Bisc 120 Exam 3 (USC, Egan, Fall 2025)

1 What is a virus?

An infectious particle consisting of nucleic acid enclosed in a protein coat and sometimes surrounded by a membranous envelope.

2 Why are viruses considered nonliving?

Because they cannot carry out metabolism or reproduce independently; they require a host cell.

3 What is a viral genome composed of?

Either DNA or RNA

4 What is a capsid?

A protein shell that encloses the viral genome.

5 What are capsomeres?

The protein subunits that make up a capsid.

6 What is a viral envelope?

A membrane derived from the host cell that surrounds the capsid of some viruses and contains viral glycoproteins.

7 How do viruses recognize host cells?

Through specific interactions between viral surface proteins and host cell receptor molecules.

8 What determines a virus’s host range?

The compatibility between viral proteins and host receptor molecules.

9 What is the difference between a broad and narrow host range?

Broad infects multiple species (e.g.

10 What is a bacteriophage?

A virus that infects bacteria.

11 How do bacteriophages replicate?

By injecting their genome into a bacterial cell and hijacking its machinery to produce viral components.

12 What are the two main phage replication cycles?

The lytic cycle and the lysogenic cycle.

13 What happens in the lytic cycle?

The virus replicates rapidly

14 What is a virulent phage?

A phage that reproduces only by the lytic cycle.

15 What is a lysogenic cycle?

A viral replication cycle where the viral DNA integrates into the host genome and replicates silently with the host cell.

16 What is a prophage?

The phage genome inserted into a bacterial chromosome during lysogeny.

17 What can trigger a prophage to enter the lytic cycle?

Environmental stressors such as UV radiation or chemical damage.

18 What is a temperate phage?

A phage capable of both lytic and lysogenic cycles.

19 How do viruses replicate in animal cells?

They enter by endocytosis or fusion

20 What is the role of reverse transcriptase in retroviruses?

It transcribes RNA into DNA.

21 What is a retrovirus?

An RNA virus that uses reverse transcriptase to convert its RNA genome into DNA inside the host.

22 What is an example of a retrovirus?

HIV (Human Immunodeficiency Virus).

23 What is a provirus?

Viral DNA integrated permanently into the host genome (e.g.

24 How does an enveloped virus exit the host cell?

By budding through the host membrane

25 What are glycoproteins on viral envelopes used for?

Binding to host cell receptors for entry.

26 What is antigenic drift?

Gradual accumulation of mutations in viral genes that alter surface proteins

27 What is antigenic shift?

Abrupt reassortment of genes from different viral strains

28 Why do RNA viruses evolve faster than DNA viruses?

RNA replication lacks proofreading

29 What is zoonosis?

Transmission of a virus from an animal to a human host.

30 Give an example of a zoonotic virus.

Influenza

31 What are emerging viruses?

Newly identified viruses that have jumped species or mutated to become more transmissible.

32 How can viruses spread within a population?

Through direct contact

33 What is a viral reservoir?

A population or environment in which a virus naturally persists and multiplies.

34 What are viroids?

Small circular RNA molecules that infect plants and interfere with growth.

35 What are prions?

Infectious misfolded proteins that cause neurodegenerative diseases.

36 How do prions propagate?

By converting normal proteins into misfolded versions.

37 Give an example of a prion disease.

Creutzfeldt–Jakob disease or mad cow disease.

38 How do vaccines prevent viral infection?

By stimulating adaptive immunity and memory cell formation.

39 What is an attenuated vaccine?

A vaccine made from a weakened form of the pathogen.

40 What is an inactivated vaccine?

A vaccine made from killed or non-replicating virus particles.

41 What is the main difference between DNA and RNA viruses in replication?

DNA viruses use host DNA polymerases; RNA viruses use viral RNA-dependent polymerases.

42 Why are antiviral drugs difficult to develop?

Viruses use host cell machinery

43 What are bacteriophages being explored for today?

Phage therapy — using phages to treat antibiotic-resistant bacterial infections.

44 What are the general sizes of viruses?

10–300 nanometers

45 What is a viral latency period?

A dormant phase where viral replication is halted but the genome remains in the host.

46 What is the Baltimore classification system?

A system that groups viruses based on their type of nucleic acid and replication strategy.

47 What is the difference between positive-sense and negative-sense RNA viruses?

Positive-sense RNA acts as mRNA; negative-sense must be transcribed into mRNA first.

48 What are giant viruses?

Large DNA viruses that blur the line between viruses and cells (e.g.

49 What is viral reassortment?

Exchange of genome segments between different viral strains infecting the same cell.

50 Why are viruses considered “obligate intracellular parasites”?

They rely entirely on host cells for replication and metabolism.

