“Take Two and Call Me in the
Morning”
A Case Study in Cell Structure and Function
by
Peggy Brickman
University of Georgia
Part I - Mysterious Illness
• Well, Becky thought, being a dorm counselor for
freshmen was not going to be that bad. She got a free
room for the year and the food was plentiful - free
steaks last week at an outdoor BBQ followed by a hay
ride in a horse-drawn wagon in their welcome
celebration.
• But, then again, it wasn’t perfect: she had ended up
covered in bug bites; some of the students got sick
from eating steak that was burned on the outside and
raw in the middle; the horses had mucked up the
courtyard; and pigeons had roosted on the dorm roof.
• At least tonight the students were finally settling in and
quieting down, she mused.
2
Part I, continued…
• The quiet was shattered a few minutes later when one
of the other counselors, Ann, yelled through her door:
• “Becky, we’ve got a problem. One of the students
found a homeless kitten, and the girl has been
keeping her in her room. I only found out because the
girl, Ellie, just came to my room complaining of being
sick. I felt sick too when I saw the mess that kitten
made. I thought cats were born housebroken, but I
guess not.”
• “Anyway, now I think Ellie might really be sick,” Ann
continued. “She’s feverish and says she’s going to
throw up.”
3
Part I, continued
• “What do you want me to do?” Becky asked.
• “I’m freaking out!” Ann answered. “Forget about the
mess, just help me figure out what to tell them at the
health center. I don’t know what she’s been exposed to.
Or what we’ve been exposed to for that matter! This is
the second girl this week with aches, fever, and
nausea.”
• “My Mom sent me a bunch of medicine,” Becky
answered. “I’ll make a list of where we’ve been, what
we’ve eaten, and what we’ve possibly been exposed to.
Then we can start taking something right away to keep
from getting it, too.”
4
Becky’s Task
•
The Health Center will be using the
differences between organisms to diagnose
and treat Ellie.
•
In the next 2 minutes, list the clues in the
story that help you identify how Ellie could
have contracted a disease with flu-like
symptoms. Come up with possible
suspects (organisms) that could cause her
to be sick.
5
Your Task
•
Becky did an Internet search and found 5 possible
suspects that could be causing Ellie’s illness.
•
During this class session we will investigate the
differences between them.
•
Organisms are usually distinguished by the
characteristics you listed in your homework answers
to Table 1. Add any details you missed so that
when you hear the results of the Health Center tests
you will be able to figure out what was making Ellie
sick.
•
Fill in possible drug treatments in Table 2.
6
Ellie’s Diagnosis
• Initial Identification: The Health Center collected
blood samples from Ellie and observed her cells
under a microscope. They identified foreign
structures with DNA and outer membranes. The
cells were gram negative and about 1/10 the size of
her cells.
• “Ah, ha!” said Becky. “That matches one of my
suspects. I knew those were a health hazard. I just
need to re-check the size thing. This internet chart
compares our cells to viruses and stuff.”
7
Metric Review
10-3
1 mm
Cells like
ours
10-4
meters
• 1 meter (m) = ~3 feet
• 1 meter (m) = 1000
millimeter (mm)
• 1 millimeter (mm) = 1000
micrometer (µm) (smallest
size distinguished by naked
eye)
• 1 micrometer (µm) = 1000
nanometer (nm) (only seen
with light microscope)
• 1 mm poppy seed = (1000
µm/mm) = 1000 µm
10-5
10-6
Bacteria
1 µm
10-7
Viruses
10-8
Proteins
10-9
1 nm
Atoms
8
Becky’s Internet Search
Results – List of Suspects
Suspect 1: Coxiella
burnetii causes Q-fever.
Coxiella are often found in
livestock and are excreted in
milk, urine, and feces. Infection
occurs 2-3 weeks after
inhalation of barnyard dust.
They are 0.3-0.5 µm gramnegative bacterium
(prokaryotes) that must invade
and reside inside human cells
to cause infection.
1µm
9
Prokaryotes
• Unicellular
• Reproduce asexually
• Composition
– Protected interior (cytoplasm) that
contains genetic material (one circle of
DNA) as well as complexes of protein
enzymes to carry out necessary functions
of gathering energy, manufacturing
proteins (ribosomes), etc.
10
Prokaryotes
• Size
– 0.2-10 micrometer (µm)
• Composition
– Phospholipid membrane, many contain
cell wall composed of peptidoglycan
(positive for chemical Gram stain), those
with little or no peptidoglycan called Gram
negative (like Coxiella).
11
CQ1: “That’s great,” Becky said. “My Mom sent
me 3 different antibiotics to kill bacteria.”
Given the description of Ellie’s test results,
which antibiotic will definitely NOT work:
A: Amoxicillin, Penicillin, and other ß-lactams
– Blocks the enzyme that normally creates links in
peptidoglycan molecules.
B: Streptomycin
– Blocks prokaryotic ribosomes.
