mammalian cells. Human mitochon- 
drial DNA is a closed, circular mole- 
cule 16,569 nucleotide pairs long. 
Although this is less than 1 percent of 
the total DNA in a human cell, each 
mitochondrion has enough to code 
for several of the key inner membrane 
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 characteristic of 
human mitochondria is the fact that all 
of a person's mitochondria are descen- 
dants of those of his or her mother; 
no paternal mitochondria are present. 
This fact has proved useful to evolu- 
tionary biologists, who can study the 
passage of mitochondrial DNA from 
generation to generation while ignor- 
ing the "interfering" information con- 
tained 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 
mitochondrial gene prevents the optic 
nerves from producing enough ATP, 
and the nerves, which need huge 
amounts of ATP and are thus particu- 
larly 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 
musculoskeletal 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 motile 
mechanisms for the cell. 
39 
