Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

802 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
nucleus
RNA
13 mtDNAencoded
proteins
TOM
TIM
cytoplasmic
ribosome
~1000 different
nuclear-encoded
mitochondrial proteins
mitochondrial DNA
(mtDNA)
discussed in detail in Chapter 18, during apoptosis the mitochondrion releases
proteins (most notably cytochrome c) from the crista space through its outer
mitochondrial membrane, as part of an elaborate signaling pathway that is triggered
to cause cells to undergo programmed cell death.
The Fission and Fusion of Mitochondria
MBoC6 n14.329/14.61
Are Topologically Complex
Processes
In mammalian cells, mitochondrial DNA makes up less than 1% of the total cellular
DNA. In other cells, however, such as the leaves of higher plants or the very
large egg cells of amphibians, a much larger fraction of the cellular DNA may be
present in mitochondria or chloroplasts (Table 14–2), and a large fraction of the
total RNA and protein synthesis takes place in the organelles.
Mitochondria and chloroplasts are large enough to be visible by light microscopy
in living cells. For example, mitochondria can be visualized by expressing in
cells a genetically engineered fusion of a mitochondrial protein linked to green
RNA
mitochondrial
ribosome
Figure 14–61 Biogenesis of the
respiratory-chain proteins in human
mitochondria. Most of the protein
components of the mitochondrial
respiratory chain are encoded by nuclear
DNA, with only a small number encoded by
mitochondrial DNA (mtDNA). Transcription
of mtDNA produces 13 mRNAs, all of
which encode subunits of the oxidative
phosphorylation system, and the 24 RNAs
(22 transfer RNAs and 2 ribosomal RNAs)
needed for translation of these mRNAs on
the mitochondrial ribosomes (brown).
The mRNAs produced by transcription
of nuclear genes are translated on
cytoplasmic ribosomes (green), which
are distinct from the mitochondrial
ribosomes. The nuclear-encoded
mitochondrial proteins (dark green) are
imported into mitochondria through two
protein translocases called TOM and
TIM, and constitute the vast majority
of the approximately 1000 different
protein species present in mammalian
mitochondria. The nuclear-encoded
mitochondrial proteins in humans
include the majority of the oxidative
phosphorylation system subunits, all
proteins needed for expression and
maintenance of mtDNA, and all proteins of
the mitochondrial ribosomes.
The mtDNA-encoded subunits (orange)
assemble together with the nuclear
subunits to form a functional oxidative
phosphorylation system. (Adapted from
N.G. Larsson, Annu. Rev. Biochem.
79:683–706, 2010.)
Table 14–2 Relative Amounts of Organelle DNA in Some Cells and Tissues
Organism Tissue or cell type DNA molecules
per organelle
Mitochondrial DNA
Organelles
per cell
Organelle DNA as percentage
of total cellular DNA
Rat Liver 5–10 1000 1
Yeast* Vegetative 2–50 1–50 15
Frog Egg 5–10 10 7 99
Chloroplast DNA
Chlamydomonas Vegetative 80 1 7
Maize Leaves 0–300** 20–40 0–15**
*The large variation in the number and size of mitochondria per cell in yeasts is due to mitochondrial fusion and fission.**In maize, the amount of
chloroplast DNA drops precipitously in mature leaves, after cell division ceases: the chloroplast DNA is degraded and stable mRNAs persist to
provide for protein synthesis.