BIO

Viruses

• ⭐ Acellular → not cells

• No metabolism, cannot grow

• Must use a host cell to replicate

• Ultimate parasite → uses host machinery

• Origin uncertain; may be pieces of nucleic acid collected from other sources

• Can infect almost all life forms, often species- and tissue-specific

1892: Tobacco mosaic virus

• → filtered plant extract still caused disease → smaller than bacteria ⭐

1930s: Electron microscope (EM)

• makes viruses visible

• Most viruses too small for light microscope (~20 nm)

• EM magnification: 500,000x, resolution ~0.05 nm ⭐

Virus Evolution Hypotheses

1. Devolution: from free-living cells

2. Escape: nucleic acids escaped from cells

3. Self-replicating molecules: started as independent RNA/DNA

Devolution

from free-living cells

Escape

nucleic acids escaped from cells

Self-replicating molecules

started as independent RNA/DNA

Virus Structure

• Core: DNA or RNA (⭐ one type only)

  • Single or double-stranded

  • Circular or linear

  • One piece or segmented

• Protein capsid → protects genome ⭐

• Envelope (optional) → similar to cell membrane

• Shapes: helical, polyhedral, complex ⭐

Core

• DNA or RNA (⭐ one type only)

  • Single or double-stranded

  • Circular or linear

  • One piece or segmented

Protein capsid

→ protects genome

Envelope (optional)

→ similar to cell membrane

Shapes

 helical, polyhedral, complex ⭐

Bacteriophage T4

Host	Features	Most Tested
Bacteria (E. coli)	DNA genome, tail fibers attach to host	DNA virus, bacteriophage

Adenovirus

Human respiratory tract

Non-enveloped, spikes on capsid

Non-enveloped virus

⭐HIV (Retrovirus)

Humans

RNA → DNA via reverse transcription, enveloped

Retrovirus, envelope

Virus Genome Notes

• DNA → host makes new viruses

• RNA → can be directly transcribed or replicated first

• RNA viruses mutate faster → more errors ⭐

Virus Classification (4 main ways)

1. Nucleic acid type & function ⭐

2. Capsid structure ⭐

3. Enveloped or non-enveloped ⭐

4. Genome structure (linear, circular, segmented)

Baltimore classification

• based on type of nucleic acid + how it makes mRNA

First virus discovered

→ Tobacco mosaic virus ⭐

Virus vs cell: name 3 differences

→ acellular, no metabolism, needs host ⭐

Bacteriophage T4 infects?

 → E. coli ⭐

HIV unique feature?

→ RNA → DNA (retrovirus), envelope ⭐

RNA viruses mutate faster because…

→ replication enzymes make more errors ⭐

Capsid shapes?

→ helical, polyhedral, complex ⭐

Enveloped vs non-enveloped virus example?

→ Envelope: HIV; Non-envelope: Adenovirus ⭐

Virus genome can be…

→ DNA or RNA, single/double-stranded, circular/linear, segmented ⭐

Three virus origin hypotheses?

→ Devolution, Escape, Self-replicating ⭐

Virus Classification by Genome (Core)

Viruses are classified by genetic material (DNA/RNA), strands, shape, and segments:

Core Type	Strand	Shape	Segments	Examples
DNA	ss	Linear	Non-segmented	Herpesvirus, smallpox
DNA	ds	Circular	Non-segmented	Papillomavirus, many bacteriophages
RNA	ss	Linear	Non-segmented	Rabies, retroviruses
RNA	ss	Linear	Segmented	Influenza
RNA	ds	Linear	Non-segmented	Parainfluenza

⭐ Most Tested

• Rabies = ssRNA, enveloped helical;

• Smallpox = dsDNA, complex;

• Influenza = segmented ssRNA

Icosahedral (20 faces)

• Polio virus (naked)

• Epstein-Barr virus (enveloped)

Helical

• Tobacco mosaic virus (naked)

• Mumps virus (enveloped)

Complex

• Herpesvirus

• Bacteriophage T4

Tip: Capsid + genome type =

most common exam Q

Virus Effects on Cells (Cytopathic Effects)

Viruses can damage host cells:

