16,569 nucleotide pairs long. 
Although this is less than 1 
percent of the total DNA in a 
human cell, each mitochon- 
drion has enough DNA to 
code for several of its key 
inner membrane proteins and 
its own rihosomal proteins. 
(All of the other proteins in a 
mitochondrion are coded for 
in the nucleus, made on free 
ribosomes in the cytoplasm, 
and imported into the 
organelle.) 
Another curious characteris- 
tic of human mitochondria is 
the fact that all of a person's 
mitochondria are descendants 
of those of his or her mother; 
no paternal mitochondria are 
present. This fact has proved 
useful to evolutionary 
biologists, who can study the 
passage of mitochondrial 
DNA from generation to 
generation while ignoring the 
“interfering” information 
contained in the nuclear 
DNA, which records the 
genetic contributions of both 
parents. 
Scientists have long suspected 
that defects in mitochondrial 
genes could lead to inherited 
disease in the same way that 
mistakes in nuclear DNA do. 
This hunch was not proven 
until 1988, when Douglas 
Wallace of Emory University 
showed that a rare eye disease 
called Leber’s hereditary 
optic neuropathy is caused by 
a mutation in mitochondrial 
DNA. The defective mito- 
chondrial gene prevents the 
optic nerves from producing 
enough ATP and the nerves, 
which need huge amounts of 
ATP and are thus particularly 
sensitive to any deprivation, 
die. When he announced 
these findings, Wallace said, 
“We feel that these alterations 
[in mitochondrial DNA] may 
be responsible for a wide 
spectrum of diseases in the 
brain, the central nervous 
system, and the musculoskele- 
tal system.” 
Mitochondria, chloroplasts, 
and the other organelles 
described thus far are 
surrounded by membranes. 
But cells can also contain 
threadlike organelles that 
lack membranes. These 
extremely fine structures 
serve as buttresses, highways, 
and movement mechanisms 
for the cell. 
39 
A mitochondrion is 
shown us if it hud been 
sliced longitudinally. 
The inward folds of the 
