DNA is the instruction manual stored in almost every cell. A gene is one section of it that holds the code for a protein or a working RNA.
DNA structure
DNA is a double helix built from four bases: adenine, thymine, guanine and cytosine. A pairs with T and G pairs with C, so each strand can serve as a template to copy the other.
Chromosomes
Human cells hold 46 chromosomes in 23 pairs, one of each pair from each parent. The 23rd pair sets sex: XX or XY.
Transcription
In the nucleus, a gene is copied into messenger RNA (mRNA). The mRNA then leaves the nucleus and travels to the ribosomes.
Translation
Ribosomes read the mRNA three bases at a time, and each of these codons calls for one amino acid. The finished chain of amino acids folds into a working protein.
Mutations and inheritance
A mutation is a change in the DNA sequence that can alter or stop a protein. Some are inherited in dominant, recessive or X-linked patterns, while others arise during life, as in most cancers.
At the bedside
A careful family history across three generations can reveal inherited patterns, such as several relatives with early heart disease or breast cancer. Drawing it as a pedigree makes the pattern easy to see.
Related guides
Written by Making Synapses. Topic reference: OpenStax, Anatomy and Physiology 2e (no text adapted).