Where in the world is your genome, anyway? In two different places within your cells: the nucleus and the mitochondria.
1 In the nucleus: Your body is made of cells that have a structure in the middle called the nucleus. This is where most of your genetic material resides. The nucleus isn't just one long piece of DNA, but an arranged series of individual pieces called chromosomes (refer to the earlier section "Chromosomes: Where your DNA is" for information on what chromosomes do).
In the mitochondria: Another structure in your cells contains DNA. This structure is called mitochondria, and each of your cells contains several dozens to hundreds of these structures. Mitochondria serve as the power plants of the cell; they're involved in metabolism and energy production.
Mitochondrial DNA doesn't contain many genes, but it's fascinating that mitochondria have any DNA at all. Why do these small things inside your cells have their own genome, and how did they get there? In later chapters, I get back to mitochondria, examining how they evolved and looking at what various studies of mitochondrial DNA tell scientists about human history. For now, just remember that you have some mitochondria and that your mitochondria have some DNA.
Not all organisms have cells with nuclei. In bacteria, the genetic material is happy to float around inside the cell, hanging out with everything else, and doesn't need its own special home. Furthermore, bacteria tend to keep their genome in one piece. It probably would be biochemically complicated for humans to have their entire genome in one segment, as the DNA would be extremely long. Bacteria can get away with keeping their genome together only because their genomes are much smaller. But small things don't necessarily have genomes that are just one piece; many viruses store their genomes in sections.
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