Viruses

Big Idea 3: Living systems store, retrieve, transmit, and respond to info essential to life processes.

Essential Knowledge

• 3C3: Viral replication results in genetic variation, and viral infection can introduce genetic variation into

the hosts.

Structure of Viruses• Viruses are not cells

small infectious particles consisting of nucleic acid in a protein coat or a membranous

envelope

Viral Genomes

Made of either:

– Double- or single-stranded DNA, or

– Double- or single-stranded RNA

RNA Viruses

Need Reverse Transcriptase: enzyme that makes double stranded DNA to match w/ host cell

Lack replication error-checking mechanisms higher rates of mutation (HIV!)

Herpes Virus-DNA

Ebola Virus-RNA

Capsids and Envelopes• A protein shell that encloses viral genome

• Built from protein subunits called capsomeres

Fig. 19-3

RNA

Capsomere

Capsomereof capsid

DNA

Glycoprotein

18 250 nm 70–90 nm (diameter)

Glycoproteins

80–200 nm (diameter) 80 225 nm

Membranousenvelope RNA

Capsid

HeadDNA

Tailsheath

Tailfiber

50 nm50 nm50 nm20 nm

(a) Tobacco mosaic virus

(b) Adenoviruses (c) Influenza viruses (d) Bacteriophage T4

• Highly efficient allow for rapid evolution and acquisition of new phenotypes!

Viral Replication

• AKA: phages, viruses that infect bacteria

• Have a long capsid head that encloses DNA

• A protein tail attaches phage to host and injects DNA inside

Bacteriophage

Fig. 19-1

0.5 µm

Viruses Need Hosts!• Intracellular

parasites that have a host

range (a limited number of host cells that it can

infect)

Viral Reproductive Cycles

Virus VC

Transcriptionand manufactureof capsid proteins

Self-assembly of new virus particles and their exit from the cell

Entry anduncoating

Fig. 19-4VIRUS1

2

3

DNA

Capsid

4

Replication

HOST CELL

Viral DNA

mRNA

Capsidproteins

Viral DNA

Reproductive Cycles of Phages

• 1st Rep Cycle: lytic cycle Death of host cell• Virulent phage: virus that only reproduces via lytic cycle

Fig. 19-5-1

Attachment1

Fig. 19-5-2

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

Fig. 19-5-3

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

3

Fig. 19-5-4

Phage assembly

Assembly

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

4

Head Tail Tail fibers

3

Fig. 19-5-5

Phage assembly

Head Tail Tail fibers

Assembly

Release

Synthesis of viralgenomes andproteins

Entry of phageDNA anddegradation ofhost DNA

Attachment1

2

4

5

3

• 2nd reprod. Cycle: latent/lysogenic cycle

• No destroying host cell viral DNA is mixed into host cell’s chromosome prophage

• Gives host cell properties like increased pathogenicity in bacteria

Reproductive Cycles of Phages

• Phages that use both lytic and lysogenic cycles

• Env. signal can trigger switch

Temperate Phages

Lambda Phage Attacks E. Coli

Fig. 19-6

PhageDNA

Phage

The phage injects its DNA.

Bacterialchromosome

Phage DNAcircularizes.

Daughter cellwith prophage

Occasionally, a prophageexits the bacterialchromosome,initiating a lytic cycle.

Cell divisionsproducepopulation ofbacteria infectedwith the prophage.

The cell lyses, releasing phages.

Lytic cycle

Lytic cycleis induced or Lysogenic cycle

is entered

Lysogenic cycle

Prophage

The bacterium reproduces,copying the prophage andtransmitting it to daughter cells.

Phage DNA integrates intothe bacterial chromosome,becoming a prophage.

New phage DNA and proteinsare synthesized andassembled into phages.

Evolution of Viruses

• Maybe plasmids?? (circular DNA in bacteria and yeasts) and transposons,

small mobile DNA segments

• Mimivirus (double-stranded DNA) is 2nd largest virus discovered

• Virus evolved before or after cells?

Megavirus (2 strand DNA) Oct. 2011, largest virus

• Harmless derivatives of pathogenic microbes that stimulate immune system to mount

defenses against actual pathogen

• Viral infections cannot be treated by antibiotics

Vaccines

With your partner, make a list of vaccines you have

received!

• Epidemic: New cases of a disease appear in a local area aka outbreak

• If epidemic spreads to another locale pandemic

Epidemics vs Pandemics

Plant Viruses

Most have an RNA genome– Horizontal transmission, entering through

damaged cell walls – Vertical transmission, inheriting the virus from a

parent

Viroids and Prions: Simple

• Viroids: circular RNA molecules that infect plants and disrupt growth

• Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in

mammals

Ex: Prion Mad Cow Disease

Ex: Prion Creutzfeldt Jakob Syndrome

Understanding Viruses Moviehttp://www.schooltube.com/video/360cafa902fb60e55f61/Understanding-Viruses