Viruses

Acellular

Not composed of cells

Obligate intracellular parasites

Cannot multiply unless they invade a host cell and instruct genetic and metabolic machinery to make and release new viruses

Animal virus

Viruses that infect eukaryotes

Bacteriophage

Viruses that infect bacteria

Virion

Fully formed virus able to establish an infection in a host cell

Retrovirus

RNA virus that reproduces by transcribing RNA into DNA and inserting DNA into a chromosome

Provirus

Viral DNA that inserts into a host genome

Components of all viruses

Genome, capsid, attachment proteins

Capsid

Protein coat that protects viral genome from damage when outside host cell

Attachment proteins

Proteins in outer-most layer of virion (capsid or envelope) that binding to host cell

Spike attachment proteins

Allow tighter binding to host cell

Components of some viruses

Envelope, matrix enzymes

Cell envelope

Phospholipid bilayer containing viral proteins obtained from host's cell membrane that plays a role in entry and exit from host cell

Matrix enzymes

Viral enzymes in viral core with special roles in multiplication not provided by host cell's enzymes

Matrix enzyme benefits

Higher rates of spontaneous mutation allowing for viral antigenic variation and development of drug resistance

Antigenic variation

Ability to change appearance to avoid immune system

Helical capsid

Rod-shaped cylinder

Icosahedral capsid

20-sided sphere

Atypical capsid

Complex viruses

Enveloped helical viruses

Helical nucleocapsid placed within an envelope (influenza)

Naked helical viruses

Long rods, cylindrical capsid with helical structure (tobacco mosaic virus)

Enveloped icosahedral viruses

Naked icosahedral viruses

Complex viruses

Virions composed of several parts, each with separate shapes and symmetries

Virus multiplication steps

Adsorption, penetration, uncoating, synthesis, assembly, release

Adsorption

Attachment proteins or spikes of virion specifically bind to proteins/sugars on the host cell membrane

Penetration

Virus entry into host cell's cytoplasm

Uncoating

Viral genome (nucleic acids) freed from capsid and other surrounding layers into cytoplasm of host cell

Synthesis

Replication of viral components by host cell and viral matrix enzymes (if present)

Assembly

Viral genome is packaged into capsids and attachment proteins/spikes are placed in capsid or host's cell membrane

Release

Virus exits host cell

Endocytosis

Host's cell membrane surrounds & engulfs the entire virion

Fusion

Viral envelope combines with host's cell membrane, the nucleocapsid enters host cell's cytoplasm; some enveloped viruses

Lysis

Virions burst host cell and immediately kills host cell; naked and complex viruses

Budding

Host's cell membrane (containing attachment proteins) wraps around nucleocapsid forming viral envelope and releasing virion; slowly kills host cell; enveloped viuses

Retroviruses

Enveloped RNA viruses with matrix enzyme, reverse transcriptase

Retrovirus multiplication

Adsorption, penetration, uncoating, provirus formation, synthesis, assembly, release by budding

Acute infections

Short term infection of host

Persistent infections

Permanent infection of host by avoidance of host cell defenses by becoming a latent or chronic disease

Latent infections

Sporadic symptoms; periods of active and inactive viral multiplication

Chronic infections

Symptoms gradually appear; viral multiplication is slow

Signs

Objective evidence of disease; fever, rash, cough

Symptoms

Subjective evidence of disease; headache, dizziness, nausea

Asymptomatic

Without symptoms