viruses, for instance, have just a few thousand letters of DNA (or RNA, since some viral genomes contain no DNA) and a small handful of genes. Bacterial genomes, by contrast, are millions of letters long and contain around 4,000 genes. Fly genomes contain around 14,000 genes spread out across hundreds of millions of DNA base pairs. The human genome comprises about 3.2 billion letters of DNA, with around 21,000 protein-coding genes. Interestingly, a genome’s size is not an accurate predictor of an organism’s complexity; the human genome is roughly the same length as a mouse or frog genome,
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