Biology Reference
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must be transcribed, translated, and acquire a sequence that targets the pro-
tein produced in the cytoplasm back into the mitochondrion (
Ryan et al. 1997
).
Furthermore, the new nuclear gene must be properly regulated. On reaching
the mitochondrion, the protein must be properly folded, modified, and assem-
bled into a larger protein complex. Proteins destined for mitochondria are main-
tained in their proper form by
molecular chaperones
, proteins that bind to and
assist in the folding of proteins into their functional states. Chaperones do not
form part of the final protein structure nor do they contain information specify-
ing a particular folding or assembly pathway. Examples of molecular chaperones
include proteins produced by the
Hsp70
and
chaperonin
gene families (
Ryan
et al. 1997
).
A double membrane surrounds each mitochondrion. The inner membrane is
highly invaginated, with projections called cristae that are tubular or lamellar.
These cristae are the sites of oxidative phosphorylation that result in the forma-
tion of
adenosine triphosphate
(ATP), the primary molecule for storing chemical
energy in a cell (
Saraste 1999
). Mitochondrial DNA is a significant component of
the total DNA in insect cells. Approximately half of the DNA in an unfertilized
D. melanogaster
egg is mtDNA.
The coexistence of more than one type of mtDNA within a cell or individual
(
heteroplasmy
) is thought to be rare in natural populations. Paternal mtDNA
either is not transmitted at fertilization, or it contributes only a small fraction of
the mtDNA in the developing embryo, and the paternally derived mitochondria
typically are lost during development. Because insect mitochondria are trans-
mitted from mother to progeny, they are inherited asexually (
Birky 1995
). Until
recently, it was assumed that inheritance of mitochondria was a passive process,
a consequence of their random diffusion throughout the cytoplasm. Now, it
is known that mitochondria are associated with the cytoskeleton and move in
coordinated ways during cell division and differentiation (
Yaffe 1999
).
Mitochondria contain distinctive ribosomes, tRNAs, and aminoacyl-tRNA syn-
thetases (
Gray 1989, Sogin 1997, Kobayashi 1998
). Mitochondria have their own
genetic code that differs slightly from the universal genetic code in the nucleus.
The mitochondrion of
Drosophila yakuba
codes for 37 genes: two genes are rRNA,
22 are tRNA, and 13 are protein genes that code for subunits of enzymes function-
ing in electron transport or ATP synthesis (
Clary and Wolstenholm 1985
,
Figure 3.6
).
The organization and evolution of insect mitochondrial genomes is being derived
from analysis of the complete sequences of mitochondria isolated from an increas-
ing number of species. Sequences of mitochondria have been obtained from many
insects and are deposited in GenBank and other databases.