51 What is the main structural difference between bacteriophages and animal viruses?

Phages have a tail apparatus for injecting DNA; animal viruses lack tails but often have envelopes.

52 What are the stages of the viral life cycle?

Attachment

53 What is the main method used to study viruses?

Culturing in host cells or embryonated eggs and molecular analysis.

54 How does CRISPR provide bacterial immunity to viruses?

By storing viral DNA fragments and using them to recognize and cut invading phage DNA.

55 How has CRISPR been adapted for biotechnology?

As a genome-editing tool that can precisely cut and modify DNA.

56 What enzyme is essential for CRISPR’s DNA cutting?

Cas9.

57 What is the guide RNA in CRISPR?

An RNA sequence that directs Cas9 to the complementary DNA site.

58 How are viruses used in gene therapy?

As vectors to deliver corrected genes into human cells.

59 What are the two main types of viral vectors used in gene therapy?

Adenoviral and lentiviral vectors.

60 What is a potential risk of using viral vectors?

Insertional mutagenesis — the viral DNA may disrupt essential host genes.

61 What are prokaryotes?

Single-celled organisms lacking a membrane-bound nucleus or organelles.

62 What domains include prokaryotes?

Bacteria and Archaea.

63 What is the average size of a prokaryotic cell?

0.5–5 µm in diameter.

64 What structure maintains prokaryotic cell shape and protects it?

The cell wall.

65 What is peptidoglycan?

A polymer of sugars and amino acids forming the bacterial cell wall.

66 What is the difference between Gram-positive and Gram-negative bacteria?

Gram-positive have thick peptidoglycan; Gram-negative have thin peptidoglycan and an outer membrane.

67 What is the function of the Gram stain?

To distinguish bacteria based on cell wall composition.

68 Why are Gram-negative bacteria often more resistant to antibiotics?

Their outer membrane acts as an additional barrier.

69 What is lipopolysaccharide (LPS)?

A toxic molecule found in the outer membrane of Gram-negative bacteria.

70 What is a capsule?

A sticky polysaccharide layer that protects bacteria and aids in adhesion.

71 What is a fimbria?

A hairlike appendage used for attachment to surfaces or host cells.

72 What is a pilus?

A longer appendage used for DNA transfer (conjugation) or motility.

73 What is chemotaxis?

Movement of a cell toward or away from a chemical stimulus.

74 What is the structure responsible for bacterial motility?

The flagellum.

75 How is the prokaryotic flagellum different from the eukaryotic one?

It’s composed of flagellin and rotates

76 What is the nucleoid?

The region in a prokaryotic cell where DNA is concentrated.

77 What are plasmids?

Small

78 How do bacteria reproduce?

By binary fission — an asexual process producing identical offspring.

79 Why can bacterial populations evolve rapidly?

Because of short generation times and high mutation rates.

80 What are the three mechanisms of genetic recombination in prokaryotes?

Transformation

81 What is transformation?

Uptake of foreign DNA from the environment.

82 What is transduction?

Transfer of DNA between bacteria via bacteriophages.

83 What is conjugation?

Direct transfer of DNA between bacterial cells via a pilus.

84 What is an F plasmid?

A fertility factor that allows conjugation to occur.

85 What is an R plasmid?

A plasmid carrying antibiotic resistance genes.

86 What is an endospore?

A dormant

87 Why are endospores medically significant?

They enable survival of pathogens like Bacillus anthracis or Clostridium botulinum.

88 What is nitrogen fixation?

Conversion of atmospheric nitrogen (N₂) into ammonia (NH₃) by prokaryotes.

89 What enzyme performs nitrogen fixation?

Nitrogenase.

90 What are obligate aerobes?

Organisms that require oxygen for respiration.

91 What are obligate anaerobes?

Organisms poisoned by oxygen that rely on fermentation or anaerobic respiration.

92 What are facultative anaerobes?

Organisms that can switch between aerobic and anaerobic metabolism.

93 What are photoautotrophs?

Organisms that use light as an energy source and CO₂ as a carbon source.

94 What are chemoautotrophs?

Organisms that use inorganic chemicals as an energy source and CO₂ as a carbon source.

95 What are photoheterotrophs?

Organisms that use light for energy but must consume organic carbon.

96 What are chemoheterotrophs?

Organisms that consume organic molecules for both energy and carbon.

97 What is the importance of metabolic diversity in prokaryotes?

It allows them to inhabit almost every environment on Earth.

98 What is biofilm formation?

A cooperative community of microorganisms attached to a surface within a slime-like matrix.

99 Why are biofilms medically relevant?

They protect bacteria from antibiotics and immune defenses.

100 What is quorum sensing?

Cell-to-cell communication in bacteria via chemical signals to coordinate behavior.