Size isnt everything Sizing up the genome

Different organisms have radically different genome sizes, but not necessarily in the way you might expect. You have a much bigger genome than a bacterium or a mushroom does, and at first, that seems to make a lot of sense. After all, humans certainly appear to be more complex than bacteria or mushrooms. We have a lot more parts — arms, eyes, complicated nervous systems, and so on — so it seems reasonable that our genome would be bigger. Right? Well . . . maybe, and maybe not.

The range of genome sizes varies among several major groups of organisms. Your genome is much bigger than the genome of yeast or Escherichia coli (E. coli) bacteria, for example, yet many plants have more DNA that people do. And just in case your ego hasn't taken enough of a hit, you should be aware that some plants don't just have more DNA than we do, but more genes as well. Poplars, for example, have twice as many genes as people do.

For details about genome size and coding and non-coding DNA, head to Chapter 15.

Base pairs

The unit of genome size is the number of base pairs (abbreviated bp). Why base pairs? It's simple math, really — and understanding how the DNA sequence works. Think about the size of the human genome, which contains about 12 billion nucleotides. Because of the paired structure of DNA (A always pairs with T, C always pairs with G, and so on), when you know the 6 billion on one strand, you automatically know the 6 billion on the other strand. From an information standpoint, only 6 billion independent bits of information exist. As a result, scientists refer to the size of the human genome as 6 billion bp.

Junk DNA

Some DNA doesn't seem to contain much information; it's what scientists call junk DNA. Some junk DNA consists of long sections in which short sequences of nucleotides repeat over and over and over. Both plants and mammals have lots of junk DNA, but plants seem to have more (which explains why the fern hanging in your kitchen has more DNA than you do). That's quite a feat, considering that we humans have a lot of junk ourselves. And just how much junk do we have? As a matter of fact, it appears that more than 95 percent of human DNA doesn't contain any information about how to make a person! Researchers are still trying to figure out why so much of human DNA is junk — a problem that I discuss in Chapter 15.

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