Carl Zimmer in his excellent blog, The Loom:
The human genome contains about 3.2 billion base pairs. Last year,scientists at the University of Leceister printed the sequence out in 130 massive reference-book-sized volumes for a museum exhibit. From start to finish, they would take nearly a century to read.
A typical gene is made up of a few thousand bases. The human genome contains about 21,000 genes that encode proteins. There are other genes in the human genome that encode molecules known as RNA, but how many of those RNA molecules actually do anything useful in the cell is a matter of intense debate. A lot of the human genome is made of neither protein- or RNA-coding genes. Much (maybe most) of it is made up of dead genes and parasite-like stretches of DNA that do little more than making copies of themselves.
As I wrote recently in the New York Times, 3.2 billion base pairs and 21,000 genes are not essential requirements for something to stay alive. E. coli is doing very well, thank you, with a genome about 4.6 million base pairs. That’s .14% the size of our genome. Depending on the strain, the microbe has around 4100 protein-genes. That’s about a fifth the number of protein-coding genes that we carry. The high ratio of genes to genome size in E. coli is the result of its stripped-down, efficient genetics. Mutations that chop out non-functional DNA spread a lot faster in microbes than in animals.
E. coli, in turn, has proven to be positively gargantuan, genetically speaking, compared to some other species.
More here.