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Take Two and Call Me in the Morning

发布时间:2013-12-05 14:02:31  

“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

?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

10-3

1 mm

10-4

10-510-61 μm

10-710-810-9

1 nmCells likeoursBacteria

Viruses

Proteins

Atoms

8

meter

s

Becky’s Internet Search Results –List of SuspectsSuspect 1: Coxiella

burnetiicauses Q-fever.

Coxiellaare often found in

livestock and are excreted in

milk, urine, and feces. Infection occurs 2-3 weeks after

inhalation of barnyard dust.

They are0.3-0.5 μm gram-

negative bacterium

(prokaryotes) that must invade and reside inside human cells to cause infection.1μm9

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 ?-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?”A: Yes

B: No10-31 mmCells like10-4ourss

r

10-5

eBacteria

t

e10-61 μm

m

10-7Viruses

10-8Proteins

10-91 nmAtoms

13

Part II: Microscope AnalysisBecky 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

EukaryotesProkaryotesDNASize

Organization

Metabolism

Organelles

Examples

Becky’s Internet Search Results –List of SuspectsEukaryote -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

Suspect 3:

4-6 μm single-celled

Most likely acquired through

19Eukaryote -Toxoplasma gondii?protozoan parasite of mammals & birds.?ingesting cysts in undercooked meat.

Becky’s Internet Search Results –List of SuspectsSuspect 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

―Invasion 4‖ and ―Escape 2‖ at

http://www.uvm.edu/microbiology/videos_ward.php?id=2320

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 ?-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

NucleusMitochondrion

GolgiChloroplast24

CQ4: Becky’s Anti-Eukaryotic Medicines:

?Pyrimethamine, Sulfonamides: Interfere with enzymes used to make the folic acid needed to make thymine and uracil nucleotides.

?Polyenescombine 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 ?-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

4.Electron micrographsInfluenza Virus: Spread primarily through respiratory droplets from sneezing or coughing. Virus has single strand of RNA surrounded by phospholipids/protein envelope (80-120nm).

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).5.27

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 protein enzymes required for duplication.

29

Virus hijacking host system

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 NucleusDNAA

+

-

Divides asexually

+

Size1μm

Cell Sexual WallReproduction+

-

BCD

---

+-+

+-+

10μm0.1μm5μm

--+

+-+

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 TTGGTCCGTCAGC: Coxiella bacteria

D: Toxoplasmanucleus AACGTGGTAGTTE: Cryptococcus nucleus ATGGTGGCAATGATGGTCTCAATGTTGGTCGGTCAG

35

Strange Similarities

―What’s weird is that Ellie’s mitochondrial DNA matches the Coxiellasequence so closely.‖Ellie’s mitochondria TTGGTCCGTCAGCoxiella bacteriaTTGGTCGGTCAG

―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

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.

373.Both divide like prokaryotes.

Eukaryotic Organelles & Endosymbiosis

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 agoSingle-celled eukaryotes2 billion years ago39

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

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