Virology - Basics of Viruses
Virology is the study of viruses - tiny infectious agents that can only multiply inside living host cells. These notes cover what viruses are, their structure and classification, how they replicate, how they cause disease, and how infections are prevented, followed by a 20-question practice set.
What Viruses Are
A virus is an obligate intracellular parasite: it carries genetic material (DNA or RNA) but cannot make energy or proteins on its own, so it must hijack a host cell to reproduce.
- Viruses are much smaller than bacteria and are not cells.
- They contain either DNA or RNA - never both.
- Outside a host, a complete virus particle is called a virion.
- They are not affected by antibiotics, which target bacteria.
Structure
- Genome: the nucleic acid core (DNA or RNA), single- or double-stranded.
- Capsid: the protein coat that surrounds and protects the genome.
- Nucleocapsid: the genome plus capsid together.
- Envelope: some viruses have an outer lipid membrane (enveloped); others do not (naked). Enveloped viruses are more easily destroyed by soap and drying.
Classification and Replication
- Viruses are grouped by genome type (DNA vs RNA), strandedness, and the Baltimore classification.
- Retroviruses (e.g. HIV) use reverse transcriptase to copy RNA into DNA.
- The replication cycle: attachment -> entry (penetration) -> uncoating -> replication of genome and synthesis of proteins -> assembly -> release (by lysis or budding).
- Bacteriophages are viruses that infect bacteria.
Disease and Prevention
- Important human viruses: influenza, SARS-CoV-2, HIV, hepatitis B and C, measles, dengue, rabies, HPV.
- Some viruses are oncogenic (cancer-causing), e.g. HPV (cervical cancer) and hepatitis B (liver cancer).
- Prevention: vaccines (e.g. measles, hepatitis B, HPV) and hygiene.
- Treatment: antiviral drugs (e.g. acyclovir for herpes, antiretrovirals for HIV) - not antibiotics.
Key Terms
- Virion
- A complete, infectious virus particle outside the host cell.
- Capsid
- The protein coat that encloses and protects a virus's genome.
- Obligate intracellular parasite
- An organism that can only reproduce inside a living host cell - true of all viruses.
- Reverse transcriptase
- The enzyme used by retroviruses such as HIV to make DNA from their RNA genome.
Practice Quiz — 20 Questions
-
A virus can reproduce only:
- A.In soil
- B.Inside a living host cell
- C.In water
- D.On dead surfaces
B. Inside a living host cell — Viruses are obligate intracellular parasites. -
The genetic material of a virus is:
- A.Always DNA
- B.Always RNA
- C.Either DNA or RNA
- D.Both DNA and RNA
C. Either DNA or RNA — A virus contains either DNA or RNA, never both. -
The protein coat of a virus is called the:
- A.Envelope
- B.Capsid
- C.Membrane
- D.Cell wall
B. Capsid — The capsid is the protective protein coat. -
A complete, infectious virus particle is a:
- A.Virion
- B.Prion
- C.Plasmid
- D.Ribosome
A. Virion — The extracellular infectious particle is a virion. -
Viruses that infect bacteria are called:
- A.Retroviruses
- B.Bacteriophages
- C.Prions
- D.Protozoa
B. Bacteriophages — Bacteriophages (phages) infect bacteria. -
Antibiotics are ineffective against viruses because viruses:
- A.Are too large
- B.Lack the bacterial targets antibiotics act on
- C.Live in the blood
- D.Have thick cell walls
B. Lack the bacterial targets antibiotics act on — Antibiotics target bacterial structures/processes that viruses lack. -
Which enzyme is characteristic of retroviruses like HIV?
- A.DNA polymerase
- B.Reverse transcriptase
- C.Lysozyme
- D.Protease only
B. Reverse transcriptase — Reverse transcriptase copies viral RNA into DNA. -
An enveloped virus has an outer layer made mainly of:
- A.Peptidoglycan
- B.Lipid membrane
- C.Cellulose
- D.Chitin
B. Lipid membrane — The envelope is a lipid membrane taken from the host cell. -
The first step of the viral replication cycle is:
- A.Assembly
- B.Attachment to the host cell
- C.Release
- D.Uncoating
B. Attachment to the host cell — The virus first attaches to specific receptors on the host cell. -
Release of new enveloped viruses often occurs by:
- A.Budding
- B.Binary fission
- C.Sporulation
- D.Conjugation
A. Budding — Enveloped viruses usually leave the cell by budding. -
Which virus is a well-known cause of cervical cancer?
- A.HIV
- B.HPV
- C.Influenza
- D.Rabies
B. HPV — Human papillomavirus (HPV) causes most cervical cancers. -
Hepatitis B virus is strongly linked to cancer of the:
- A.Lung
- B.Liver
- C.Skin
- D.Bone
B. Liver — Chronic hepatitis B can lead to liver (hepatocellular) cancer. -
Which of these is an antiviral drug?
- A.Amoxicillin
- B.Acyclovir
- C.Ciprofloxacin
- D.Metronidazole
B. Acyclovir — Acyclovir treats herpes virus infections; the others are antibacterials. -
The genome plus the capsid together are called the:
- A.Envelope
- B.Nucleocapsid
- C.Virion sac
- D.Matrix
B. Nucleocapsid — Genome + capsid = nucleocapsid. -
Enveloped viruses are generally:
- A.More resistant to soap and drying
- B.More easily destroyed by soap and drying
- C.Unaffected by disinfectants
- D.Larger than bacteria
B. More easily destroyed by soap and drying — The lipid envelope is disrupted by soap and drying, inactivating the virus. -
Which is a naked (non-enveloped) property?
- A.Having a lipid membrane
- B.Lacking a lipid envelope
- C.Budding release only
- D.Containing both DNA and RNA
B. Lacking a lipid envelope — Naked viruses lack the outer lipid envelope. -
A vaccine prevents viral disease by:
- A.Killing existing viruses with antibiotics
- B.Training the immune system to recognise the virus
- C.Coating the skin
- D.Replacing host DNA
B. Training the immune system to recognise the virus — Vaccines prime the immune system to respond to the virus. -
SARS-CoV-2, the cause of COVID-19, is a:
- A.Bacterium
- B.Coronavirus (RNA virus)
- C.Fungus
- D.Protozoan
B. Coronavirus (RNA virus) — SARS-CoV-2 is an enveloped RNA coronavirus. -
Uncoating in the replication cycle refers to:
- A.Building new capsids
- B.Releasing the viral genome inside the host
- C.Attaching to the cell
- D.Leaving the cell
B. Releasing the viral genome inside the host — Uncoating frees the genome from the capsid inside the host cell. -
Viruses are best described as:
- A.Living cells
- B.Acellular infectious agents
- C.A type of bacteria
- D.Single-celled fungi
B. Acellular infectious agents — Viruses are acellular and are not classified as cells.
References
- Jawetz, Melnick & Adelberg's Medical Microbiology (standard reference)
- NCBI Bookshelf - Medical Microbiology, Virology chapters