Genes and DNA

One human chromosome set holds about 3 billion DNA base pairs; about 1 percent of its sequence codes for proteins.

Instructions passed from parent to child

Children often have a parent's eye colour or the shape of a grandparent's nose. Something must carry that information from one generation to the next, and it has to be copied every time a cell divides. That something is deoxyribonucleic acid, or DNA. NHGRI, the genome institute of the US National Institutes of Health, describes DNA as the molecule that holds the biological instructions that make each species unique. Those instructions are passed from adults to their offspring during reproduction.

In plants, animals and fungi (the eukaryotes), most DNA sits inside the nucleus of each cell. A cell is tiny and holds many long DNA molecules, so each molecule is packed tightly into a structure called a chromosome. The DNA in the nucleus is the nuclear genome; the complete genome also includes DNA outside the nucleus. There is also a little DNA in the mitochondria, the parts of the cell that release energy, and that DNA is inherited only from the mother because only the egg keeps its mitochondria at fertilization.

Four letters on a sugar and phosphate chain

DNA is a chain of building blocks called nucleotides. Each nucleotide has three parts: a phosphate group, a sugar called deoxyribose and one of four nitrogen-containing bases. The bases are adenine (A), thymine (T), guanine (G) and cytosine (C). Sugars and phosphates alternate to form the long backbone, and the bases stick out from it like the teeth of a comb. The order of the bases along the chain, called the sequence, is the information. In the 3D model the beige beads are sugars, the orange beads are phosphates and the coloured slabs are bases.

According to NHGRI, one human chromosome set contains about 3 billion base pairs and about 20,000 protein-coding genes on 23 chromosomes. A gene is a stretch of DNA with instructions for a protein or a functional RNA. Protein-coding regions make up about 1 percent of the DNA sequence. Some noncoding regions help decide when, where and how much of each protein is made.

From gene to protein in two steps

Proteins do most of the work in the body: they build hair and muscle, carry oxygen and speed up chemical reactions. A cell turns a gene into a protein in two steps. First, in transcription, an enzyme reads one strand of the DNA and builds a matching molecule of messenger RNA (mRNA). RNA uses the base uracil (U) where DNA uses thymine. Second, in translation, the cell's protein-making machinery reads the mRNA three bases at a time. Each group of three, called a codon, stands for one of 20 kinds of amino acids or for "stop". The amino acids are joined in that order to make the protein.

You can count why three letters are needed. With two bases per codon there would be only 4 × 4 = 16 combinations, too few for 20 amino acids. With three there are 4 × 4 × 4 = 64, more than enough, which is why several codons often mean the same amino acid. The model on this page starts with ATG, the start codon that stands for methionine. In the View menu, choose Transcription and press Play to watch an mRNA copy grow along the gene.