Virus Basics

• Virus: A biological particle composed of nucleic acid and protein

• Intracellular Parasites: organism that must “live” inside a host

What is a Virus?

viruses50-200 nm

prokaryotics cells200-10,000 nm

prion2-10 nm

viroids5-150 nm

eukaryotics cells10,000-100,000 nm

100 nm

1 nanometer (nm) = one billionth of a meter

• Reproduce• Have nucleic acid• Adapt to

surroundings• Have

organization

• Not made of cells or organelles

• Can’t reproduce on own• Don’t metabolize

energy• Don’t perform cellular

processes

Are viruses alive?

• All Have:– 1) Capsid: coat of

protein that surrounds nucleic acid

– 2) Nucleic Acid: RNA or DNA

• Some Have:– Tail Fibers: Used

for attachment (not legs)

• Shape & size varies

Virus Video

• Bacteriophages (viruses that attack bacteria) are often studied

• Replication is similar with many animal viruses

• Two “life” cycles: Lytic & Lysogenic

Dozens of phage viruses attacking an E. coli bacteria cellThe bacteria cell is destroyed

and new viruses escape

1st Step: Attachment– Virus lands on cell

membrane– Virus attaches to a

cell receptor– Virus acts as key;

receptor acts as lock• No attachment = No

infection

2nd Step: Entry–Virus enzyme weakens cell membrane–Genetic material (DNA or RNA) enters host

cell

endocytosis

3rd Step: Replication– Virus DNA/RNA

uses ribosomes to make virus proteins

– Virus proteins created by transcription/ translation

transcription translationViral DNA Viral RNA Viral Proteins

4th Step: Assembly–New virus

proteins are assembled in the cytoplasm

5th Step: Release– Virus enzyme causes cell membrane lyse (burst)– Viruses are released to find new host– Cycle repeats

Attachment

Entry

Replication & Assembly

Release

All stages on one slide

1st step: Attachment = Same as lytic cycle

2nd Step: Entry = Same as lytic cycle

cell DNAPro-virus

3rd step: Replication –Provirus Created

• Virus DNA combines with cell DNA

– Infected cell divides by mitosis and copies the provirus

–Each new cell will contain the provirus

Virus DNA

Pro-virus

Pro-virus

4th Step: Assembly:Same, except many new viruses are assembled in many cells

5th Step: Release

Same, except many cells burst releasing many more viruses

The viral DNA become active and starts making new viral proteins

The infected cells burst…releasing the new viruses

1. True or False: All viruses have a capsid, nucleic acid, and tail fibers.

2. List 3 reasons why viruses may be living? Nonliving?

3. What is the virus capsid made from?

4. What type of organism do phages infect?

5. In order list the stages of the lytic/lysogenic cycles.

6. In which stage…• does the host cell explode?• are virus proteins built to make new viruses?• is a provirus created?• does a virus connect with the host cell’s receptors?

7. Which two objects make up a provirus?

8. Which virus “life” cycle is considered the most harmful? Why?