VIRUSES Viruses Are Not Cells There are several

VIRUSES

Viruses Are Not Cells! There are several structural and functional differences between cells and viruses n The structural differences include: n – – – n No nucleus No cell membrane No organelles The functional differences include: – – Do not grow Do not respire Do not develop Do not reproduce

Viral Structure n Viruses are made up of 2 parts: – Capsid—a protein coat which enables the virus to bind to a cell – Nucleic acid—either DNA or RNA n Some viruses have an additional protective coating called an envelope

Replication vs. Reproduction n Viruses do not reproduce n Viruses replicate n Reproduction requires cell division and replication does not n A virus requires a host for it to replicate n A host is a living cell that provides all the materials a virus needs to replicate

Classification of Viruses n Shape: allows viruses to attach to only a few kinds of cells; like two pieces of a jigsaw puzzle – Filovirus—has no distinct shape § Ebola- Zaire Ebola Virus, has the highest case-fatality rate, up to 90%. Massive blood loss leads to death. – Polyhedral virus—multifaceted geometric shape § Herpesvirus- Cold sores, shingles, virus lays dormant along nerve fibers

− Binal virus—has a polyhedral capsid and a helical tail • Bacteriophage T 4 - virus which infects E. Coli. for molecular biology research − Helical virus—tightly coiled DNA or RNA inside a long, narrow capsid • Tobacco Mosaic Virus-infect members of nine plant families, 125 individual species

Classification of Viruses (cont. ) n Host: – Plant virus – Animal virus – Bacterial virus—these are also called bacteriophages

Lytic Cycle Bacteriophage Bacterial chromosome DNA or RNA Entry Bacterial host cell Attachment Lysis and Release Replication Assembly

Lysogenic Cycle Entry: Lysis & Release: Assembly: Not pictured Prophage Replication:

Retrovirus n Contains an RNA core that is replicated by 1 st transcribing its RNA into DNA n Ex: HIV—the virus that infects white blood cells and causes AIDS 1. RNA & reverse transcriptase enter the host cell 4. Viral DNA produces new viruses 2. Enzymes copy the viral RNA into DNA 3. Viral DNA attaches to cell DNA

Nonviral Particles n Viroid: – No capsid – Contains RNA – Cause plant diseases n Prion: – Protein molecule with no RNA or DNA – Cause animal diseases (i. e. mad cow disease)