Chapter 5

Virus

Acellular infectious particle composed of nucleic acid surrounded by a protein coat that requires a host cell to replicate.

Virus (Latin meaning)

Poison.

Acellular

Not made of cells; lacks organelles, cytoplasm, and a cell membrane.

Obligate intracellular parasite

An organism that must live and replicate inside a host cell.

Capsid

Protein shell surrounding and protecting the viral genome.

Capsomere

Repeating protein subunits that make up a capsid.

Envelope

Lipid membrane surrounding some viruses that contains attachment proteins.

Adhesin

Viral attachment protein that binds to specific receptors on host cells.

Viral genome

Genetic material of a virus; can be DNA or RNA, single- or double-stranded.

Virion

A complete infectious viral particle.

General characteristics of viruses

Acellular, contain DNA or RNA, have a capsid, lack ribosomes and metabolism, do not grow, and require host cells for replication.

Can viruses perform translation?

No, because they lack ribosomes.

Can viruses perform metabolism?

No, because they lack metabolic enzymes.

Why are viruses considered obligate intracellular parasites?

They depend entirely on host cells for replication.

Three viral capsid shapes

Helical, Polyhedral (Icosahedral), and Complex.

Helical virus

Virus with a spiral-shaped capsid.

Polyhedral (Icosahedral) virus

Virus with a symmetrical 20-sided capsid.

Complex virus

Virus with a complicated structure, such as bacteriophages.

What determines viral shape?

The capsid, not the envelope.

Naked virus

Virus without an envelope; capsid attaches directly to host cells.

Enveloped virus

Virus surrounded by a lipid envelope that contains attachment proteins.

Difference between naked and enveloped viruses

Naked viruses use capsid proteins for attachment; enveloped viruses use envelope adhesins.

Historical discovery of viruses

Tobacco Mosaic Disease passed through bacteria-proof filters, proving viruses were smaller than bacteria.

First virus discovered

Tobacco Mosaic Virus (TMV).

Electron microscope importance

Allowed scientists to visualize viruses.

ICTV

International Committee on Taxonomy of Viruses.

How viruses are classified

Envelope, capsid shape, size, and genome type.

Virus family ending

-viridae.

Virus genus ending

-virus.

Virus species

Common virus name ending in "virus."

Seven viral genome types

dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA, ssRNA-RT, dsDNA-RT.

Positive-sense RNA (+RNA)

Functions directly as mRNA.

Negative-sense RNA (-RNA)

Must first be copied into positive-sense RNA.

Why genome type matters

It determines how the virus replicates and synthesizes proteins.

Why RNA viruses require special enzymes

Host cells cannot copy RNA from RNA templates.

Five stages of viral replication

Attachment

Attachment (Adsorption)

Virus binds specific receptors on the host cell.

Entry (Penetration)

Virus enters the host cell and releases its genome.

Synthesis

Host machinery produces viral genomes and proteins.

Assembly

Viral genomes and proteins assemble into new virions.

Release

New viruses leave the host cell.

Release of naked viruses

Cell lysis (bursting).

Release of enveloped viruses

Budding through the host membrane.

Bacteriophage

Virus that infects bacteria.

Are bacteriophages enveloped?

No.

Plaque

Clear zone where bacteriophages have lysed bacteria.

Bacteriophage T4

Complex, dsDNA bacteriophage that infects E. coli using the lytic cycle.

Host of bacteriophage T4

Escherichia coli (E. coli).

Lytic cycle

Virus immediately replicates and lyses the host cell.

Steps of the lytic cycle

Attachment

How T4 enters bacteria

Injects DNA through the bacterial cell wall while the capsid remains outside.

Lysozyme

Viral enzyme that breaks down bacterial peptidoglycan.

Lysogenic cycle

Viral DNA integrates into the host genome and remains dormant.

Prophage

Viral DNA integrated into a bacterial chromosome.

Temperate phage

Bacteriophage capable of both lysogenic and lytic cycles.

Bacteriophage Lambda

Temperate dsDNA bacteriophage capable of lysogeny.

Difference between lytic and lysogenic cycles

Lytic kills the host immediately; lysogenic integrates into host DNA and remains dormant.

What triggers a prophage to enter the lytic cycle?

Stress or damage to the host DNA.

Difference between bacterial and animal viruses

Animal viruses infect eukaryotic cells and may be enveloped; bacteriophages infect bacteria and are never enveloped.

How bacteriophages infect bacteria

Inject genome into the cell while the capsid stays outside.

Animal virus entry methods

Membrane fusion, endocytosis, or direct penetration.

HPV genome type

Double-stranded DNA (dsDNA).

HPV disease

Causes warts and can lead to several cancers.

HPV transmission

Direct contact, indirect contact, and autoinoculation.

Latent virus

Virus that remains dormant in host cells.

Oncogenic virus

Virus capable of causing cancer.

How oncogenic viruses cause cancer

Viral DNA disrupts genes controlling normal cell division.

Influenza genome type

Negative-sense single-stranded RNA (-ssRNA).

Why influenza carries RNA-dependent RNA polymerase

Host cells cannot make RNA from RNA templates.

RNA-dependent RNA polymerase

Enzyme that synthesizes RNA from an RNA template.

Antigenic drift

Small mutations that gradually change influenza viruses.

Cause of antigenic drift

Error-prone RNA polymerase.

Antigenic shift

Major genetic change caused by mixing genome segments from different influenza viruses.

Why antigenic shift is dangerous

Produces a new virus that the immune system does not recognize.

Retrovirus

RNA virus that first converts RNA into DNA.

Reverse transcriptase

Enzyme that synthesizes DNA from RNA.

Integrase

Enzyme that inserts viral DNA into the host genome.

Protease

Viral enzyme involved in processing viral proteins.

HIV genome type

Positive-sense single-stranded RNA retrovirus.

Host cells infected by HIV

Helper T cells (CD4+ T cells).

How HIV weakens the immune system

Destroys Helper T cells and continuously mutates.

AIDS

Acquired Immune Deficiency Syndrome caused by advanced HIV infection.

Why HIV is difficult to treat

Reverse transcriptase makes frequent mutations, allowing rapid viral evolution.

Antiretroviral therapy (ART)

Combination of drugs that suppress HIV replication.

Major HIV drug targets

Reverse transcriptase, integrase, protease, and viral DNA synthesis.

Why viruses are difficult to treat

They use host cell machinery, mutate rapidly, and some remain latent inside host cells.

Exam Tip: Viral replication order

Attachment - Entry - Synthesis - Assembly - Release.

Exam Tip: Lytic vs Lysogenic

Lytic = immediate replication and cell death; Lysogenic = viral DNA integrates and remains dormant.

Exam Tip: Naked vs Enveloped

Naked viruses lyse cells; enveloped viruses usually bud from cells.

Exam Tip: Positive vs Negative RNA

+RNA acts as mRNA; -RNA must first be copied into +RNA.

Exam Tip: Retrovirus

RNA - DNA - Host genome - RNA - Protein.