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

800 Chapter 14: Energy Conversion: Mitochondria and Chloroplasts
THE GENETIC SYSTEMS OF MITOCHONDRIA AND
CHLOROPLASTS
As we discussed in Chapter 1, mitochondria and chloroplasts are thought to have
evolved from endosymbiotic bacteria (see Figures 1–29 and 1–31). Both types
of organelles still contain their own genomes (Figure 14–59). As we will discuss
shortly, they also retain their own biosynthetic machinery for making RNA and
organellar proteins.
Like bacteria, mitochondria and chloroplasts proliferate by growth and division
of an existing organelle. In actively dividing cells, each type of organelle must
double in mass in each cell generation and then be distributed into each daughter
cell. In addition, nondividing cells must replenish organelles that are degraded as
part of the continual process of organelle turnover, or produce additional organelles
as the need arises. Organelle growth and proliferation are therefore carefully
controlled. The process is complicated because mitochondrial and chloroplast
proteins are encoded in two places: the nuclear genome and the separate
genomes harbored in the organelles themselves. The biogenesis of mitochondria
and chloroplasts thus requires contributions from two separate genetic systems,
which must be closely coordinated.
Most organellar proteins are encoded by the nuclear DNA. The organelle
imports these proteins from the cytosol, after they have been synthesized on cytosolic
ribosomes, through the mitochondrial protein translocases of the outer and
inner mitochondrial membrane—TOM and TIM. In Chapter 12, we discussed
how this happens. Here, we describe the organelle genomes and genetic systems,
and consider the consequences of separate organelle genomes for the cell and the
organism as a whole.
The Genetic Systems of Mitochondria and Chloroplasts Resemble
Those of Prokaryotes
As discussed in Chapter 12, it is thought that eukaryotic cells originated through
a symbiotic relationship between an archaeon and an aerobic bacterium (a proteobacterium).
The two organisms are postulated to have merged to form the
ancestor of all nucleated cells, with the archeaon providing the nucleus and the
proteobacterium serving as a respiring, ATP-producing endosymbiont—one that
would eventually evolve into the mitochondrion (see Figure 12–3). This most likely
occurred roughly 1.6 billion years ago, when oxygen had entered the atmosphere
in substantial amounts (see Figure 14–56). The chloroplast was derived later, after
the plant and animal lineages diverged, through endocytosis of an oxygen-producing
cyanobacterium.
This endosymbiont hypothesis of organelle development receives strong support
from the observation that the genetic systems of mitochondria and chloroplasts
are similar to those of present-day bacteria. For example, chloroplast ribosomes
are very similar to bacterial ribosomes, both in their structure and in their
sensitivity to various antibiotics (such as chloramphenicol, streptomycin, erythromycin,
and tetracyclin). In addition, protein synthesis in chloroplasts starts with
N-formylmethionine, as in bacteria, and not with methionine as in the cytosol of
eukaryotic cells. Although mitochondrial genetic systems are much less similar to
those of present-day bacteria than are the genetic systems of chloroplasts, their
ribosomes are also sensitive to antibacterial antibiotics, and protein synthesis in
mitochondria also starts with N-formylmethionine.
Figure 14–59 Staining of nuclear and mitochondrial DNA. In this confocal
micrograph of a human fibroblast, the nuclear DNA is stained with the dye DAPI
(blue) and mitochondrial DNA is visualized with fluorescent antibodies that bind
DNA (green). The mitochondria are stained with fluorescent antibodies that
recognize a specialized protein translocase specific to the outer mitochondrial
membrane (red). Numerous copies of the mitochondrial genome are distributed
in distinct nucleoids throughout the mitochondria that snake through the
cytoplasm. (From C. Kukat et al., Proc. Natl Acad. Sci. USA 108:13534–13539,
2011. With permission from the National Academy of Sciences.)
5 μm