Biology 1200b – Test Two
Lecture 10 – Evolution of Eukaryotes
All morphologically complex life is eukaryotic. All eukaryotes share common complex traits – nucleus,
trafficking, cytoskeleton, sex, phagocytosis, organelles. Prokaryotes show almost no tendency to
develop this kind of complexity. If evolution through natural selection is gradual then why don’t these
simple organisms evolve?
The evolution of eukaryotes was driven by oxygen. The earliest bacteria were anaerobic (four billion
years ago) and as such used fermentation to continue glycolysis. This produces very little ATP while still
breaking down glucose.
About 2 billion years ago cyanobacteria evolved. These bacteria can split H2O into oxygen in a process
called oxygenic photosynthesis. They are the only single celled organisms that can evolve CO2 and are
responsible for oxygen in the atmosphere. This led to aerobic respiration.
Oxidative phosphorylation in bacteria occurs on the cellular membrane. The problem with this is that
the larger a bacteria becomes (more centers of phosphorylation) the more proteins and energy is
required for support. Volume increases much faster than surface area and soon the cell will not be able
to support its own volume.
Eukaryotic cells have very small plasma membrane surface area to volume ratio. This is not a problem
since these cells have mitochondria internalized. Eukaryotes as such have far more energy and therefore
code for many more proteins than prokaryotes. Genome size varies by huge amounts amongst
eukaryotes but is far greater than prokaryotes (eukaryotes have the ATP to support it).
Primitive cells developed an endoplasmic membrane (reticulum) from the infolding of the plasma
membrane. Cells developed a nuclear envelope to separate DNA from the rest of the cell in order to
control transcription. Cells then took in mitochondria (and later chloroplasts) through endosymbiosis.
Mitochondria are thought to have formerly been an aerobic bacterium. Chloroplast would have been
Evidence for endosymbiosis – Morphology (mitochondria and chloroplast look like cyanobacteria),
formation (division of these organelles is very similar to bacteria), electron transport chains (only
chloroplast and mitochondria have these components), genomes (mitochondria and chloroplast have
their own genomes and can synthesise proteins).
The mitochondria must become fully part of the developing cell. One belief is that over millions of years
genes have moved from the organelles to the nucleus in a process called lateral gene transfer. The
function of the transferred gene is still retained.
Southern blotting is used to test if a genome has a particular gene and how many copies. Genomic DNA
is isolated and ran on a gel. A DNA probe hybridizes with the single stranded DNA to indicate that this
sequence is similar to the probe. Thus a certain gene can be detected. DNA can be isolated from the
mitochondria and run on a gel. A protein that is mitochondrial can be tested as being coded from either
the mitochondrial or nuclear genome.
Some eukaryotes do not have mitochondria. These tend to cause disease and are dangerous. These are
not evolutionary intermediates between bacteria and eukaryotes. cpn60 is a mitochondrial protein that
is absolutely essential and is still found in giardia, a mitochondrial-less eukaryote. This shows that
giardia evolved from mitochondrial eukaryotes and that lateral gene transfer of cpn60 occurred very
early in evolution.
Lecture 11 – Intro to Prokaryotic Genes
Modern endosymbiotic genomes are greatly diminished. In Chlamy there are 120,000 base pairs in the
nucleus, 200 in the chloroplast and 16 in the mitochondria. Both the chloroplast and mitochondria have
circular DNA. E.coli, a prokaryote, has far more kilobases (5000) than either chloroplast or mitochondria.
Not all genes code for proteins. Some code for RNA (tRNA, ribosomal). Mitochondria and chloroplast
have so few protein coding genes because they have no need for movement or enzymes like helicase
that now occur in the cytoplasm of the cell. Redundant genes are removed by mutation and deletion or
lateral gene transfer to decrease genome size as much as possible.
Lateral gene transfer occurs due to coordinate control (allows the nucleus more control over the cell)
and because DNA is favourably removed from organelles where ROS (reactive oxygen species) are
created and liable to harm the genetic material.
Nuclear DNA is different than mitochondrial in that it undergoes sexual recombination. This allows it to
have a greater level of diversity and is thus more favourable.
Why haven’t ALL organelle genes moved to the nucleus? Various reasons – It is too difficult to transfer
from mitochondria to the nucleus, some local control might be necessary for function, some genes are
too large and changes might be required for a gene to exist in a eukaryotic environment rather than a
Elysia are sea slugs that feed on algae called Vaucheria. They have no cell walls and are coenocytic,
meaning a sea slug can simply bite off the end and absorb the entire cytoplasm of the algae. The
chloroplasts from the algae enter the digestive lining of the sea slug and undergo photosynthesis
meaning elysia is an autotrophic animal.
mRNA (and tRNA) pairs with itself (complementary base pairing) to form structure by folding and
bending. Ribosomal RNA base pairs with itself and is catalytic and structural. DNA polymerase is
interesting because it replicates the genes that code for itself.
In a gene the promoter is a sequence that signals for RNA polymerase when to start transcription.
All eukaryotes share common complex traits nucleus, trafficking, cytoskeleton, sex, phagocytosis, organelles. Prokaryotes show almost no tendency to develop this kind of complexity. The evolution of eukaryotes was driven by oxygen. The earliest bacteria were anaerobic (four billion years ago) and as such used fermentation to continue glycolysis. This produces very little atp while still breaking down glucose. These bacteria can split h2o into oxygen in a process called oxygenic photosynthesis. They are the only single celled organisms that can evolve co2 and are responsible for oxygen in the atmosphere. Oxidative phosphorylation in bacteria occurs on the cellular membrane. The problem with this is that the larger a bacteria becomes (more centers of phosphorylation) the more proteins and energy is required for support. Volume increases much faster than surface area and soon the cell will not be able to support its own volume. Eukaryotic cells have very small plasma membrane surface area to volume ratio.