• Lysis → Cell bursts ⭐

• Apoptosis → Programmed cell death

• Immune response → Symptoms

• Budding → Virus leaves without killing cell

Lysis

→ Cell bursts ⭐

Apoptosis

 → Programmed cell death

Immune response

→ Symptoms

Budding

→ Virus leaves without killing cell

Steps of Virus Infection

1. Attachment

2. Entry

3. Replication and Assembly

4. Egress (Release)

Attachment

• Virus binds host cell receptors

• Very specific to species/tissue ⭐

Entry

• Bacteriophages → DNA injected directly

• Eukaryotic viruses → Endocytosis or membrane fusion if enveloped

Replication & Assembly

Virus Type

Process

DNA

• Make mRNA → viral proteins; duplicate DNA → new genomes

RNA

• Make complementary RNA if needed; transcribe mRNA → proteins; copy RNA → new genomes

RNA retrovirus

• Reverse transcribe RNA → DNA → integrate into host; host makes viral proteins/genomes

Egress (Release)

• Lysis → kills host

• Budding → host survives

Most Tested

Influenza cycle: glycoproteins attach → engulfed → RNA/proteins made → new virions

Bacteriophages (Phages)

• Viruses that infect bacteria

• Tail fibers inject DNA into host

• Have two cycles

Lytic cycle

Virus replicates → cell bursts

Lysogenic cycle

Viral DNA integrates → replicates with host; stress triggers lytic cycle ⭐

Rabies virus has which genome type?

ssRNA, enveloped helical ⭐

Smallpox virus capsid type?

dsDNA, complex ⭐

Steps of viral infection?

Attachment → Entry → Replication/Assembly → Egress ⭐

Latent

Virus hides in tissue; may reactivate

Herpes simplex, Varicella-zoster (chickenpox → shingles) ⭐

Oncogenesis

Can cause cancer

Hepatitis B & C, HPV ⭐

Varicella-Zoster Virus (Chickenpox/Shingles)

• Childhood → chickenpox → latent in nerves → may reactivate → shingles

• dsDNA, enveloped, icosahedral capsid

HPV (Human Papillomavirus)

• Naked icosahedral capsid, dsDNA

• Sexually transmitted → can cause cervical cancer ⭐

Influenza Virus

• Infects lungs → fever, cough, aches

• Responsible for 1918 Spanish flu (20–50M deaths worldwide)

• 3 types:

  • Type A → serious

  • Type B & C → mild

• Proteins: H = entry, N = exit

• Antigenic changes:

  • Drift → small mutations, yearly flu vaccine needed

  • Shift → new strain from 2+ viruses combining → may cause pandemic ⭐

HIV (Retrovirus)

• Enveloped, icosahedral, ssRNA

• Attaches to CD4 receptor → fuses with cell → RNA → DNA (reverse transcription) → integrates into host genome ⭐

Plant Viruses – Transmission & Effects

• Horizontal: Enters via damaged tissue, pollen, insects

• Vertical: Parent → offspring

• Effects:

  • Hypoplasia (stunted growth)

  • Necrosis (death of tissue)

• Human impact: Can indirectly affect food supply via crops

Vaccines – Preventing Viral Infection

• Boost immune protection

• Types:

  • Attenuated live virus ⭐

  • Killed virus

  • Molecular subunits

• Small risk of infection with live vaccines

Prions – Infectious Proteins

• Very small, no nucleic acid

• Cause fatal neurodegenerative diseases:

  • Mad cow (BSE), Creutzfeldt-Jakob (humans), Kuru (humans), Scrapie (sheep), Chronic wasting disease (deer, moose, elk)

• Resistant to cooking

• Mechanism: Abnormal PrP → converts normal PrP → abnormal PrP ⭐

Viroids – Plant RNA Pathogens

• Tiny circular RNA

• Infect only plants

• Replicate inside cells but do not make proteins

• Spread via infected tools or plant parts

• Can cause crop failures ⭐

Which virus causes shingles?

Varicella-zoster (latent dsDNA virus) ⭐

Difference between antigenic drift and shift?

Drift = small mutations; Shift = new viral subtype

Name 2 prion diseases in humans

Creutzfeldt-Jakob, Kuru ⭐