Viruses are obligate intracellular parasites that consist of a nucleic acid core surrounded by a protein coat called a capsid. They can only reproduce inside a living host cell by using the host's materials and enzymes. Viruses come in a wide range of sizes but are generally 20-400 nanometers. Some viruses have an envelope formed from the host cell membrane that may contain spikes. The complete, infectious viral particle is called a virion. Bacteriophages are viruses that infect bacteria and have a more complex structure including a head and tail used to inject viral DNA. They can undergo lytic replication, releasing new virus particles, or lysogenic replication where their DNA inserts into the host chromosome.

1. Virus

2. Virus Viruses are obligate intracellular parasites and can be seen at magnifications provided by the electron microscope.

3. Virus All viruses essentially consist of a nucleic acid core (DNA or RNA) and a protein coat called the capsid. Capsid is made of protein subunits called capsomeres .

4. Virus Viruses cannot grow or replicate on their own (inactive particles). Viruses only reproduce inside a living host cell using its raw materials & enzymes. Tobacco Mosaic Virus Leaf infected with TMV

5. Virus They lack ribosomes & enzymes needed for protein synthesis or metabolism. Viruses are extremely small particles ranging from 20 - 400 nanometers on average

6. Structure of Virus Nucleocapsid includes the viral nucleic acid & its capsid.

7. Structure of Virus Some viruses have lipid rich covering around capsid called the envelope. Envelope usually is formed from host cell membrane. Envelope may have spikes to help chemically recognize & attach to the host cell.

8. Structure of Virus Structurally complete, mature and infectious virus is called the ‘virion’.

9. Viroids Smallest particle able to replicate. Made of a short, single strand of RNA with no capsid. Cause disease in plants.

10. Prions No nucleic acid or capsids. Made of protein particles that have folded incorrectly. Attacks the central nervous system. Cause animal diseases in cows (Mad Cow disease), sheep, & humans.

11. Bacteriophage Bacteriophage is one of the most complex viruses. They attack bacterial cells.

12. Bacteriophage The virus has a head, tail, base plate, & tail fibers. A long DNA molecule is found inside the head. Tail is used to inject the viral DNA into the host cell. Tail fibers are used to attach to the host.

13. Bacteriophage Replication

14. Bacteriophage Replication In the lytic cycle, the host cell bursts, releasing new phage particles. Some phage can also undergo a lysogenic cycle, in which their DNA is inserted into the host chromosome, where it replicates for generations. When conditions are appropriate, the phage DNA exits the host chromosome and enters a lytic cycle.

15. Some viruses have promoters for host RNA polymerase, which they use to transcribe their own genes.

16. Some RNA viruses use their RNA to make mRNA to code for enzymes and replicate their genomes without using DNA.

18. Retroviruses, such as HIV, have RNA genomes that they reproduce through a complementary DNA intermediate.