What Is DNA? An Interactive Introduction for Beginners

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13 steps · 2 quizzes · about 60 minutes · open any step, or read it here

1. 1
   What is DNA?Start hereIn this lesson you learn what DNA is, what it does in your body and how it works, starting from zero. DNA is the instruction book inside your cells: a very long molecule that carries the instructions for building and running a living thing.InteractiveHands-on · 4 min

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   In this lesson you learn what DNA is, what it does in your body and how it works, starting from zero. DNA is the instruction book inside your cells: a very long molecule that carries the instructions for building and running a living thing.

   Every plant, animal, mushroom and bacterium has DNA. Yours came from your parents, and it is part of the reason you have your eye colour, your blood type and much more.

   Below is a small piece of DNA. It looks like a twisted ladder. Spin it, unzip it and look at the letters on the rungs.

   Interactive · A double helix, up close — try it in the lesson

   In this lesson you'll:

   • find where DNA lives in your body, and how 2 metres of it fits inside a single cell
   • read DNA's four-letter code and the one rule that pairs its letters
   • follow a gene as it becomes a protein, the molecules that do the work in your body
   • see how DNA copies itself, what happens when a letter changes, and why you are a mix of your parents

   It takes about 60 minutes. Each step has an animation to play with, and each one uses what you learned in the step before.

   Takeaway: DNA is a long, twisted ladder of letter pairs that holds the instructions for building you.

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   Where DNA lives: from your body to your cellsYour body is not one solid thing. It is built from tiny living units called cells, about 30 trillion of them, and each one is far too small to see by eye. Most cells have a control centre called the nucleus.InteractiveHands-on · 4 min

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   Your body is not one solid thing. It is built from tiny living units called cells, about 30 trillion of them, and each one is far too small to see by eye.

   Most cells have a control centre called the nucleus. Inside it is your DNA, split into pieces called chromosomes. Almost every cell in your body carries a full copy of your DNA: a skin cell has the same DNA as a muscle cell.

   Try it: zoom in from your whole body, one level at a time, and read the size at each level.

   Interactive · Zoom from your body to your DNA — try it in the lesson

   If a skin cell and a nerve cell carry the same DNA, why are they so different? Each kind of cell uses different parts of the instructions, like two cooks using different recipes from the same cookbook.

   Takeaway: your DNA sits in the nucleus of almost every cell, packed into 46 chromosomes.

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   DNA is a code written with four lettersZoom right in and DNA turns out to be a chain of small building blocks called nucleotides. Each one has three parts: a phosphate, a sugar and a base. The phosphates and sugars link up into a long chain, the backbone.InteractiveHands-on · 4 min

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   Zoom right in and DNA turns out to be a chain of small building blocks called nucleotides. Each one has three parts: a phosphate, a sugar and a base. The phosphates and sugars link up into a long chain, the backbone. The bases stick out from it, and they come in only four kinds:

   Letter	Base
   A	adenine
   T	thymine
   G	guanine
   C	cytosine

   The order of the bases along the chain is the code, the way the order of letters in a sentence carries its meaning. ATGC and CGTA use the same four letters but say different things.

   Try it: build a chain with the letter buttons. Watch how many different messages a chain of that length could spell.

   Interactive · Write with four letters — try it in the lesson

   With only four letters, a chain of 10 can be arranged in more than a million ways. Your DNA has about 3 billion letters in each set, and you have two sets, one from each parent. That's about 6 billion letters in a single cell.

   Takeaway: DNA's information is the order of its four bases: A, T, G and C.

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   The pairing rule: A with T, G with CDNA has two chains side by side, joined through their bases like the two halves of a zip. And the bases don't join just any partner: A always pairs with T · G always pairs with C · Why?InteractiveHands-on · 4 min

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   DNA has two chains side by side, joined through their bases like the two halves of a zip. And the bases don't join just any partner:

   • A always pairs with T
   • G always pairs with C

   Why? It comes down to their shapes and the weak links between them (chemists call them hydrogen bonds). A and G are larger bases; T and C are smaller. Every pair is one large and one small base, so every rung of the ladder is the same width. A and T hold on with 2 weak links; G and C with 3.

   Try it: build the partner strand. Pick the letter that fits each piece, then try a wrong one to see why it doesn't fit.

   Interactive · Find the partner: A with T, G with C — try it in the lesson

   Because the rule never changes, one strand tells you the other. If one side reads ATGC, the other side must read TACG. DNA uses exactly this trick to copy itself, as you'll see later in the lesson.

   Takeaway: A pairs with T and G pairs with C, so each strand is a pattern for the other.

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   The double helix: a twisted ladderPut the two strands together and you get a ladder: the sugar-phosphate backbones are its sides and the base pairs are its rungs. Then the whole ladder twists, like a rope ladder turned around and around.InteractiveHands-on · 4 min

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   Put the two strands together and you get a ladder: the sugar-phosphate backbones are its sides and the base pairs are its rungs. Then the whole ladder twists, like a rope ladder turned around and around. This shape is called the double helix; "helix" means spiral.

   Try it: drag the Twist slider from 0% to 100%, then spin the result.

   Interactive · From ladder to double helix — try it in the lesson

   The twist makes DNA compact and keeps the bases tucked inside the spiral, protected. One full turn takes about 10 base pairs, and the whole spiral is only 2 nanometres wide.

   The shape was worked out in 1953 by James Watson and Francis Crick, using X-ray images of DNA from Rosalind Franklin's lab.

