DNA and Genetics Basics

The code of life

Every living thing — from a bacterium to a blue whale to you — is built and run by a set of instructions written in a molecule called DNA (deoxyribonucleic acid). DNA is sometimes called the "code of life" because it holds the information needed to make and maintain an organism, and to pass that information to the next generation.

The study of how this information is stored, expressed and inherited is called genetics.

The structure of DNA

In 1953, James Watson and Francis Crick (using crucial X-ray data from Rosalind Franklin and Maurice Wilkins) described the structure of DNA: a double helix, like a ladder twisted into a spiral.

The two long sides of the ladder are made of sugar and phosphate. The "rungs" are made of pairs of chemicals called bases, of which there are four:

• A – adenine
• T – thymine
• C – cytosine
• G – guanine

The pairing follows strict rules: A always pairs with T, and C always pairs with G. This is called complementary base pairing, and it is the secret to how DNA copies itself accurately. When a cell divides, the two strands unzip, and each acts as a template to build a perfect new partner strand.

Genes and chromosomes

The order of the bases along the DNA spells out instructions, a bit like letters spelling out words. A gene is a section of DNA that contains the instructions to build a particular protein. Proteins do most of the work in your body — they build tissues, carry oxygen, fight infection and speed up chemical reactions.

DNA does not float loose in the cell. It is wound up tightly into structures called chromosomes. Humans have 46 chromosomes arranged in 23 pairs. You inherit one chromosome of each pair from each parent, which means you carry two copies of nearly every gene.

DNA → genes → chromosomes → genome. The full set of an organism's DNA is its genome.

How traits are inherited

Because you have two copies of each gene, you may inherit two slightly different versions. These different versions are called alleles.

Some alleles are dominant and some are recessive:

• A dominant allele shows its effect even if only one copy is present.
• A recessive allele only shows its effect when both copies are recessive.

Consider a simplified example with a flower colour gene where purple (P) is dominant and white (p) is recessive:

Alleles inherited	Genotype	Flower colour
P and P	PP	Purple
P and p	Pp	Purple
p and p	pp	White

A tool called a Punnett square is used to predict the chances of offspring inheriting each combination. If two Pp parents are crossed, the offspring are expected in a ratio of about 3 purple to 1 white.

This kind of pattern was first worked out by Gregor Mendel in the 1860s, studying pea plants — long before anyone knew DNA existed.

Variation, mutation and evolution

No two individuals (except identical twins) have exactly the same DNA. This genetic variation comes from the mixing of parents' genes during reproduction, and from mutations — changes in the DNA sequence.

Most mutations are harmless or are repaired. A few are harmful. But occasionally a mutation is beneficial, giving an individual a survival advantage. Over many generations, helpful variations become more common — this is natural selection, the engine of evolution.

Genetic variation is also the foundation of biodiversity and conservation: the more genetic diversity a species has, the better it can adapt to threats like disease and a changing climate.

Genetics in the modern world

Understanding DNA has transformed our world. It allows:

• Medicine — diagnosing genetic diseases and developing targeted treatments.
• Forensics — identifying people from tiny DNA samples.
• Agriculture — breeding hardier, higher-yielding crops.
• Ancestry — tracing how populations are related and how humans spread across the globe.

Try it yourself: extract DNA from a strawberry

DNA is real and you can actually see it with kitchen equipment.

1. Place one strawberry in a sealable plastic bag and mash it thoroughly.
2. Make an extraction liquid: mix half a cup of water, a teaspoon of washing-up liquid and a pinch of salt.
3. Add two spoonfuls of this liquid to the bag and gently mash again for a minute. (The soap breaks open the cell membranes; the salt helps the DNA clump together.)
4. Filter the mixture through a coffee filter or kitchen sieve into a clear glass.
5. Slowly pour cold rubbing alcohol (isopropanol) down the side of the glass so it forms a layer on top. Do not stir.
6. Watch the boundary between the layers. Within a minute you should see stringy, white, cloudy strands rising — that is strawberry DNA.

Strawberries work especially well because they have eight copies of each chromosome, giving lots of DNA to see. You are looking at the same kind of molecule that carries the instructions for every living thing on Earth.