BLAST

This is the Title of the Book, eMatter Edition

Copyright © 2012 O’Reilly & Associates, Inc. All rights reserved.

Genomes and Genes

|

35

teria. So the next time you munch on a carrot, you might consider how many

genomes are really in there.

So far, this chapter has neglected viruses. Where do they fit in? By most definitions,

viruses aren’t even alive; they don’t grow or have repair processes. Viruses seem to

break every rule of biology. Some viruses infect prokaryotes and others that parasit-

ize eukaryotes. Viruses come in many different shapes and have wildly different life-

styles. Some have genomes made from RNA instead of DNA, and others have single-

stranded rather than double-stranded genomes.

Genomes and Genes

In general, the genomic structure of prokaryotes is very different from that of eukary-

otes (Figure 2-5). Genomes are organized into chromosomes. Prokaryotes often have

a single circular chromosome, and eukaryotes usually have multiple linear chromo-

somes. People are sometimes surprised to find that genome size and chromosome

number aren’t reflected in organismal complexity. For example, the single-celled

Amoeba dubia has a genome that is about 200 times larger than the human genome.

Although dogs and cats have very similar genome sizes, dogs have twice as many

chromosomes. One rule to keep in mind when thinking about genomic organization

is that genomes of viruses and prokaryotic ...