General Biology II – Viruses: Basic Characteristics, Identification & Classification

36 question-and-answer flashcards covering discovery, structure, life status, replication cycles, identification methods, and classification of viruses based on morphology, chemistry, and replication mode.

Who first discovered viruses and in what year, according to the lecture notes?

Russian botanist D. I. Ivanovsky in 1852.

Which scientist coined the term “virus” and when?

Duthman Beijerink in 1898.

What are viruses chemically identified as?

Nucleoproteins – nucleic acid combined with proteins.

Why were viruses among the first specimens studied with an electron microscope?

They are too small (20–300 nm) to be resolved by a light microscope.

What is the typical size range of viruses?

Approximately 20 nm to 300 nm in diameter.

Name the two basic structural components found in all viruses.

A nucleic-acid core (DNA or RNA) and a protein coat (capsid).

What additional structural layer is present in some viruses outside the capsid?

A lipoprotein envelope.

Do viruses possess organelles or cellular structures?

No, they lack cellular structures and organelles.

Why are viruses described as obligate endoparasites?

They can reproduce only inside host cells and at the host’s expense.

What is the single living function viruses perform inside a host?

Reproduction (replication).

Explain why viruses are called “borderline” entities between living and non-living worlds.

They possess genetic material and can reproduce (living traits) but lack cellular structure (non-living trait).

What must a virus do before entering a host cell?

Attach to a specific receptor site on the host cell’s plasma membrane.

Where can a virus’s attachment protein be located?

On the surface, within the capsid, or in the envelope.

Name the two main forms of viral replication cycles.

Lytic cycle and lysogenic cycle.

What key event distinguishes the lytic cycle from the lysogenic cycle?

The lytic cycle ends with host-cell lysis; the lysogenic cycle integrates viral nucleic acid into the host genome without immediate lysis.

List the five general stages of the lytic cycle in order.

Attachment, Entry, Replication & Gene Expression, Assembly, Release.

Describe two ways a virus can enter a host cell during the entry stage.

Injection of its nucleic acid, or fusion/endocytosis of the whole virion (common with enveloped viruses).

After viral genome replication, where are viral proteins synthesized?

At the host cell’s ribosomes.

Give three mechanisms by which newly formed virions exit a host cell.

Lysis, exocytosis, budding (forming an envelope).

In the lysogenic cycle, what is the integrated viral genome called?

A prophage.

What happens to the prophage during normal host-cell division?

It is replicated along with the host chromosome and passed to daughter cells.

Can a lysogenic infection switch to a lytic infection?

Yes, through induction triggered by certain factors.

Name four electron microscopes used to view viruses.

Transmission Electron Microscope (TEM), Scanning Transmission Electron Microscope (STEM), Scanning Electron Microscope (SEM), Low-Energy Electron Microscope (LEEM).

What is the most widely used viral culture method mentioned?

Embryonated goose eggs inoculation.

List at least three animals commonly used in animal inoculation methods for virus culture.

Mice, rats, monkeys.

Give three immunological or molecular methods for virus identification.

ELISA, PCR/RT-PCR, haemagglutination assay.

State the three major criteria used in classifying viruses.

Morphology, chemical composition, and mode of replication.

Define helical viral morphology.

Capsid proteins wrapped helically around the viral nucleic acid, forming filamentous or pleomorphic shapes.

What geometric shape characterizes icosahedral viruses?

A polyhedron with 20 equilateral triangular faces and 12 vertices.

Describe the structure of a complex virus such as bacteriophage T4.

An icosahedral head attached to a helical tail with a base plate and tail fibres, functioning like a molecular syringe.

How are viruses classified based on their nucleic-acid type?

ssRNA, dsRNA, ssDNA, or dsDNA viruses.

Where do most RNA viruses replicate and assemble, and what is the main exception?

In the host-cell cytoplasm; influenza virus is the exception (replicates in the nucleus).

Where do most DNA viruses replicate and assemble, and what is the notable exception?

In the host-cell nucleus; pox virus is the exception (assembly in cytoplasm).

Which viruses replicate through a double-stranded DNA intermediate?

All DNA viruses, retroviruses, and some tumour-causing RNA viruses.

Which viruses replicate through a single-stranded RNA intermediate?

All RNA viruses except Reo virus and tumour-causing RNA viruses.

Provide four examples of viruses listed in the lecture notes.

HIV, Hepatitis B, Ebola virus, Influenza virus.