BIOI 121 cell and tissues



LESSON - 29

THE BASIC STRUCTURE OF VIRUSES

A. Objectives

— Give a characterization of a virus; know the name of some viruses important to us; tell how viruses were discovered; relate the size of viruses to the size of cells and bacteria.

— Give a picture of the structure of a generalized (animal) virus and note the function of the different parts; explain what the origin of the envelope is; what determines the structure of the glycoproteins.

— Discuss the diversity of viruses; make a picture of a bacteriophage; explain a lysogenic and lytic cycle; define prophage.

— Discuss the differences between DNA and RNA viruses; give a review of the replication of an animal virus.

B. Lecture outline

Read: Viruses (SBM p501-510)

1. VIRUSES ARE INTRACELLULAR PARASITES.

2. VIRUSES ARE VERY SMALL.

3. VIRUSES HAVE A SIMPLE STRUCTURE

Read: The structure of viruses (R&J p676)

a. nucleocapsid

b. envelope

4. DIVERSITY OF VIRUSES

a. bacteriophages: lytic and lysogenic cycles

Read: Figure24.2 and 24.3 (SBM p504)

b. animal viruses: DNA and RNA viruses

5. REPLICATION OF ANIMAL VIRUSES

— budding of enveloped viruses

C. Study Questions.

1. Living organisms that consist of protein coat surrounding nucleic acid are called

A. living entities.

B. non—living cells.

c. precursors to life.

D. rare particles.

E. viruses.

2. A typical virus measures between

A. 10 and 200 pico-meter.

B. 10 and 200 nano-meter.

c. 10 and 200 micro-meter.

D. 10 and 200 milli-meter.

E. 10 and 200 kilo-meter.

3. Viruses can do all of the following EXCEPT:

A. alter transcription and translation in a host cell.

B. carry genes coding for specific proteins.

C. cause cellular ribosomes to produce viral proteins.

D. code for enzymes different from those of the host.

E. grow and replicate on their own.

4. Human viruses are genetically most closely related to

A. animal viruses.

B. plant viruses.

C. bacteriophages.

D. prophages.

E. humans.

5. What technique(s) could be used to distinguish between viruses and bacteria? In contrast to viruses, bacteria

A. can grow in a cel-free medium.

B. possess both RNA and DNA.

C. are retained by a filter of 0.45 μm pores.

D. are visible with a light microscope.

E. all of the above.

6. Living organisms that contain only one type of nucleic acid are

A. animals.

B. plants.

C. viruses.

D. bacteria.

E. eukaryotes.

7. The agent of puppy diarrhea has been isolated recently. The organisms was found to contain the organic, cyclic, nitrogen bases adenine, guanine, cytosine and uracil. It was therefore concluded that puppy diarrhea was caused by a

A. gram—negative bacterium.

B. RNA-virus.

C. DNA-virus.

D. animal cell.

E. edible mushroom.

8. The agent of infectious hepatitis (liver infection) of the dog contains adenine, guanine, thymine and cytosine and, therefore, is a A. gram-positive bacterium.

B. RNA-virus.

C. DNA-virus.

D. animal cell.

E. cyanobacterium.

9. The layer of protein that surrounds the nucleic acid of a virus is called the

A. capsid.

B. casing.

C. envelope.

D. membrane.

E. virion.

10. The layer of phoepholipids and glycoproteins that forms the outer surface of some viruses is called the A. capsid.

B. casing.

C. envelope.

D. membrane.

E. virion.

11. Some viruses can become incorporated in the nucleic acid of the host cell and be replicated with host DNA generation after generation. Such viruses are called

