DNA Decoded
Crack the molecule of life: the double helix, the four bases and how their order spells out every protein you make.
Work it through together, step by step
A free interactive activity that works the method through with a class, step by step.
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DNA Decoded
Inside almost every one of your cells is a molecule that spells out how to build **you**: **DNA**. Decode its structure, the four-letter alphabet it uses, and how the order of those letters becomes the proteins that run your body.
DNA, genes and chromosomes
Four words that examiners deliberately swap around. Learn them against each other, not one at a time.
Zoom in
Put these in order, from the largest structure down to the smallest.
- A body cell
- The nucleus inside that cell
- A single chromosome
- A gene, one section of that chromosome
- A triplet of three bases
The double helix
DNA is a **polymer** made of **two strands** coiled into a **double helix**. Each strand is a chain of small units called **nucleotides**. A nucleotide has three parts: a **sugar**, a **phosphate** and a **base**. The sugars and phosphates form the **backbone**, and the bases stick inwards. There are **four** bases: **A, T, C and G**. Look at how they meet in the middle. The pairing is **fixed**, and this is called **complementary base pairing**: **A** always pairs with **T**, and **C** always pairs with **G**. So if you know one strand, you know the other.
Label the DNA
Drag each label onto the right part of the DNA molecule.
Complete the strand
One strand of DNA reads A, C, G, T. Because the pairing is fixed, the strand opposite it must read _____, _____, _____, _____.
The code of three
How does a base sequence build a protein? The **order** of the bases is a code. Every group of **three bases** (a **codon**) codes for **one amino acid**. Chain the amino acids in that order and you get the protein. Interestingly, **most** of your DNA does **not** code for proteins. Much of it controls **gene expression** (which genes are switched on).
Count the bases
A protein is 120 amino acids long. How many bases of DNA are needed to code for it?
When a mutation matters
A mutation changes a single base inside a gene. Why might that change how the finished protein works?
- It can change one amino acid, which alters the protein's shape and so what it can do
- It destroys the whole chromosome the gene sits on
- It always stops the protein being made at all
- It doubles the number of amino acids in the chain
Why map the genome?
- Search for genes linked to disease
- Understand and treat inherited disorders
- Trace patterns of human migration
- Identify who is at higher risk of a condition, so it can be watched for early
- Work out what a faulty gene does, which is the first step towards treating it
- Compare genomes between populations to see how people spread across the world
Write it out
Explain why scientists mapped the human genome, in about 50 words. A full-mark answer covers all three uses:
- searching for genes linked to particular diseases
- understanding and treating inherited disorders
- tracing patterns of human migration from the past
True of DNA?
Pick the TWO correct statements.
- The genome is all of an organism's genetic material
- Most mutations have no effect on the protein
- The base G pairs with the base A
- One base codes for one amino acid
In the exam
Decoded. Grade-9 habits for DNA: • **DNA** = a double-helix polymer of two strands; a **gene** codes for one protein; the **genome** is all of it. • A nucleotide = **sugar + phosphate + base**; bases pair **A–T** and **C–G**. • **Three bases (a codon) → one amino acid**; most DNA is non-coding; **mutations** change the base sequence (usually with no effect).