Chromosomes, DNA and Genes Revision Notes | GCSE AQA Higher Biology

1. The Genetic Filing System

Your body contains the instructions for everything about you - your eye colour, height, blood type, even how your enzymes work. But where are these instructions stored, and how are they organised?

Think of your genetic information like a massive library. The building is your cell nucleus. Inside are filing cabinets called chromosomes. Each filing cabinet contains thousands of instruction manuals called genes. The paper these manuals are written on? That's DNA.

Trap: Many students think genes and chromosomes are the same thing. Actually, one chromosome contains many genes - like one filing cabinet containing many instruction manuals.

2. The Hierarchy of Genetic Information

Let's zoom in from the biggest structure to the smallest. In your cell nucleus, you'll find 46 chromosomes. Each chromosome is made of one incredibly long DNA molecule, coiled up tightly to fit.

Along this DNA molecule are thousands of genes - specific sections that code for particular characteristics. Think of it like this: if DNA is a recipe book, then genes are individual recipes, and chromosomes are the chapters that group related recipes together.

The key relationship is: Nucleus contains chromosomes → Chromosomes are made of DNA → DNA contains genes. Each level contains the next, like Russian dolls.

3. Chromosome Pairs in Body Cells

Here's where it gets interesting. Human body cells don't just have 46 random chromosomes scattered about. They have 23 pairs - that's 46 chromosomes arranged in matching couples.

Each pair consists of two chromosomes that carry genes for the same characteristics. For example, both chromosomes in pair 1 have genes for eye colour - but one might code for brown eyes while the other codes for blue eyes.

Why pairs? Because you inherited one chromosome from each parent. Your mum gave you 23 chromosomes, your dad gave you 23 chromosomes, and they paired up to give you 23 pairs (46 total).

Trap: Students often think humans have 23 chromosomes total. Actually, we have 23 pairs = 46 chromosomes in body cells. Only gametes (sex cells) have 23 individual chromosomes.

4. Visualising the Genetic Hierarchy

Let's put this together with a concrete example. Imagine you're looking at chromosome pair 15 in one of your muscle cells.

Worked example: Tracing Genetic Information from Cell to Gene

Follow the genetic information for eye colour from the cell nucleus down to the specific gene.

1. Cell nucleus contains 46 chromosomes in 23 pairs — Every body cell has this complete set
2. Chromosome pair 15 contains two chromosomes — One from mum, one from dad
3. Each chromosome 15 is made of one long DNA molecule — Tightly coiled to fit in the nucleus
4. The eye colour gene is at a specific location on this DNA — Same position on both chromosomes in the pair

Answer: One gene for eye colour exists at the same position on both chromosomes in pair 15

This hierarchy explains why you might have brown eyes like your dad but your sister has blue eyes like your mum - you both got different versions of the eye colour gene from your parents.

5. Common Exam Mistakes and How to Avoid Them

Examiners love to test whether you understand the relationship between chromosomes, DNA and genes. Here are the mistakes that cost students marks, and how to avoid them.

Exam depth extension: In chromosomes, dna and genes, connect the genetic filing system to the hierarchy of genetic information using clear cause-and-effect statements. When you explain biological processes, include what changes, why it changes, and the consequence for cells, organisms, or populations. If calculations appear, show each step, include units, and end with a sentence that interprets the result in biological context.

Mark-winning phrases: When describing chromosome structure, always mention that chromosomes are 'made of DNA molecules' and that they 'carry many genes'. This shows you understand the relationship.

For 'explain' questions about inheritance, remember to mention that 'chromosomes exist in pairs in body cells' - this sets up why offspring get characteristics from both parents.