DNA in Plain Words: The Instruction Manual Inside Every Cell
Almost every cell in your body carries a full copy of the same instructions. Learn what DNA is made of, how it works, and why you are 99.9 percent the same as everyone else.
Inside nearly every cell in your body is a long molecule called DNA, short for deoxyribonucleic acid. It holds the instructions for building and running you.
What DNA looks like
DNA is shaped like a twisted ladder, called a double helix. The sides are made of sugar and phosphate. The rungs are pairs of four chemical "letters" called bases: A (adenine), T (thymine), C (cytosine), and G (guanine). A always pairs with T, and C always pairs with G.
The order of these letters is the code. It is a bit like how a few letters in an alphabet can write any book.
Genes, chromosomes, and the genome
- Gene: a stretch of DNA that carries the instructions for making a particular protein or controlling other genes.
- Chromosome: a tightly packed bundle of DNA. Humans normally have 23 pairs, so 46 in total.
- Genome: the full set of DNA in an organism. The human genome has about 3 billion letter pairs.
If the DNA in a single cell were stretched out, it would be about two metres long. It fits inside a nucleus smaller than a speck of dust because it is coiled up very tightly.
How a gene becomes a protein
The cell reads a gene
A copy of the gene's code is made, as a related molecule called RNA.
The copy leaves the nucleus
It travels to the cell's protein factories.
The code is translated
Each group of three letters tells the cell which building block to add next.
A protein is built
The chain folds into a specific shape that can do a job, such as carrying oxygen or digesting food.
Why people differ
Any two people share about 99.9 percent of their DNA. The tiny differences, and how genes are switched on and off by the environment, make each person unique. Identical twins begin with the same DNA, but their lives and environments still make them different.
A short history
- In 1953, James Watson and Francis Crick described the double helix, building on X-ray images by Rosalind Franklin and work by Maurice Wilkins.
- The Human Genome Project read the human genetic code and announced its completion in 2003.
- Today, tools such as genome sequencing and gene editing are used in medicine and farming.
What most people miss
- Most DNA is not genes. Only a small part of the genome codes for proteins. Much of the rest helps control when and how genes work.
- Genes are not destiny. Many traits and diseases come from a mix of genes, lifestyle, and chance.
- You also carry DNA in the cell's power plants. This small set, called mitochondrial DNA, is inherited from the mother.
