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Everyone
has the dream where you show up for a class
that
you didn’t know was on your schedule, only to be
having
a test. But in second place is the dream where you
are
back in elementary school, or maybe the principal’s
office.
Above is a picture of every teacher I had in
elementary
school.
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But your study of word roots may help you survive. Homo- means same, while -nym means word. OK, it’s coming back to
you. Homonyms can be words that are spelled but have different meanings and
origins (called homographs) or words that are pronounced the same but have
different spellings and meanings (called homophones). Yes! The class cheers, and Mrs. Belcher
is more than mildly surprised. Crisis averted.
I have no idea how you got transported back to grammar
school, but your question and answer is very timely to our discussion today. Homographs,
like minute (min-it, a short time)
and minute (my-noot, a small
amount), look the same, but have different meanings. Homophones, like to,
too, and two sound the same but
mean different things and have different origins. This is very much like the differences in prokaryotic
flagella and eukaryotic undulipodia. Too much of a stretch for you... maybe.
We have seen that bacterial flagella are long, whip-like
structures protruding from the cell that can aid in motility. So are eukaryotic
undulipodia. They look very similar, yet we are going to see they have very
different structures, mechanisms of function, and origins – just like homophone
and homonyms. So maybe the analogy
wasn’t so far off.
Both flagella and undulipodia extend from the cell surface
with a long tail. But in the prokaryote, this was made of small subunits of flagellin proteins. In the undulipodia, the structure is called the axoneme, and is made of long microtubules of tubulin protein.
Already we have significant differences between two things that look very
similar.
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A
cross section of an undulipodium axoneme looks like
this
– although I’m not sure they’re color coded in the
cell.
Notice how the inner are connects the nested tubule
of
one doublet to the outside tubule of the next doublet.
The
inner arms are responsible for the degree of bend,
the
outer arms are involved in the rate of movement. There
are
other proteins involved, but we aren’t getting that
detailed.
Maybe you want to do that on your own.
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The way the axoneme is built is also the key to how it
works. The different microtubule doublets are cross-linked by protein complexes
called dynein arms. There are inner
arms and outer arms. An inner arm connects one microtubule from one doublet to
another microtubule of the adjacent doublet. When one doublet slides further
out from the cell body and the connected adjacent doublet doesn’t, this creates
stress and the whole thing must bend to maintain the connections.
So, walking proteins are how the undulipodia create their
whip-like action. There is a walking system analogous to dynein arm movement
that has developed in many animals. We have looked at muscle contraction before. Just like myosin heads walking along actin filaments that are anchored
to the muscle cell membrane, the dynein arms on some doublets start to crawl up
or down the adjacent microtubule of an undulipodia creating a bend and then
they can reverse to whip back the other direction. This is very different from
the spinning motor of the bacterial flagellum.
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Pay
attention to these cartoons, they show how the undulipodia
bend.
ATP powers the sliding of the dynein arms. As they move
down
one tubule, that filament moves up. If the filaments are
anchored
in the membrane, as they are in undulipodia, the
movement
creates tension and a bend. The cartoon on the
right
is a good summary.
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The undulipodium motion often occurs in just one plane, back
and forth instead of all around, but that doesn’t mean it has too limit itself
to that. It can spin too; it just takes a very coordinated sliding back and
forth of microtubules.
Another difference between prokaryotic flagella and
eukaryotic undulipodia is in how they are powered. We saw that flagella in bacteria get their force from the spinning motor, and the motor gets its energy
from an ion gradient across the membrane. But in eukaryotes, it was seen early
on that if you strip the membrane off of an undulipodium, and added ATP,
they start to move.
Yes, all undulipodia are held within the membrane. Some bacterial flagella are membrane
covered, but all eukaryotic versions
are sheathed in plasma membrane. But back to the ATP. Exposing the naked undulipodium
to ATP, even on a dead cell, can initiate the dynein walking and microtubule
sliding, so it is definitely ATP powered.
