Download Sex and the Simpleton: Evolution of Sex and the Rise of the

Two Main Types of Organisms
Sex and the Simpleton:
Evolution of Sex and the Rise of the Eukaryotes
Prokaryotes (include Archaea and Bacteria),
distinguished by lack of a true nucleus
-term derived from Greek pros meaning before and
karyon meaning nut (referring to the nucleus).
And
Eukaryotes (Eukarya only),
Distinguished by possession of a true nucleus
-term derived from the Greek eus or true and karyon,
meaning nut (again referring to the nucleus)
Prokaryotes
Very simple cells enclosed
by cell wall containing an
inner part of amino acids and
sugars and an outer part of
lipids.
Prokaryotes have a single
“chromosome”, contained
within a “nucleoid region”
rather than a distinct
membrane-bound nucleus
Prokaryotes: Reproduction
Prokaryotes reproduce by the
simple process of binary fission
The cell makes an identical
copy of its genetic material, and
each of the two copies ends up
in each daughter cell
The daughter cells are clones
of their parent
Critical Question: What are the
advantages and disadvantages
of this approach ?
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Eukaryotes: Reproduction
Eukaryotes
In eukaryotes, genetic material is contained within
chromosomes, which are housed in the nucleus
(humans have 23 pairs)
Eukaryotes are complex cells
with a membrane-bound
nucleus, and other structures
such as mitochondria (in animal
and plant cells) and plastids (in
plant cells).
Cells of eukaryotes are also
much larger than those of
prokaryotes
Eukaryotes have two instruction books on reproduction.
We will call these “Mitosis” and “Meiosis”
Note:
“n” denotes
number of
chromosomes
Mitosis involves
replication of genetic
material and splitting to
form clones
Meiosis involves splitting of
genetic material that can
later be recombined (via
sexual reproduction) to
restore full genetic code
Functions of Mitosis and Meiosis in Eukaryotes
Mitosis: the main process involved in
maintaining tissue growth, and to some
extent reproduction (e.g. splitting of
amoebas or asexual budding to make
colonies of clones- e.g. in corals).
Meiosis: the fundamental process
involved in reproduction among
eukaryotes (meeting of cells [sperm and
egg in animals] from two parents
produce offspring)
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Sources of Variation: Asexual vs. Sexual Reproduction
Asexual
Sexual
Mutation
Mutation
Errors in code transcription
Errors in code transcription
Crossing over and trading
of genetic material between
chromosome pairs
Splitting and recombination
of genetic material
Natural Selection acts on these sources of variation to
weed out the bad and retain the good. Sexual reproduction
in eukaryotes ensures lots of variation and lots of change.
But not all eukaryotes are alike
Why do we have plants and animals ?
Answer probably lies in the types of organelles eukaryotes
possess
Origin of Nucleus ?
Making a nucleus was probably not too difficult for
the first eukaryote. It has been speculated that the
nucleus was produced by the infolding of the cell
membrane, and engulfing of genetic material within
the cell. More complex models exist, but this is the
basic idea.
Genetic material in
chromosome-like
clump
Infolded
membrane
surrounds genetic
material
Endosymbiosis Theory
(Are Eukaryotes Composite Prokaryotes ?)
Animals and plants have mitochondria
-the “power plants” of cells
-provide the energy a cell needs to move, divide, and produce secretory
products
-food (sugar) is combined with oxygen to produce ATP (adenosine
triphosphate) - the primary energy source for the cell.
-similar to some bacteria in form and function (quite similar to the
bacterium Rickettsia- responsible for typhus)
Plants have plastids
-the “food factories” of plant cells
-it is here that photosynthesis occurs (energy from sunlight is harnessed to
produce sugar)
-similar to cyanobacteria in form and function
It has been suggested that mitochondria and plastids
are actually bacteria that decided to reside in large
host prokaryotes- additional evidence for this ?
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Note that it is possible for a eukaryote to live
without a mitochondrion (Giardia doesn’t have
one)
Serial Endosymbiosis Hypothesis (Lynn Margulis)
cyanobacteria
S
S
plant cell
animal cell
purple bacteria
Giardia: Single Celled Eukaryote (Protist)
with no mitochondrion
-Likes anaerobic environment
-eats anaerobic bacteria
Evidence for Origin of Mitochondia and Chloroplasts
As “Slave Bacteria”
The basic thoughts on the origin of organelles
-purple bacterium becomes mitochondrion
-cyanobacterium becomes plastid
Perhaps something like this might have become a protoeukaryotic cell upon formation of a nucleus and acquisition of
symbiotic bacteria
Mitochondria and Chloroplasts are of similar size as bacteria.
Mitochondria and Chloroplasts have complex double
membrane systems, similar to bacteria.
Mitochondria and Chloroplasts are somewhat self-contained,
as if they derived from functional cells.
Mitochondria and Chloroplasts divide by binary fission,
similar to bacteria.
Largest known bacterium: 0.2 mm dia
Thiomargarita namibiensis
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Back to Reality…Oldest Evidence of Eukaryotes
Note:
“Ga” means billions
of years before
present
(“giga annum”)
Oldest Known Eukaryote Fossils:
Grypania spiralis
carbonized algal structures 2 mm wide and 10 cm long
Found in rocks 2.1 Ga in Michigan and
abundantly in rocks 1.4 Ga in China, Montana and Michigan
Oldest known Acritarchs (algal cysts)
up to 150 microns (0.15 mm) in diameter
1.6 to 1.4 Ga
Up to about 575 million years eukaryotes remained
relatively simple in form (cysts, blobs, and strands)
Then something remarkable happened…
END OF LECTURE
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