 Virus = an infectious particle consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope  NOT living (unable to replicate its genes or regenerate its own supply of ATP)  Very tiny – smaller than a ribosome  Genome may be: double-stranded DNA, single-stranded DNA, double-stranded RNA, or single-stranded RNA  Single linear or circular molecule of nucleic acid

 Caspid = the protein shell enclosing the viral genome (variety of shapes)  Viral envelopes = accessory structure that helps a virus infect its host derived from the membrane of the host cell (containing both host cell and viral proteins and glycoproteins)  Bacteriophages (“phages”) = viruses that infect bacteria

 Caspid = the protein shell enclosing the viral genome (variety of shapes)  Viral envelopes = accessory structure that helps a virus infect its host derived from the membrane of the host cell (containing both host cell and viral proteins and glycoproteins)  Bacteriophages (“phages”) = viruses that infect bacteria

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 Two Mechanisms (in phages):  1. Lytic Cycle – culminates in the death of the host cell (“virulent phages”)  Bacteria lyses open  Phages produced within the cell are released  Phages can infect a healthy cell  *Note: Often, when phage DNA enters a bacterium, it is recognized as foreign and cut up by restriction enzymes

 2. Lysogenic Cycle – replicates the phage genome without destroying the host  Phages capable of using both modes are called temperate phages  Viral DNA is known as a prophage when it is integrated into the bacterial chromosome, yet for the most part, it is silent (not transcribed, just replicated)  The trigger to switch from a lysogenic cycle to a lytic cycle is usually environmental (radiation or presence of certain chemicals)

 Only reproduce inside host cells  Key features that influence scheme of infection:  RNA or DNA, single or double stranded  Presence or absence of a membranous envelope derived from host cell membrane  In general…  Bacteriophages – few have an envelope or RNA genome  Animal viruses – most have RNA genome and envelope

 “Keys” to enter animal cell = glycoproteins that bind to receptor molecules  Viruses that are coated in the host cell’s plasma membrane are free to infect other cells (they are disguised)  Some viruses have other envelopes (such as the nuclear membrane) and have double- stranded DNA genome  After infection, such viruses may remain latent until some sort of physical or emotional stress causes a flare-up… this can happen throughout the person’s life.

 Retroviruses – animal viruses with the most complicated reproductive cycles  Equipped with reverse transcriptase which transcribes RNA into DNA  Example: HIV (human immunodeficiency virus) that causes AIDS (acquired immunodeficiency syndrome)  Once entering a cell, the provirus (integrated viral DNA) never leaves the host’s genome  This DNA is translated and transcribed into new virus particles that are released from the cell