Also, this points out that the power for the bacteria
movement comes from the motor in the base, but the eukaryotic movement is in
the axoneme, not the base. The basal body that anchors the eukaryotic
undulipodium into the cell membrane is amazing in its own way. The basal body
is actually a centriole, the same structure that helps to move the chromosomes apart in the spindle apparatus during mitosis. We’ll come back to this double duty organelle in a a few
weeks.
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Chlamydomonas
algae species have a double flagella
for
swimming. A single stroke as shown above. The
power
strokes on top is followed by the recovery stroke
below.
Put all the numbers together and looks like
someone
swimming underwater. The picture of the
organism
is there because it’s always better to see the
real
thing as compared to a cartoon.
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But soon after that, the protozoans were
discovered to have undulipodia as well. Organisms from the algae genus Chlamydomonas have two long undulipodia
that they use for motility. Located at the front of the cell, their movement
pulls the alga through the water. But protozoans are just as likely to have
undulipodia that push them through water. They can work both ways.
Amazingly, when a Chlamydomonas finds itself out of water,
the undulipodia resorb in short order. Nature hates to waste energy, so why
maintain a boat motor if you’re not in the water. But place them back in a
liquid environment and the two structures will reassemble with in an hour – with the same structure 9(2) + 2 and working the same exact way.
Of course, saying they all have the same 9(2) + 2 structure
is an invitation to find exceptions, and science has found them. A 2006 study
found that rabbits are quite the rule breakers. Sure, they have 9(2) + 2 axonemes,
but they don’t stop there. Some rabbit embryo undulipodia show a 9(2) + 0 structure,
where the central doublet is completely missing, yet the structure functions
just as other motile undulipodia.
What’s more, rabbits can also have 9(2) + 4 axonemes, with
double the number of central microtubules. Again, they function just fine. Is
there a reason for these variations – maybe, but maybe they are just mutations
that didn’t have a negative impact, so they were retained.
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You
know how annoying it is when you touch a cactus
and
those little bristles get stuck in your skin? Well,
don’t
touch a bristleworm. They’re lined with those painful
bristles
– hence the name. Somebody studied these worms,
and
found out they have parasitic protozoans in their gut.
Then
someone studied those parasites and found that they
have
sperm. And someone studied the sperm and found
that
they have unique axoneme structures. I love science.
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The undulipodium basal body (born as a centriole) can have
exceptions as well. The vast majority have a structure of 9(3) + 0, where
instead of doublets, they have triplets. This makes sense since they need to be
strong to support the axoneme.
But diatoms, very small algae cells protected by a silica
shell, can have sperm that look very different, according to a 2013 study. Their
basal bodies have been observed to have doublet microtubules, and are very
similar to the axoneme. Even weirder, a couple of insects feel the need to go
big with their basal bodies. Acerentomonon
microrhinus, a primitive hexapod insect has sperm tail basal bodies with 14
microtubule doublets, while Sciara
coprophila, a fungus gnat (see picture), has up to 90 singlet microtubules in its sperm
basal body.
We have talked a lot about sperm tails and protozoan
motility structures, and these undulipodia look the most like bacterial
flagella. But undulipodia come in a couple of flavors; those longer than 40 m
or so are called flagella while the shorter ones are called cilia. See the
naming problem and why Lynn Margulis came up with undulipodia?
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This
is a fungus gnat – it sounds like they eat fungus,
but
nope. Only the larval form feeds; the adults never
eat.
That’s OK, they don’t live very long at all. They
overwinter
in their adult form because they are unique
in
that they can both tolerate freezing weather and freeze
themselves
without damage. It must be important, since
their
sperm tails are loaded with 90 microtubules in the
basal
body – everyone knows that massive numbers of
microtubules
is the best way to avoid cold damage.
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Next week – Halloween is coming, so what better time to have a discussion of genetically
modified foods and an 19th century teenage girl who wrote the best
science fiction book ever.
For
more information or classroom activities, see:
Undulipodia
–
Inteeresting thoughts
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