|Figure 12-1 A phylogenetic tree based on homologies between
cytochrome c molecules in various organisms. Branch length is
represented by the most likely number of point mutations that
occurred during evolution of these species.
Organisms with a nucleus may have evolved as long ago as 3.5 billion
years, but how the first nuclear membrane arose remains a mystery.
According to the membrane proliferation hypothesis, one or more invaginations
of the plasma membrane in the progenote coalesced internally
to surround the genome, became severed from the plasma membrane,
and formed a double-layered nuclear membrane. The manner of
infolding of the plasma membrane shown in Figure 12-2 accounts for the
fact that the nucleus of modern eukaryotic cells is enclosed within a "double
membrane" consisting of two lipid bilayers. Note that a portion of the
ER is continuous with the outer membrane of the nuclear envelope.
The origin of mitochondria in younger eukaryotes may be explained
by the endosymbiotic theory. Some ancient cells were capable of ingesting
food particles by endocytic invaginations of their plasma membranes.
It is possible that at least one large, fermenting, feeder cell engulfed
one or more smaller respiratory bacteria, but failed to digest them.
This endosymbiont was able to survive in an environment where nutrients
were abundant and it could hide from other predatory cells. In turn,
the host feeder cell gained the energetic advantages of oxidative respiration
over fermentation. These complementary advantages
evolved into a symbiotic ("living together") relationship wherein neither
entity can survive without the other. Part of this mutual adaptation involved the transfer of most of the genes of the bacterial endosymbiont
into the nucleus of the host cell. Most negatively charged molecules, including
mRNAs, tRNAs, rRNAs, and some proteins, that cannot cross
the membrane of these organelles must still be encoded by the genomes
of these organelles. This process is proposed to have given rise to the mitochondria
of modern eukaryotic cells at least 1.5 billion years ago.
|Figure 12-2 Formation of a double nuclear membrane.
Astronger case can be made for the evolution of chloroplasts by endosymbiosis
than that for mitochondria. An aerobic, eukaryotic feeder
cell (one that had already evolved mitochondria) is proposed to have engulfed
one or more eubacteria (related to cyanobacteria) that were capable
of oxygenic photosynthesis. In the process of evolving into chloroplasts,
the endosymbionts relinquished some of their genes to the nuclear
genome, but not as many as did the endosymbionts that evolved into mitochondria.
Like the mitochondria, the protochloroplasts had to retain all
of the genes specifying tRNAs and rRNAs for protein synthesis within
Much evidence supports the endosymbiotic theory for the origin of
chloroplasts and mitochondria. These organelles are approximately the
same size as bacteria. The genomes reside within a single, circular DNA
molecule that is devoid of histone proteins, like bacteria. Both organelles
reproduce asexually by growth and division of existing organelles in a
manner similar to binary fission. Protein synthesis in mitochondria and
chloroplasts is inhibited by a variety of antibiotics that inactivate bacterial
ribosomes, but have little effect on cytoplasmic eukaryotic ribosomes.
Nascent polypeptides in bacteria, mitochondria, and chloroplasts
have N-formylmethionine at their amino ends. Mitochondria and chloroplast
genomes encode the tRNA and rRNA molecules for their own protein-
synthesizing systems. The ribosomes in both organelles resemble
bacterial ribosomes in size and structure. Lastly, the endosymbiotic theory
accounts for the fact that both organelles have double membranes.
The inner membrane corresponds to the plasma membrane of the ancestral
endosymbiont; the outer membrane represents the plasma membrane
of the ancestral feeder host cell.
A type of purple, photosynthetic bacteria that had
lost its photosynthetic ability and retained its respiratory
chain is hypothesized to represent the endocytosed
One theory suggests that the flagella
and cilia of eukaryotes originated
from motile, symbiotic bacteria on
the surface of ancestral eukaryotic