19-7 Capsid and viral genome enter the cell Capsid RNA HOST CELL Envelope (with glycoproteins) Viral genome (RNA) Template mRNA Capsid proteins ER Copy of genome (RNA) Glyco- proteins New virus The Lytic Cycle a phage reproductive cycle that culminates in the death of the host cell produces new phages and digests the hosts cell wall, releasing the progeny viruses A phage that reproduces only by the lytic cycle is called a virulent phage The Lysogenic Cycle The lysogenic cycle replicates the phage genome without destroying the host The viral DNA molecule is incorporated into the host cells chromosome prophage Every time the host divides, it copies the phage DNA and passes the copies to daughter cells An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode Temperate phages Use both lytic and lysogenic cycles Viral Hosts Host Range Each type of virus can infect & parasitize only a limited range of host cells Identify host cells via lock & key fit Between proteins on viral coat & receptors on host cell surface Broad host range Rabies = can infect all mammals Narrow host range Human cold virus = only cells lining upper respiratory tract AIDS virus = binds only to specific WBC Defenses against viruses Bacteria have defenses against phages Natural selection favors bacterial mutants with receptor sites that are no longer recognized by a particular type of phage Bacteria produce restriction enzymes that recognize & cut up foreign DNA Modifications to bacterias own DNA prevent its destruction by restriction enzymes Its an escalating war
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