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Lecture 10

Biology 1002B Lecture Notes - Lecture 10: Genome Size, Nuclear Membrane, Nuclear Dna

Course Code
BIOL 1002B
Tom Haffie

of 3
Lecture 10: Evolution of Eukaryotes
Friday, February 15, 2013
5:41 PM
Archaea don't show very much morphological complexity, not as much as eucarya
Bacteria can perform interesting biochemical functions, but this doesn't reflect intense
The paradox
All morphologically complex life is eukaryotic
All eukaryotes share common complex traits
Nucleus, trafficking, cytoskeleton, sex, phagocytosis, organelles
Prokaryotes show no tendency at all to develop these types of morphological/cellular
Why don't they if evolution is stepwise and gradual
What happened early on in the bifurcation in the tree of life that lead to only one domain
being where all the complexity happens?
What drove the evolution of eukaryotes?
Oxygen is the key, the ability to use oxygen was the drive
Earliest bacteria were anaerobic
Uses glycolysis and fermentation to make ATP (this does not make very much
ATP, so they didn't have enough ATP for major functions…)
2.2 bya cyanobacteria developed and they were able to use oxygenic photosynthesis to
split water into oxygen and protons
Produced oxygen and evolved oxygen and released it into the atmosphere
This eventually increased the concentration of oxygen in the atmosphere
Bacteria that undergoes aerobic respiration
Lots more ATP was produced and this allowed for more functions on the part of
the cell
Oxphos in bacteria
Archaea and bacteria are 1000s of times smaller than eukarya
The ETC and oxphos occur on the plasma mm'n (only on the plasma mm'n)
As the bacteria grows bigger, the surface area to cell volume ratio becomes greater
Not enough ATP can be produced per surface pn for the volume of the cell
This limits the size of the bacterium and thereby limits the number and complexity of
functions that the bacterium can perform
Eukaryotic cells - more energy
Eukaryotes are much bigger so they have a low plasma mm'n surface area/volume ratio
But the plasma mm'n is not the site of oxphos for eukarya, they have mitochondria
each with many oxphos centers
The internal mm'n structure is much larger than in bacteria
Lots of energy is produced
This allows the cell to support a larger genome and thereby lots of other functions that
are dictated by the genome
DNA replication = 2% cell energy, protein synthesis = 75% cell energy (prokarya don't
have enough ATP to synthesize novel pns
Bacteria = 13000 ribosomes, Eukarya = 13000 x 10000 ribosomes
Greater diversity of genome size as well
Ancestral bacterium
Develops the endomembrane system derived from the infolding of the plasma mm'n
forming the nuclear envelope and the ER
The ability to separately and tightly regulate transcription and translation
allowing for specialized control; something that is not possible in bacteria
Theory: mitochondria and chloroplasts were derived bya from free-living cells =
Modern day mitochondrion used to be an ancestral aerobic bacterium
Ancestral anaerobic would be at an advantage to take in mitochondrion (aerobic
bacterium) and chloroplasts (cyanobacterium) through phagocytosis
The uptake of aerobic bacterium occurred before that of cyanobacterium as suggested
by the fact that all cells have mitochondrion but not all have chloroplasts
Evidence of endosymbiosis
Mitochondria look like bacteria, look like E.coli
Mitochondria/chloroplasts divide using very similar processes as bacteria within
the cell
No mitochondria/chloroplast gene in the nucleus
Electron transport chains
Free living cells need this so mitochondria/chloroplasts should have this
Other organelles don't have ETC
They have their own genomes
Have their own genetic information
Transcription and translation machinery
No dependent on any other cell for transcriptional and translational machinery so
can be free-living
Lateral Gene Transfer
The proto-mitochondria was free-living with its own DNA info, but after being engulfed,
its info conflicted with that of the cell's own nucleus
The mitochondria has be totally coordinated on the cellular level with the rest of the
cell (glycolysis….other functions…)
One of the strategies to cope with this
Over millions of years, the organellar genes from the mitochondria have
relocated to the nucleus = lateral/horizontal gene transfer
Genes which had originally been in the mitochondria/chloroplast moves into
nucleus and the control was obtained by nucleus to develop the more integrated
Detecting lateral gene transfer
Southern blot
Single stranded Genomic DNA is run on a gel and hybridized with single-stranded probe
is used to identify the presence of a particular gene
Isolate mtDNA and nDNA and run it on the gel
Note: hgt is still did not stop
Mitochondrion-less eukaryotes
There are euk. That don't have mito
They causes diseases in humans (Giardia, trichomonas)
Evolutionary intermediates?
Cpn60 is an essential mitochondrial pn whose genes are in the nucleus (hgt)
Giardia doesn't have mito but has cpn60
The mitochondrial gene is in the nucleus shows that the cell has a mito at one
point but then lost it and the spn60 no longer has any function in the cell even
though its still there
Ancestor has mito and then this split into two groups
Giardia like orgs that lost their mito
Other eukaryotes that kept their mito