cells only exist for a few days in a developing embryo and then vanish forever. In reality, adults still carry stem cells in places like bone marrow, where they continually produce fresh blood cells throughout your entire life. What changes is not that stem cells disappear, but that adult stem cells are usually far more limited in what they can become compared with embryonic ones.
Reading Assignment
Read One Genome, Many Jobs: Differentiation and the Stem Cell and The Half Embryo That Broke the Rules.
Here is a good place to stop if you need to.
Hands-On Activity — The Cookbook Model
What you need: A notebook or a few sheets of paper, and coloured pens or highlighters.
What to do:
Draw a simple "book" shape on your page and label it The Genome — the complete set of genetic instructions found in (almost) every cell in your body.
Inside the book, write out five made-up "recipe" names representing genes: for example, Haemoglobin, Keratin, Insulin, Myosin, Neurotransmitter Receptor.
Now draw three small circles representing three different specialised cells: a red blood cell, a skin cell, and a muscle cell. For each one, highlight or circle only the recipe names that cell would actually use, and cross out or leave un-highlighted the ones it ignores.
In your journal, write two sentences explaining why the red blood cell circle and the skin cell circle both contain the entire genome, even though they only "use" completely different parts of it.
The Brilliant Mistake — Stories Science Gets Wrong
This Week's Story: The Frog That Broke a Fifty-Year Assumption
For much of the early twentieth century, biologists widely assumed that once a cell specialised, the process was permanent and irreversible — a one-way street. A skin cell was a skin cell forever; there was no going back. It seemed like common sense: after all, no one had ever seen it happen in reverse.
In 1962, a British developmental biologist named John Gurdon set out to test this assumption directly, using frogs. He took the nucleus from a fully specialised intestinal cell of an adult frog and transplanted it into a frog egg cell that had had its own nucleus removed. If the old assumption were correct, nothing useful should have happened; the transplanted nucleus was supposed to be permanently "locked" into being an intestinal cell.
Instead, some of these eggs developed into completely normal, healthy tadpoles. The DNA from a fully specialised adult cell had been "reset," and it went on to build an entire new frog from scratch. Gurdon had shown that specialisation is not a one-way trip after all. The instructions for becoming any cell in the body are still sitting there, intact, inside a specialised cell's nucleus; they are simply switched off, not deleted.
This discovery took decades to reach its full significance. In 2012, fifty years after his frog experiment, John Gurdon shared the Nobel Prize in Physiology or Medicine for the doors his work eventually helped open — including techniques for reprogramming adult human cells back into a stem-cell-like state.
Journal Prompt:
Why do you think biologists were confident, for decades, that specialisation was irreversible, even without direct proof?
Gurdon's result seemed to overturn "common sense." Can you think of another idea in this course so far that also overturned what seemed like common sense at the time?
What does it tell you about assumptions in science generally, when something believed for fifty years turns out to be wrong?
Scientist Portrait
Sir John Gurdon (1933–2025)
Add John Gurdon to your Scientist journal. Draw or print a portrait. Beneath it, write: "He put an adult cell's nucleus into an empty egg and grew a whole new frog, proving specialisation could be undone."
Return to Your Predictions
Go back to the two statements from the start of the week:
"Once a cell in your body is 'made,' it stays exactly the same for the rest of your life."
"Stem cells can only be found in very young embryos."
Rewrite each one now, using what you've learned this week.
The Big Question — Final Answer
Return to this week's big question:
"If every cell in your body has the exact same DNA, how does a skin cell end up different from a nerve cell?"
Write a paragraph in your learning journal. A strong answer will mention that all cells carry the same genome, but different cells switch on (express) different genes, and that this process is called differentiation.
Practice Questions — 10 Question Set
What is cell differentiation?
Explain why nearly every cell in your body contains the same DNA, even though cells look and behave so differently from one another.
What is a stem cell?
Give one place in the adult human body where stem cells are still found.
What is the difference between a gene being present in a cell and a gene being expressed by that cell?
Explain why red blood cells losing their nucleus is a good example of specialisation.
What did biologists assume about cell specialisation before 1962?
Describe what John Gurdon did in his 1962 experiment, and what the result showed.
Why did it take fifty years for the full significance of Gurdon's discovery to be recognised with a Nobel Prize?
A student says, "Stem cells are magic cells that can do anything, forever." Using this week's learning, explain what's wrong with this statement, and what a more accurate description would be.
Week 6 Checklist
Before moving to Week 7, tick off each item:
Watched Crash Course Biology #41: Multicellular Function
Sketched three specialised cells and noted a feature of each
Watched the Amoeba Sisters video: How Cells Become Specialized
Completed the Stem Cell / Specialised Cell table
Completed the Cookbook Model activity
Read the John Gurdon story and answered the journal prompts
Added John Gurdon to your Scientist journal
Rewritten your two predictions from the start of the week
Answered the 10 practice questions
Written a final answer to the Big Question
Looking Ahead — Week 7 Preview
We've spent six weeks building up from atoms to molecules to cells. Next week we take the next step up the ladder of organisation: how cells, once specialised, actually get arranged and organised into the working machinery of a whole body.
Prepare by writing this question in your journal: "If a cell is like a single brick, how do you get from a pile of bricks to an entire working building?"
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