1. VIRUSES
Viruses – are segments of nucleic acids contained in a protein coat; they are not cells; they are smaller than prokaryotes and range in size from 20nm to 250 nm

2. Is A Virus Alive? NO, viruses do not have all the properties of life, so biologists do not consider them to be living; viruses do not grow, do not have homeostasis, and do not metabolize
Viruses are pathogens – agents that cause disease; they replicate by infecting cells and using the cell to make more viruses

3. Viral Structure:
capsid– the virus protein coat; may contain either RNA or DNA, but not both
envelope – viral membrane; it helps the virus enter cells; it consists of protein, lipids, and glycoproteins
nucleic acid – DNA or RNA
RNA viruses: HIV (Human
Immunodeficiency Virus), Influenza, rabies
DNA viruses: warts, chickenpox,
mononucleosis

4. Viral Structure Glycoprotein viral membrane protein coat
used to attach to surface of cell
DNA or
RNA

5. Viral Shapes
Rod
Helix
Polyhedral
Sphere
Complex

6. Viral Reproduction:
Viruses lack enzymes necessary for metabolism and have no structures to make proteins.
Viruses must rely on living cells (host cells) for replication.
*How do viruses enter a cell?
Plant cell – through tiny tears in the cell wall
Animal cell - by endocytosis
Bacteria – it punches a hole in the bacterial wall and injects its DNA into the cell

7. Once inside a cell, the virus can take one of two different paths: the lytic cycle or the lysogenic cycle.
Lytic cycle – is the cycle of viral infection, replication, and cell destruction

8. Sometimes the DNA is incorporated into the host genome without destroying the cell where it can be passed on to subsequent generations.
Lysogenic cycle – the viral gene is inserted into the host chromosome; the viral gene is copied without destroying the host cell; a change in the environment however can cause the viral gene to go into the lytic cycle

9. Lytic vs. Lysogenic Cycle