Viruses

First virus discovered

Tobacco mosaic virus (TMV)

Virus structure

Nucleic acid genome either DNA or RNA

Protein coat made up of capsomeres

Viral structure: helical

Hollow rods

Flexible or ridged

Viral structure: polyhedral

Many sides

Most have 20 sides = icosahedral

Viral structure: Complex

Variable shape

Can have additional parts beyond capsids

Example: bacteriophage

Viral structure: enveloped

W/ lipid layer that covers capsids

Can contain proteins or carbohydrates

May have glycoprotein spikes

Stolen from human host

Viral structure: Nonenveloped

No lipid layer

Human virome

All the viruses that live in the body

Lytic cycle

Mechanism of bacteriophage multiplication that results in host cell lysis

Mechanism of lytic cycle

1. Attachment: bacteriophage attaches to host cell w/ weak interactions

2. Penetration: phage lysozymes breaks down bacterial cell wall and pushes DNA through w/ sheath like a syringe

3. Biosynthesis: synthesis of nucleic acid and protein to make more viruses

4. Maturation: assembly and packaging viruses

5. Release: virions are released from host cell when plasm membrane opens and cell lysis occurs

Lysogenic cycle

Mechanism of bacteriophage multiplication that involves stages in viral development that result in the incorporation of viral DNA into the host DNA 🧬

Lysogenic mechanism

1. Phage attaches to the host cell

2. Injects DNA

3. Phage DNA integrates w/i the bacterial chromosome by recombination becoming a prophage

4. Lysogenic bacterium reproduces normally

5. Prophage is copied w/ bacterial chromosome

6. Many cell divisions produce many infected bacteria

7. Occasionally prophage may exercise from the bacterial chromosome by another recombination event initiating a lytic cycle

Animal virus replication steps

1. Attachment

2. Entry

3. Uncoating

4. Biosynthesis

5. Maturation

6. Release

Animal virus replication: attachment

Virus binds to protein receptor sites on the plasma membrane

Animal virus replication: entry

Either by:

Receptor mediated endocytosis brings in virus or

Fusion

Animal virus replication: entry → receptor mediated endocytosis to bring in viruses

Nonenveloped viruses

Cell’s plasma membrane continuously folds inward to form vesicles

Animal virus replication: entry → fusion

Enveloped viruses

Viral envelope fuses w/ the plasma membrane and releases the capsid into the cell’s cytoplasm

Animal virus replication: uncoating

Capsid is broken down and viral genome is released separating the viral nucleic acid from its protein coat

Sometimes by host enzymes

Sometimes by viral enzymes that are quickly synthesized upon entry

Animal virus replication: biosynthesis

Varies by genome type, DNA and RNA

Replication of genome

Assembly of viral proteins

Animal virus replication: DNA virus biosynthesis

Transcription of a portion of the viral DNA

Translation

Replicate DNA in nucleus of host using host enzymes

Synthesis capsid and other proteins in the cytoplasm using host enzymes

Animal virus replication: RNA viruses biosynthesis

Can have viral RNA polymerases to copy RNA genome and can use RNA as a template

Multiply in host cell cytoplasm

Several mRNA formation mechanisms occur among different groups of RNA viruses

RNA dependent RNA polymerase

Not encoded in any cell’s genome

Viral genes cause the enzyme to be made by a host cell

Enzyme catalyzes the synthesis of another strand of RNA which is complementary in base sequence to the original infecting strand

Animal virus replication: maturation

Proteins migrate into the nucleus and are joined w/ the newly synthesized DNA to form virions

Virons mature

Animal virus replication: release

Varies depending on the virus and whether it has a viral envelope

Animal virus replication: release → rupture

Host plasma membrane is broken apart as virus is release

Lysis of host cell

Occurs w/ nonenveloped viruses

Animal virus replication: release → budding

Host plasma surrounds virus which is pinched off

Host cell not immediately killed, can survive

Steals phospholipids from host to create envelope for viral protein release

Host range

Cell types that viruses can infect

Determined by virus and host cell interaction/attachment

Bacteriophage replication basics

Attachment: tail fibers attach to cell wall

Entry: viral DNA is injected into host cell

Uncoating: not required

Biosynthesis: in cytoplasm

Chronic infection: lysogeny

Release: host cell is lysed

Animal viruses replication

Attachment: attachment sites are plasma membrane proteins and glycoproteins

Entry: capsid enters by receptor mediated endocytosis or fusion

Uncoating: enzymatic removal of capsid proteins

Biosynthesis: in nucleus (DNA viruses) or cytoplasm (RNA viruses)

Chronic infection: latency, slow viral infections, cancer

Release: enveloped → budding, nonenveloped → rupture through the plasma membrane

Phage therapy

The use of bacteriophages to fight infections

Plaque

Clearing in a bacteria lawn resulting from lysis of phages

Count number of deaths

More concentrated = more plaques

Virus evolution

Make more mutations in spike proteins to infect more hosts

Transduction

Bacterial DNA horizontally transfer from a donor cell to a recipient cell made inside a virus that infects bacteria

Mediated by a lysogenic phage which packages bacterial DNA along w/ its own DNA in the same capsid

Bacterial DNA is transferred from a donor cell to a recipient cell inside a virus that infects bacteria

Beginning of the lytic cycle w/ a twist at maturation

Includes homologous recombination

Transduction mechanism

1. Attachment

2. Penetration

3. Biosynthesis: copying viral genome and viral proteins

4. Maturation: Instead of packaging viral DNA, host DNA gets packaged

5. Relax: host cell lyses and viral releases new infection in the same generation

Magic bullet

Kills bacteria w/o harming human cells

Harms only specific pathogens not anything in its pathway

Selective toxicity

Magic bullet relation to treating viral infections and bacterial infections

Few antiviral drugs b/c viruses use host cell’s machinery making it harder to fight

Many antibiotics b/c bacteria use their own cell’s machinery