Virus Basics
Dec 18, 2015
• 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
• 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
5th Step: Release– Virus enzyme causes cell membrane lyse (burst)– Viruses are released to find new host– Cycle repeats
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?