What is DNA?

The name describes what DNA is made of. Deoxyribo refers to its sugar, deoxyribose. Nucleic means it was first found in the nucleus of cells. Acid comes from the phosphate groups along its backbone, which make it acidic. Almost every cell in your body holds a full copy of your DNA, about 3 billion letters long in each set of chromosomes.

What is DNA made of?

DNA is a polymer: a long chain of repeating units called nucleotides. Each nucleotide has three parts:

1. A phosphate group, which links one nucleotide to the next.
2. A sugar called deoxyribose.
3. A base, one of four: adenine (A), thymine (T), cytosine (C) or guanine (G).

The sugars and phosphates alternate to form a backbone, and the bases stick out from it like the teeth of a comb. Only the bases differ from one nucleotide to the next, so they carry the information. The backbone just holds them in order.

What sugar is found in DNA?

The sugar in DNA is deoxyribose, a sugar with five carbon atoms. "Deoxy" means "missing an oxygen": deoxyribose is ribose, the sugar in RNA, with one oxygen atom removed from its second carbon. That small change makes DNA more chemically stable than RNA, which suits a molecule that has to store information for a lifetime.

The DNA double helix

DNA has two strands, wound around each other into a double helix, like a twisted ladder. The backbones are the sides of the ladder and pairs of bases are the rungs.

• Base pairing: A always pairs with T, and C always pairs with G. These are called complementary base pairs.
• Hydrogen bonds hold each pair together: two between A and T, and three between C and G. Each bond is weak, but millions together hold the strands firmly, and they can still be unzipped when the cell needs to read or copy the DNA.
• The strands run in opposite directions. Each strand has a 5′ end and a 3′ end, named after carbons in the sugar, and the two strands are antiparallel: one runs 5′ to 3′ and its partner runs 3′ to 5′.

Because the pairs always match, one strand fixes the other. If you know one strand is ATGGCTTAC, the other must be TACCGAATG. That is the key to how DNA is copied: split the strands, and each one is a template for a new partner.

Try building a partner strand yourself, then press Mark to check it in our Genetics and DNA course.

What is the function of DNA?

DNA contains the instructions an organism needs to develop, survive and reproduce. It does three jobs.

1. It stores the instructions for making proteins

A gene is a section of DNA whose bases code for a protein (or, for some genes, a working RNA molecule). Proteins do most of the work in a cell: enzymes speed up reactions, haemoglobin (hemoglobin in the US) carries oxygen, keratin builds hair, and antibodies fight infection. Your genome has about 20,000 genes, which make up only about 1% of your DNA. Much of the rest helps control when and where genes are switched on.

2. It is copied so new cells get the instructions

Before a cell divides, it copies all of its DNA, a process called DNA replication. The two strands unzip, and enzymes build a new partner for each one using the base pairing rules. Each new double helix keeps one old strand and gains one new one, so the copying is called semi-conservative.

3. It passes traits from parents to children

You inherit one set of chromosomes from each parent, 23 from each, making 46 in 23 pairs. That is why you share features with your family, and why genetic conditions can run in families.

How DNA makes proteins

DNA stays in the nucleus, but proteins are made outside it, at ribosomes. So the cell works in two steps.

1. Transcription: the cell copies one gene into a messenger RNA (mRNA) molecule. RNA uses uracil (U) where DNA uses thymine (T).
2. Translation: a ribosome reads the mRNA three bases at a time. Each group of three, called a codon, stands for one amino acid, and the ribosome joins the amino acids into a chain that folds into a protein.

With four bases taken three at a time, there are 4 × 4 × 4 = 64 possible codons. Living things use 20 amino acids, so most amino acids have more than one codon, and three codons mean "stop".

Example: from DNA to protein

A gene's coding strand begins ATG GCT TAC. What are the mRNA and the first three amino acids?

• The mRNA has the same sequence as the coding strand, with U in place of T: AUG GCU UAC.
• Look up each codon in the genetic code: AUG is methionine, GCU is alanine and UAC is tyrosine.
• So the protein starts methionine, alanine, tyrosine. AUG is also the start codon, which tells the ribosome where to begin.

Where is DNA found?

In plants, animals and fungi, almost all DNA is in the nucleus, wrapped around proteins called histones and packed into chromosomes. A little is in the mitochondria, which have their own small loop of DNA, 16,569 base pairs long in humans and passed down only from your mother. Plant cells also have DNA in their chloroplasts. Bacteria lack a nucleus. Their main chromosome is one circle of DNA lying free in the cytoplasm, often with extra small rings called plasmids.

There is a lot of it. A human body cell holds about 6.2 billion base pairs, two copies of the genome, and each base pair adds about 0.34 nanometres of length. So the DNA in one cell, stretched out, would be about 6.2 × 10⁹ × 0.34 × 10⁻⁹ = 2.1 metres long, packed into a nucleus a few thousandths of a millimetre across.

DNA vs RNA

When was DNA discovered?

DNA was discovered in 1869, but its structure wasn't worked out until 1953. The story took almost a century.

So who discovered DNA? Miescher found the substance. Watson and Crick worked out its shape in their 1953 paper, and Rosalind Franklin's X-ray work was essential to it. Franklin died in 1958, four years before the 1962 Nobel Prize, and Nobel Prizes aren't awarded to people who have died.

Why is DNA important?

• It makes you, you. Your DNA shapes your eye colour, blood group and much more, together with your environment and lifestyle.
• Medicine: genetic tests diagnose inherited conditions, and some cancer treatments are chosen based on the DNA changes in a tumour. Genetic engineering lets bacteria make human insulin for people with diabetes.
• Forensics: any two people's DNA is about 99.9% identical, but the small differences make DNA profiling powerful enough to identify people from tiny samples.
• Evolution and ancestry: comparing DNA shows how species are related and traces family history.
• Gene editing: tools like CRISPR can change DNA at a chosen spot, opening up treatments for some genetic diseases.

Worked examples

Example 1: base percentages

In a sample of double-stranded DNA, 30% of the bases are adenine. What percentage are cytosine?

A pairs with T, so T is also 30%. That leaves 100% − 30% − 30% = 40% for C and G together. Every C has a G partner, which splits the 40% evenly: C = 40% ÷ 2 = 20%.

Example 2: counting hydrogen bonds

A 10 base pair stretch of DNA has 4 C-G pairs and 6 A-T pairs. How many hydrogen bonds hold it together? Each C-G pair has 3 and each A-T pair has 2, so 4 × 3 + 6 × 2 = 12 + 12 = 24. DNA with more C-G pairs needs more energy to separate, which is why it holds together at higher temperatures.

Example 3: the partner strand, with directions

One strand reads 5′ ATGGCTTAC 3′. Write its partner strand, 5′ to 3′.

Pair each base: the partner reads 3′ TACCGAATG 5′. The strands are antiparallel, so to write it 5′ to 3′ you reverse it: 5′ GTAAGCCAT 3′. Forgetting to reverse is one of the most common slips in exam answers.

Common mistakes

• Pairing A with G or C with T. It is always A with T and C with G. A quick memory aid: the letters with straight lines, A and T, pair together, and the curved letters, C and G, pair together.
• Saying DNA contains ribose or uracil. DNA has deoxyribose and thymine. Ribose and uracil belong to RNA.
• Calling a gene a chromosome. Each chromosome is a single, very long double helix wound around proteins, carrying hundreds or thousands of genes.
• Thinking all DNA codes for proteins. Only about 1% of human DNA is in protein-coding genes.
• Saying Watson and Crick discovered DNA. They discovered its structure. DNA itself was found in 1869.

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