DNA and Genetics

DNA (short for deoxyribonucleic acid) is the molecule that carries the instructions for building and running every living thing on Earth. Inside almost every one of your cells is a complete set of your DNA: about 3 billion letters of chemical code, organised into 23 pairs of chromosomes. Your DNA decides everything from your eye colour to the shape of your toes. Genetics is the science of how DNA passes on those features from parent to child.

• DNA shapeDouble helixTwo strands twisted into a spiral ladder
• DNA letters4A, T, G and C (adenine, thymine, guanine, cytosine)
• Human genome sizeapprox. 3 billion base pairsAbout 750 MB of data if written out
• Number of human genesapprox. 20,000Roughly the same as a banana
• Human chromosomes23 pairs46 total, half from each parent
• Discovery date1953Watson, Crick, Franklin, Wilkins

What does DNA look like?

DNA is shaped like a long twisted ladder, a structure called a double helix. The two long strands are made of sugar and phosphate, joined together by pairs of chemicals called bases. There are four different bases: A, T, G and C. They always pair up the same way: A with T, and G with C. The order of those four letters along a DNA strand is the genetic code that tells your cells what to do.

Genes and chromosomes

A gene is a short section of DNA that contains the instructions for making one particular protein (the molecules that do most of the work in a cell). You have around 20,000 genes: ones that decide your hair colour, ones that build your eyes, ones that tell your stomach how to digest food, and many more.

To fit inside the tiny nucleus of a cell, DNA is wound up tightly into structures called chromosomes. Humans have 23 pairs of chromosomes (46 in total). You inherited one of each pair from your mother and one from your father, which is why you look like a mixture of your parents.

How traits are inherited

The first person to work out the basic rules of inheritance was a 19th-century Moravian monk called Gregor Mendel, who carried out years of careful experiments on pea plants in his monastery garden. By counting how many of each kind of pea plant he got after crossing different parents, he showed that traits like flower colour and pea shape were passed on according to predictable patterns. He never used the word "gene" (it had not been invented yet) but his rules turned out to be exactly right.

Fact Humans share roughly 99% of their DNA with chimpanzees, our closest living relatives. We share about 60% with a banana, 50% with a fruit fly, and even about 25% with a daffodil. Lots of basic biological machinery, like how cells divide or how energy is released from food, is the same in all living things, so the DNA for it has been preserved almost unchanged for billions of years.

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