A. lethal.

B. potent.

C. temperate.

D. virulent.

E. well-tempered.

12. A helical arrangement of capsid protein molecules causes the shape of the virion to be

A. circular.

B. cubic.

C. randomly coiled.

D. rod-shaped.

E. spherical.

13. A virion that has 20 equal triangular sides has a

A. circular shape.

B. cubic shape.

C. complex shape.

D. icosahedral shape.

E. spherical shape.

14. When a virus infects an animal cell, it must first bind to a specific receptor located on the

A. chromosomes.

B. lysosomes.

C. nuclear membrane.

D. plasma membrane.

E. viral nucleic acid.

15. A virus that is integrated in the genome of the host cell is called a A. lethal virus.

B. lysogenic cell.

C. lytic virus.

D. prophage.

E. virulent virus.

16. A (bacterial) cell, in which a prophage exists is called a

A. lethal virus.

B. Lysogenic cell.

C. lytic virus.

D. prophage.

E. virulent virus.

17. The membrane that surrounds a virus is called a(n)

A. membrane.

B. plasma membrane.

C. envelope.

D. coat.

E. capsid.

18. The nucleocapsid of a virus contains A. protein.

B. glycol.ipid.

C. DNA and RNA.

D. phospholipid.

E. proteins and nucleic acid.

19. The flu virus is an enveloped RNA virus that infects humans, swine, horse and chicken. The virus attaches to host cells by means of envelope glycoproteins, the hemagglutinin. When the virus is isolated from pig cells, the hemagglutinin is coded for by nucleic acid of

A. flu virus.

B. humans.

C. swine.

D. horses.

E. chickens.

20. The direction of the RNA molecule of a single—stranded, (+) sense RNA virus is

A. from 3’ to 5’.

B. the same as that of DNA.

C. opposite to mRNA.

D. the same as mRNA.

E. from left to right.

21. The lipid composition of the Herpes virus that causes cold blisters in humans resembles the membrane of

A. human cells.

B. rat cells.

C. bovine cells.

D. cat cells.

E. other Herpes viruses.

22. Bacteriophages are viruses that

A. infect eukaryotic cells.

B. infect prokaryotic cells.

C. have an additional phosphol.ipid envelope.

D. are specific for certain cells. E. are difficult to grow.

23. What description is characteristic of a lytic infection? A lytic infection

A. involves the integration of viral DNA into host cell DNA.

B. involves production of lysozyme by the virus.

C. involves the phagocytosis of viruses by the host cell.

D. causes lysis of the host cell after infection and viral replication.

E. involves attachment of the virus to the host cell.

24. What description is characteristic of a lysogenic infection? A lysogenic infection

A. involves the phagocytosis of virus by the host cells.

B. kills cells immediately after infection and viral

replication.

C. results in formation of prophage.

D. occurs when only viral nucleic acid enters the host cell.

E. involves attachment of the virus to the host cell.

25. Which of the following properties can be used to classify viruses? The

A. type of nucleic acid.

B. shape of the capsid.

C. size of the virion.

D. type of infection.

E. all of the above.

26. Which of the following statements about bacteria is false?

A. A small percentage of bacteria are pathogenic.

B. Some bacteria can photosynthesize.

C. Bacteria are important decomposers.

D. Bacteria are not cellular and are sometimes not classified as life forms.

E. Bacteria convert atmospheric nitrogen into ammonia and then nitrates that can be used by plants.

27. Many microscopic organisms or entities have been identified as pathogens, including all of the following except:

A. protozoa.

B. viruses.

C. fungi.

D. bacteria.

E. None of these; all may be pathogenic.

28. Viruses were first identified:

A. because they caused an infection and formed colonies on nutrient agar plates.

B. because they were seen in the light microscope.

C. because they caused an infection and were small enough to pass through filters that trapped bacteria.

D. because they were smaller than bacteria and had all the properties of living organisms.

E. because they caused an infection and were seen in the electron microscope.

29. Viruses:

A. are very tiny cells on the order of 500 nm.

B. contain both RNA and DNA.

C. cannot independently perform metabolic activities.

D. have a standard morphology that includes a capsid, tail, and tail fibers.

E. All of the above.

30. The protein coat of a virus is called the:

A. capsule.

B. capsid.

C. exospore.

D. phage.

E. pilus.

31. Viruses that attack bacteria are called:

A. phages.

B. bacteriods

C. prions.

D. virons.

E. viroids.

32, 33. Use the figure to answer the corresponding questions.