   Takeaway: DNA is a double helix: a ladder of base pairs twisted into a spiral.

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   Packing 2 metres of DNA into one cellHere is a surprise. If you took the DNA out of just one of your cells and stretched it out, it would be about 2 metres long. Yet the nucleus it lives in is only about 0.006 mm across.InteractiveHands-on · 4 min

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   Here is a surprise. If you took the DNA out of just one of your cells and stretched it out, it would be about 2 metres long. Yet the nucleus it lives in is only about 0.006 mm across.

   DNA fits because it is wound and folded, again and again:

   1. It winds around spool-like proteins called histones.
   2. The wound-up string coils and folds into loops.
   3. When a cell is getting ready to divide, each piece is folded into a compact chromosome.

   Try it: press Next to pack the thread, one stage at a time.

   Interactive · Packing 2 metres of DNA into one cell — try it in the lesson

   Each of your cells has 46 chromosomes, in 23 pairs. Each chromosome is one very long DNA molecule, packed tight.

   Takeaway: about 2 metres of DNA fits in a cell because it is wound around histones and folded into chromosomes.

7. Quiz: the building blocks of DNAFour questions on what you have learned so far.Quiz · 4 questions

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   Four questions on what you have learned so far.

   Question 1. Where in a cell is most of your DNA kept?

   • In the nucleus
   • In the cell's outer skin
   • In the blood around the cell
   • Only in brain cells

   Question 2. Which four letters make up DNA's code?

   • A, T, G and C
   • A, B, C and D
   • A, U, G and C
   • X, Y, Z and W

   Question 3. One strand of DNA reads GATC. What does its partner strand read?

   • CTAG
   • GATC
   • CUAG
   • TAGC

   Question 4. How does about 2 metres of DNA fit inside a tiny nucleus?

   • It is wound around proteins and folded into chromosomes
   • It is cut into millions of tiny pieces
   • The nucleus stretches to fit it
   • Each cell keeps only a small part of your DNA
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   Genes: the recipes in your DNASo what are all these instructions for? Mostly, for making proteins. Proteins are the workers and building materials of your body: they carry oxygen in your blood, digest your food, build your hair and skin, and help fight germs.InteractiveHands-on · 4 min

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   So what are all these instructions for? Mostly, for making proteins. Proteins are the workers and building materials of your body: they carry oxygen in your blood, digest your food, build your hair and skin, and help fight germs.

   A gene is a stretch of DNA that holds the recipe for one protein. Think of your DNA as a huge cookbook and each gene as one recipe in it. You have about 20,000 genes with protein recipes.

   Try it: pick a gene to see the protein it makes and the job that protein does.

   Interactive · Genes and the proteins they make — try it in the lesson

   The protein recipes make up only about 1–2% of your DNA. Much of the rest helps decide when and where each gene is used, which is how a skin cell and a nerve cell, with the same DNA, end up so different.

   Takeaway: a gene is a stretch of DNA with the recipe for one protein, and proteins do the work in your body.

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   From DNA to RNA: copying a geneYour DNA stays safe inside the nucleus, but proteins are built outside it. So the cell sends out a copy of just the gene it needs: a messenger molecule called RNA (messenger RNA, to be exact).InteractiveHands-on · 4 min

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   Your DNA stays safe inside the nucleus, but proteins are built outside it. So the cell sends out a copy of just the gene it needs: a messenger molecule called RNA (messenger RNA, to be exact).

   An enzyme called RNA polymerase opens a short section of DNA and reads one of its strands, the template. For each letter it adds the partner letter to a growing RNA chain, with one difference: RNA uses U (uracil) where DNA would use T.

   Template DNA	RNA
   T	A
   A	U
   C	G
   G	C

   This copying is called transcription. Try it: step the enzyme along the gene, watch the RNA grow, then send it out of the nucleus.

   Interactive · Transcription: copying a gene into RNA — try it in the lesson

   The finished RNA reads the same as the DNA's other strand, with U in place of every T. A cell can make many RNA copies of one gene, so it can build lots of the same protein quickly.

   Takeaway: transcription copies a gene into RNA, using the pairing rule with U instead of T.

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    Reading the code: three letters make one amino acidOutside the nucleus, the RNA reaches a ribosome, the cell's protein builder. The ribosome reads the RNA three letters at a time. Each group of three letters is a codon, and each codon stands for one amino acid: the beads a protein is made of.InteractiveHands-on · 4 min

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    Outside the nucleus, the RNA reaches a ribosome, the cell's protein builder. The ribosome reads the RNA three letters at a time. Each group of three letters is a codon, and each codon stands for one amino acid: the beads a protein is made of.

    • There are 20 kinds of amino acid but 64 possible codons, so several codons can mean the same amino acid.
    • AUG means start. It also adds the first amino acid, methionine.
    • UAA, UAG and UGA mean stop.

    At a stop codon the chain of amino acids is finished. It folds into a 3D shape, and that shape decides what the protein can do. This reading is called translation: from the language of letters into the language of proteins.

    Try it: step the ribosome along the RNA you made in the last step. Then read the start of a real gene: hemoglobin-beta, part of the protein that carries oxygen in your blood.

    Interactive · Translation: reading three letters at a time — try it in the lesson

    Almost every living thing uses this same code, from bacteria to mushrooms to people. It is one of the strongest clues that all life on Earth is related.

    Takeaway: a ribosome reads RNA in three-letter codons; each codon adds one amino acid until a stop codon ends the protein.