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GCSE Biology - Year 1 Week 3 - DNA, Cell Cycle and Introduction to Mitosis

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"Every second, your body makes millions of new cells. Cut your finger? Grow taller? Heal from the flu? That’s mitosis in action, the biological equivalent of copy-paste, but far more precise."


Introduction

This week, we go deeper into what cells do. You’ll find out how your body knows what to build, when to grow, and how to repair itself, and it’s all hidden inside your DNA.


You’ll explore:

  • What chromosomes and DNA are

  • Why DNA is called the "blueprint of life"

  • How cells grow and divide using the cell cycle

  • How mistakes in cell division can lead to diseases like cancer


By the end of the week, you’ll be able to explain how a single cell becomes two identical cells through mitosis, and why that matters for your growth and health.


🧠 What Should I Already Know?

From previous weeks:

  • Cells are the building blocks of all life

  • Organelles like the nucleus control the cell

  • Microscopes let us see cells in detail


Recap by watching Cognito - Cell Types and Cell Structure.


📚 Learning


1. What Is DNA?

DNA = Deoxyribonucleic Acid

DNA is a long molecule shaped like a twisted ladder. This is known as the double helix. It is found inside the nucleus of eukaryotic cells and carries the genetic code for building proteins; it's like instructions for how your body works


Watch Amobea Sister - DNA Structure. As you watch it, pause and make notes in your journal about DNA. Make sure you include:

  • New vocabulary

  • Diagrams of DNA and the double helix

  • Drawings and sayings to help you remember how the bases pair.



  1. History of DNA's Discovery

As with most discoveries, there is a bit of controversy here!



Now watch TED-Ed - Rosalind Franklin


Write at least a paragraph in your journal about the discovery of the DNA structure.


  1. What Are Chromosomes?

DNA is packaged into chromosomes, and humans have 46 chromosomes in each body cell. The chromosomes come in pairs (23 from your mother, 23 from your father). During cell division, chromosomes are copied and passed to new cells.


Work through BBC Bitesize - Chromosomes, genes and DNA.


We will look more closely at chromosomes when we study heredity.


4. The Cell Cycle – What Cells Do

Cells don’t just sit there, they’re always doing something:

  • Interphase – the cell grows, and DNA is copied

  • Mitosis – the copied DNA is shared equally into two new nuclei

  • Cytokinesis – the cell splits in two, creating two identical cells


📌 Fun fact: Every second, your body makes about 25 million new cells using this cycle.

Watch the Amobea Sisters - The Cell Cycle. In this video the cell cycle only has two phases, you need to know that there are three (listed above).


The cell cycle is important for:

  • Growth (e.g. from baby to adult)

  • Repair (e.g. healing a cut)

  • Replacing worn-out cells (e.g. skin cells)


But what happens when it goes wrong?


5. Cancer – Uncontrolled Cell Division

Cancer is caused by mutations (changes in DNA) that can make cells divide out of control. Tumours form when damaged cells keep dividing. Some cells break away and spread (this is metastasis).


Tasks

Create a diagram of the cell cycle showing:

  • Interphase

  • Mitosis (just as one stage for now)

  • Cytokinesis

  • Add labels and describe what happens at each point.

  • Watch TED-Ed - The Immortal Cells of Henrietta Lacks.

  • Read BBC Bitesize - Henrietta Lacks: How her immortal cells advanced modern science.

  • Write a biography for Henrietta Lacks' cells.

  • Write a few sentences explaining whether you think it was ethical to use her cells or not.

  • Check the board notes and make sure you have recorded all the relevant information in your journal for your board.


📋 Board Notes

✅ All Exam Boards Require:


  • You must know that chromosomes are found in the nucleus and made of DNA.

  • DNA carries the genetic information that controls the development and function of an organism.

  • Human body cells contain 46 chromosomes (23 pairs).

  • Before a cell divides, the DNA must be copied.

  • The cell cycle is the process by which cells grow, copy their DNA, and divide by mitosis.

  • Mitosis is used for growth, repair, and asexual reproduction.

  • You should be able to describe the main stages of the cell cycle and link mitosis to growth and repair.


🔶 Higher Tier Only (All Boards):


  • You may be asked to recognise or label stages of the cell cycle on a diagram or graph.

  • Understand the link between uncontrolled cell division and the formation of tumours or cancer.



Conclusion

This week, you discovered that DNA holds the instructions for life, chromosomes carry that DNA, and the cell cycle allows cells to divide and grow. You’ve now got the tools to understand mitosis — and that’s what we’ll tackle next week.

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