1. 32. The structure of the virus in the accompanying figure that is common to all viruses is:

A. 2.

B. 3.

C. 4.

D. 5.

E. All of the above.

33. The function of the structure labeled 3 is:

A. attachment to a host cell.

B. locomotion.

C. mate recognition.

D. replication.

E. to take over host cell mechanisms.

[pic]

34. Viruses:

A. are usually species-specific.

B. may have multiple origins.

C. are probably related to mobile genetic elements.

D. may have escaped from cells.

E. All of these.

35. Viruses are classified by:

A. the sequence of their DNA.

B. structural similarities.

C. the sequence of their rRNA genes.

D. the traditional Linnaean binomial nomenclature system.

E. an international committee.

36. Phages are being considered for:

A. vectors in gene therapy experiments.

B. antibiotics in bacterial infections.

C. gene transfer vectors in animal cells.

D. being equivalent to the first living organisms.

E. All of the above.

37. Temperate viruses:

A. always destroy their host immediately.

B. are considered virulent.

C. do not always destroy their host.

D. do not undergo a lysogenic cycle.

E. undergo a lytic cycle.

38. Virulent (lytic) phages:

A. infect viruses.

B. destroy bacteria.

C. infect but do not destroy bacteria.

D. infect virions.

E. infect prions.

39. The five stages of a lytic infection are attachment, penetration, _____________, assembly, and release.

A. lysis

B. prophage

C. lysogenesis

D. transduction

E. replication

40. A prophage:

A. is phage DNA that is integrated into viral DNA.

B. is phage DNA that is integrated into bacterial DNA.

C. replicates only when the eukaryotic cell it infects replicates.

D. is an underdeveloped lytic virus.

E. is a bacterial cell about to lyse.

41. ____________ conversion occurs when a bacterium carrying viral genes takes on new, atypical characteristics.

A. Transducing

B. Lysogenic

C. Prophage

D. Lytic

E. Reverse

42. The bacterium that causes botulism disease is harmless until it:

A. is infected by lytic bacteria.

B. is infected by certain retroviruses.

C. contains a certain prophage DNA.

D. is irradiated by UV radiation.

E. is infected by prions.

43. The specificity of viruses to different types of cells is due to _____________ sites on the host cell.

A. prophage

B. receptor

C. transduction

D. penetration

E. transcription

44. Retroviruses differ from other RNA viruses by:

A. having reverse transcriptase instead of RNA-dependent RNA polymerase.

B. the shape of their capsid.

C. the way they infect their host cells.

D. the sugar coating on their capsids.

E. lytically destroying their hosts.

45. Which of the following diseases is not caused by a virus?

A. rabies

B. polio

C. warts in humans

D. chickenpox

E. Lyme disease

46. Which of the following identifies a group of DNA viruses?

A. retroviruses

B. herpesviruses

C. paramyxoviruses

D. filovirus

E. picornaviruses

47. Human viruses can enter human cells by fusion with the cell membrane or by:

A. diffusion into the cell.

B. passage through specific protein channels.

C. coating themselves with lipids that cloak their entrance into the cell.

D. endocytosis.

E. hitchhiking onto protein signals entering the cell.

48. Viral proteins can damage host cells by:

A. increasing the rate of fermentation within the host.

B. producing endotoxins.

C. causing a decline in coordination and increased irritability.

D. preventing transcription of viral nucleic acids.

E. overwhelming the host cell with a large number of viruses.

49. Viral infections in humans spread via the circulatory system. Viral infections in plants spread from cell to cell via:

A. endocytosis.

B. plasmodesmata.

C. infected seeds.

D. pili.

E. transformation.

50. The genome of a virus consists of

A. DNA

B. RNA

C. prions

D. DNA and RNA

E. DNA or RNA

51. The capsid of a virus consist of

A. protein

B. nucleic acid

C. helical proteins

D. a simple carbohydrate

E. RNA and lipid

52. The types of organisms a particular virus can infect are referred to as its

A. assembly

B. commensal cohort

C. host range

D. prophage range

E. restriction cycle

53. Viruses that kill host cells are

A. lysogenic

B. lytic

C. viroids

D. prophages

E. temperate

54. Arrange the following list into the correct sequence for viral reproduction.

1. Penetration 2. Assembly 3. Replication 4. Attachment 5. Release

A. 1, 2, 3, 4, 5

B. 5, 2, 3, 4, 1

C. 4, 1, 3, 2, 5

D. 4, 1, 2, 3, 5

E. 3, 1, 2, 4, 5

55. In lysogenic conversion

A. bacterial cells may exhibit new properties

B. the host cell dies

C. prions sometimes convert to viroids

D. reverse transcriptase transcribes DNA into RNA

E. prion proteins convert to infective agents

56. Retroviruses

A. are DNA viruses

B. are RNA viruses

C. use reverse transcriptase to transcribe DNA into RNA

D. use prion protein to transcribe RNA

E. are DNA viruses that cause AIDS

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