C: Ciprofloxacin hydrochloride (Cipro)
– Blocks bacterial DNA gyrase enzyme needed to
counteract excessive twisting of DNA that occurs
when circles of DNA are unwound to be copied
into DNA or RNA.
12
CQ2: “Wait a minute!” Ann said. “The doctor
said the blobs in Ellie’s blood were 1/10th the
size of her cells. Could they be Coxiella?”
10-3
1 mm
10-4
B: No
10-5
Bacteria
meters
A: Yes
Cells like
ours
10-6
1 µm
10-7
Viruses
10-8
Proteins
10-9
1 nm
Atoms
13
Part II: Microscope Analysis
Becky and Ann talked together outside the student’s room at
the student health center the next morning.
“You’re right!” Becky exclaimed after viewing photographs of
Ellie’s blood up close. “I wish I hadn’t started taking the
antibiotics. The little crescent shaped structures that I thought
were the bacteria may not be. When you zoom in on them,
they show up clearly in the electron micrograph on the right.
They aren’t too big to be bacteria, but they aren’t too small to
be mitochondria or some kind of protozoan parasite.”
“Wait a minute,” Ann replied. “The things on the right are the
pathogens? Look at their insides, they can’t be bacteria.”
“Why not?” Becky asked.
14
Part II, Continued
1µm
15
CQ3: “Well,” Becky admitted, “there should be differences
between Ellie’s cells and the little blobs they saw.
Otherwise, it might mean one of my other suspects is the
cause. These are some of the structures normally found in
all cells.”
“No,” Ann answered, “one isn’t.”
Which structure is NOT found in all cells?
A: Cytoplasm
B: DNA
C: Outer phospholipid membrane
D: Ribosomes
E: Membrane-bound organelles
16
Eukaryotes Prokaryotes
DNA
Size
Organization
linear strands within
membrane-bound nucleus
single circle in “nucleoid
region
5-100 µm
0.2-10 µm
often multicellular, some have
usually single-celled,
cell walls (no peptidoglycan) some have peptidoglycan
cell walls
Metabolism
usually need oxygen to exist
may not need oxygen to
exist
Organelles
membrane bound organelles
like mitochondria
no organelles, different
ribosomes
Examples
plants, animals, protists, fungi
bacteria, archaea
17
Becky’s Internet Search
Results – List of Suspects
Eukaryote - Suspect 2:
Cryptococcus neoformans
2.5-10 µm encapsulated fungus found
in decaying pigeon or chicken
droppings. Inhaled as spores that
eventually spread to the brain causing
meningoencephalitis. Has a black
pigmented layer that can be seen
sometimes on bird seed.
18
Becky’s Internet Search
Results – List of Suspects
Eukaryote - Suspect 3:
Toxoplasma gondii
• 4-6 µm single-celled
protozoan parasite of
mammals & birds.
• Most likely acquired through
ingesting cysts in
undercooked meat.
19
Becky’s Internet Search
Results – List of Suspects
Suspect 3:
Toxoplasma gondii
• Usually no symptoms, but
can cause flu-like
complaints.
• Sexual life cycle occurs in
cats, so infection can follow
contact with cat feces.
• See QuickTime movies
at http://sbb.uvm.edu/~gward/Movies.html
(Toxoplasma invasion #2 and motility #2)
20
Eukaryotes
• Uni- or multicellular.
• Reproduce asexually &
sexually.
• Composition:
– Genetic material (long linear strands of
DNA chromosomes) especially isolated and
enclosed in membrane (nucleus)
– Some have cell walls (plants have
cellulose, fungi ß-glucan)
21
Eukaryotes
• Size 10-100 (µm)
• Composition:
– Phospholipid membrane outside, as
well as inside.
– Interior membranes separate
functions such as gathering and
transforming cellular energy and
manufacturing macromolecules.
22
Eukaryotic Organelles
Animal Cell
Plant Cell
23
Eukaryotic Organelles
Endoplasmic reticulum
Golgi
Nucleus
Mitochondrion
Chloroplast
24
CQ4: Becky’s Anti-Eukaryotic Medicines:
• Pyrimethamine, Sulfonamides: Interfere with enzymes
used to make the folic acid needed to make thymine
and uracil nucleotides.
• Polyenes combine with a component of fungal and
some bacterial membranes, disrupt and break them.
One of these drugs specifically affects one of the two
eukaryotic suspects. Which test of Ellie’s blood would help
you tell which eukaryotic suspect she was infected with?
A: Presence of DNA.
B: Presence of ß-glucan-containing cell walls.
C: Presence of cellulose.
D: Presence of peptidoglycan cell walls.
25
Part III: Viruses
• Becky and Ann are back at the dorm waiting for the
results of more tests.
• “I’ve also got some tamiflu,” Becky volunteered. “I
mean, what if those cells in the picture aren’t really
making her sick. Maybe she just has the plain old
flu.”
• “What do you mean?” Ann asked. What’s the
difference?”
