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

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

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

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

Viral Shapes Rod Helix Polyhedral   Sphere Complex

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

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

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

Lytic vs. Lysogenic Cycle