02/27f Virus Ultrastructure

Question 1

What signals are detected by a transmission electron microscope?

Answer 1

Electrons that are not bounced back or scattered by the sample (transmitted electrons)

Question 2

What signals are detected by a scanning electron microscope?

Answer 2

Secondary electrons (bounced out from the sample)

Question 3

What happens when outer shell electrons fall down energy levels to fill vacant orbits?

Answer 3

X-rays are released

Question 4

What two specimen preparation methods are used for EM samples? For what types of specimens is each method used?

Answer 4

Negative staining - for fluid specimens

Ultrathin sectioning - for cells and solid tissues

Question 5

How is negative staining performed?

Answer 5

Fluid sample is spun down
Supernatant is placed on a waxy substance, which causes it to bead up
A tiny grid coated with a support film is placed over the sample bead
The grid is then wicked, placed on a droplet of stain, wicked again, and placed in the microscope

Question 6

What does the negative stain actually stain?

Answer 6

Background (what you’re NOT interested in)

Question 7

How is ultrathin sectioning performed?

Answer 7

Tissue is fixed and embedded in an epoxy resin
Ultrathin (60-90nm) sections are cut on a microtome
Section is then placed onto an EM grid

Question 8

What are the two basic morphologies of viruses?

Answer 8

Naked viruses

Enveloped viruses

Question 9

What are the morphological characteristics of naked viruses in negative staining?

Answer 9

Hard and hardy

Appear round

Question 10

What are the morphological characteristics of enveloped viruses in negative staining?

Answer 10

Encased in pleiomorphic membrane - may appear smooth or “spiky”
Nucleocapsid contained within the membrane

Question 11

What are the possible shapes of the nucleocapsid?

Answer 11

Spherical
Helical
Complex
Nondescript

Question 12

Where are DNA viruses usually seen in ultrathin sections? What about RNA viruses?

Answer 12

DNA viruses - nucleus

RNA viruses - cytoplasm

Question 13

Where is poxvirus (a DNA virus) formed?

Answer 13

In the CYTOPLASM (atypical)

Question 14

For what types of clinical procedures are EM particularly useful?

Answer 14

Differentiating viral infections
Identifying emerging viruses
Identifying weird, mysterious infections
Bioterrorism surveillance

Question 15

What types of things can be seen via ultrathin sectioning EM?

Answer 15

Virus location inside cells
Whether the viruses bud, and from which membrane
Capside/nucleocapside size and shape