• “Flu is a virus,” Becky answers. “I’ve actually got
two suspects that are viruses. They’re probably
the most different from the prokaryotes and
eukaryotes. They’re not even cells.”
26
Becky’s Internet Search
Results – List of Suspects
Electron micrographs
4.
Influenza Virus: Spread primarily
through respiratory droplets from
sneezing or coughing. Virus has
single strand of RNA surrounded by
phospholipids/protein envelope (80120nm).
5.
West Nile Virus: Spread by
mosquitoes that have previously
fed on infected birds. 20% of
infected people show symptoms.
Single stranded RNA,
phospholipid/protein envelope
(50nm).
27
Viruses
• Not cells
• Cannot reproduce alone
– hijacks a host cell to replicate itself.
• Composition
– Outer shell: repetitive protein often
inserted into a lipid envelope (responsible
for recognition and infection of host cell.)
28
Viruses
• Size
– Smallest Organisms (50nm)
– 100 times smaller than bacteria
• Composition
– Protected interior that contains genetic
material (DNA or RNA) with important
protein enzymes required for duplication.
29
Virus
hijacking
host system
30
Tamiflu: Blocks neuraminidase enzyme made by all influenza A
strains (cause the “flu” and avian flu.) Viruses are unable to
remove sticky sialic acid, and can’t escape.
31
Table 1
Take a few minutes to complete the
homework table comparing viruses,
bacteria, and eukaryotes.
32
CQ5: Match the description with the suspect.
Use your responses to Table 1 to identify the
row below that best describes West Nile Virus.
Circular Nucleus
DNA
Divides
asexually
Size
Cell
Wall
Sexual
Reproduction
A
+
-
+
1µm
+
-
B
-
+
+
10µm
-
+
C
-
-
-
0.1µm
-
-
D
-
+
+
5µm
+
+
33
Part IV: DNA Analysis
“Well, it isn’t viral,” Becky said, closing her cell
phone. “No neuraminidase. But, they found some
foreign DNA with the sequence: AACGTGGTCGTT.
The closest match is a gene used to make
ribosomes (rDNA). They are searching a huge DNA
database of sequences to find the organism that has
the closest match.”
34
CQ6: Foreign DNA sequence isolated from
Ellie: ACGTGGTCGTT.
Which sequence is the best match with this
foreign DNA?
A: Ellie’s nucleus
B: Ellie’s mitochondria
C: Coxiella bacteria
D: Toxoplasma nucleus
E: Cryptococcus nucleus
ATGGTCTCAATG
TTGGTCCGTCAG
TTGGTCGGTCAG
AACGTGGTAGTT
ATGGTGGCAATG
35
Strange Similarities
“What’s weird is that Ellie’s mitochondrial DNA
matches the Coxiella sequence so closely.”
Ellie’s mitochondria TTGGTCCGTCAG
Coxiella bacteria
TTGGTCGGTCAG
“That makes sense in a way,” Ann answered.
“There is a lot of evidence that points to
mitochondria being descendents of gram-negative
bacteria just like Coxiella.”
“What type of evidence?” Becky asked.
36
Eukaryotic Organelles &
Endosymbiosis
prokaryote
N
C
N
Strange similarities:
1. Chloroplasts and mitochondria are the same
size as prokaryotes.
2. Both have circular DNA without histones with
similar sequence to photosynthetic bacteria
(cyanobacteria) and obligate intracellular
rickettsia bacteria.
37
3. Both divide like prokaryotes.
Eukaryotic Organelles &
Endosymbiosis
prokaryote
N
C
N
Strange similarities:
4. Have their own protein synthesis machinery
(ribosomes) more like bacteria than
eukaryotes (sensitivity to Streptomycin).
5. Inner membrane of mitochondria contains
unusual phospholipid characteristic of
bacterial membranes.
38
Similarities Used to Group
Organisms
First prokaryotes
3.75 billion years ago
Single-celled eukaryotes
2 billion years ago
39
CQ7: Which letter best defines the place
mitochondria would take next to their
closest relatives on this family tree of living
organisms?
A
B
C
D
E
40
Finale: Ellie’s Prognosis
“Well, Ellie’s responding well to the pyrimethamines that the
doctors prescribed,” Becky commented to Ann while checking her
email a few days later at the dorm.
“Yeah, and we’re lucky the cipro we took couldn’t harm our cells,”
Ann replied. “We were so wrong! I’m never self-medicating again.
Do you think we should warn the other students. They might have
had contact with the kitten, too.”
“We don’t know if it was from cat poop,” Becky answered. “I
learned that something like 25-40% of American adults are
already infected with Toxoplasma gondii, and not because of their
cats—usually it’s from eating raw meat. Plus, apparently the
oocysts in fresh cat poop aren’t infectious for a couple of days.
So, if you scoop the box right away you don’t have to worry.”
“So now I have to know how often the cat box is cleaned?! I don’t
think I’m cut out for this job!” Ann moaned.
41