Welcome back!
Last time we met the cell and found DNA tucked inside it, the instruction manual for building and running you. Today we zoom right in on the molecule itself. What does DNA actually look like? And who worked it out?
A quick word before we start: this course is education, not medical advice. If you have a question about your own health or your family's, a doctor or genetic counsellor is the right person to ask.
Ready? Let's build a ladder.
Picture a twisted ladder
Imagine an ordinary ladder with two long side rails and lots of rungs between them. Now hold the top and give it a twist, like wringing out a towel. You've just made a spiral staircase shape. Scientists call it a double helix, and that's DNA.
Here's the diagram in words. Picture two ribbons winding around each other, like two handrails on a spiral staircase. Between them, flat rungs stretch across, stacked one above the other like the steps. Each rung is made of two pieces that meet in the middle and click together.
So what are those parts made of?
The side rails: the backbone
Each side rail is a long chain that repeats two ingredients over and over: a sugar and a phosphate. Sugar, phosphate, sugar, phosphate, all the way down. That's why it's called the sugar-phosphate backbone. It's sturdy and it's the same pattern from top to bottom. It holds everything in place, but it doesn't carry the message.
Here's one more detail worth knowing. The two strands run in opposite directions, a bit like the two lanes of a road. We'll see why that matters in a later level.
The rungs: four bases
The message lives in the rungs. Each sugar on the backbone carries one of four chemical bases:
- A: adenine
- T: thymine
- C: cytosine
- G: guanine
Each rung is two bases, one from each strand, joined in the middle by weak links called hydrogen bonds. Think of them like a zip. Each tooth holds on lightly, but together they hold firm, and you can still unzip them when you need to.
The golden rule of pairing
Now for the most important idea in this lesson. The bases don't pair up at random. A always pairs with T, and C always pairs with G.
Why? Their shapes and chemistry fit together like jigsaw pieces. A-T is one matching pair and C-G is the other. A mismatched pair simply doesn't fit in the ladder properly.
A handy memory trick: Apple in a Tree, Car in a Garage.
This means that if you know one strand, you automatically know the other. Let's try it.
Say one strand reads: A T G C
What's on the other side? Go letter by letter. A pairs with T. T pairs with A. G pairs with C. C pairs with G. So the partner strand reads T A C G.
Your turn. What pairs with G G A T? Have a think...
It's C C T A. Nicely done!
The two strands are called complementary. They aren't identical, but each one fits the other perfectly, like a key and its lock.
So who figured this out?
Nobody could see DNA directly. It's far too small. So how did anyone work out its shape? That's a story with four main characters, all working in Britain in the early 1950s.
Maurice Wilkins at King's College London was one of the first to study DNA using X-rays. Shine X-rays through a sample and they scatter, leaving a pattern on film. Read that pattern skilfully and you can work out the shape that caused it.
Rosalind Franklin joined King's in 1951. She was an expert at this technique, called X-ray crystallography. She took far sharper images of DNA than anyone before her and worked carefully through the measurements. On 2 May 1952, her PhD student Raymond Gosling, working under her direction, captured the famous Photo 51. Its striking X-shaped pattern is a tell-tale sign of a helix.
James Watson and Francis Crick at Cambridge took a different approach. They built physical models out of metal and cardboard, trying to find a shape that fitted all the known facts.
The controversial part
Here's where it gets uncomfortable. In early 1953, Wilkins showed Photo 51 to Watson without Franklin's knowledge. Watson and Crick also saw a research report that summarised her data. Her measurements helped them pin down the final model.
In April 1953 the journal Nature published Watson and Crick's double helix paper. In the same issue were papers from Wilkins's group and from Franklin and Gosling, including Photo 51.
Sadly, Franklin died of ovarian cancer in 1958, aged just 37. In 1962, Watson, Crick and Wilkins shared the Nobel Prize in Physiology or Medicine. Franklin wasn't included, and Nobel Prizes aren't given to people who have died.
So why is her credit still debated? For years the story was told as if she were a side character, or as if her data had simply been taken from her. In 2023, historians Matthew Cobb and Nathaniel Comfort looked at overlooked documents from the time. They argued she was best seen as an equal member of a four-person effort, a scientist who supplied crucial data and helped confirm the result. The conversation goes on, and it's a reminder that science is done by people, with all their rivalries and blind spots.
What this means for you
Every cell in your body that has a nucleus carries this twisted ladder. The A-T and C-G pairing rule you just used is exactly what makes DNA such a good store of information. Each strand carries the pattern for its partner. Hold on to that thought, because next time we'll see how the cell puts it to work.
Key takeaways
- DNA is a double helix: a twisted ladder made of two strands.
- The side rails are a sugar-phosphate backbone, and the rungs are pairs of bases.
- The four bases are A, T, C and G. A pairs with T, and C pairs with G.
- If you know one strand, you can work out its complementary partner.
- Franklin's X-ray work, including Photo 51, was crucial. Wilkins also did X-ray studies, and Watson and Crick built the model. The three men shared the 1962 Nobel Prize after Franklin's death.