Geology Reference
In-Depth Information
of oxygen to extract energy from food molecules. This arrangement has been working
successfully ever since it was first invented about 2,500 million years ago. Every single
oxygenbreathing cell around today that isn't a bacterium is alive because of it, but the
original bacterial predator has changed so much over time that its true identity was hid-
den from the prying eyes of scientists until relatively recently. The predator in its mod-
ern form is known to science as the mitochondrion (
Figure 38
)—the powerhouse of the
nucleated cell.
Figure 38: A nucleated cell showing its complex organisation, including mitochondria, the des-
cendants of ancient free-living bacteria. (
photo © Science Photo Library
)
Mitochondria are about the same size as bacteria, but they don't look like them. Unlike
the relatively undifferentiated innards of a normal bacterium, the insides of mitochon-
dria are organised into a highly convoluted membrane rather like the high-hedged mazes
one finds in the grounds of old English country houses. Excellent evidence has emerged
from DNA sequencing that mitochondria were once free-living bacteria, for it turns out
that mitochondrial DNA is much more closely related to that of bacteria than it is to the
DNA in the nucleus of the host cell. The association between the mitochondria and the
host nucleus has become so tightly coupled over time that the nucleus now produces
some of the proteins found in mitochondria.
