About 1.5% of the human genome codes for proteins. This small fraction consists chiefly of coding sequences within exons, the parts of genes retained in mature messenger RNA and translated into proteins.
The human genome contains roughly 3 billion DNA base pairs and around 20,000 protein-coding genes. Although those genes are essential for building and operating cells, their protein-coding portions occupy only a tiny slice of the genome. Most gene regions also contain introns, which are transcribed but removed before translation, so the full span of a gene is much larger than its coding sequence.
The remaining DNA is not simply useless “junk.” It includes regulatory regions that control when genes are active, genes for non-coding RNAs, repetitive sequences, centromeres, telomeres and other structural elements. Scientists still debate how much of this non-coding DNA has a defined biological function.
A common mix-up is confusing protein-coding DNA with the total size of protein-coding genes. Including introns and nearby regulatory sequences would produce a much larger percentage; the approximately 1.5% figure refers specifically to DNA that directly encodes protein sequences.