BIO 101 1st EditionLecture 22Outline of Last LectureI. TranscriptionII. Translationa. Initiationb. Elongationc. TerminationIII. Genetic Codea. Messenger RNAb. Transfer RNAc. RibsosomeIV. Mutationsa. Base pair substitutionsb. Base pair deletions or insertionsV. Split Genes and RNA ProcessingVI. MicroRNAs and Gene ExpressionVII. Molecular Biology of CancerOutline of Current LectureI. Virus Backgrounda. Virus particlei. Viral genomeii. Viral shellII. ReplicationIII. Host RangeIV. Bacterial Virusesa. Lytic cycleb. Lysogenic cycleV. Animal VirusesVI. Emerging VirusesGenetics of VirusesVirus- simplest of genetic systems- at the border of living and nonliving.- Not really cells (so NOT alive- remember the cell theory)- Lack many cell structures and metabolic machinery- Consist only of viral genes enclosed in a protein shell- By itself, a virus is inert- But it can enter a cell and take over the cell machinery and reprogram the cellto make lots of virus particlesVirus particle- 2 basic partsA. Viral genome (set of genes)- a type of nucleic acid- Genome is either DNA or RNA (never both)- Genome can be double stranded DNA (dsDNA)(just like the genome of cells)BIO 101 1st Edition- Or can be single stranded DNA (ssDNA) or ssRNA (like in cells-mRNA) or dsRNAB. Viral shell- made of protein, called capsid or coat protein, arranged in a particular shape around the viral genome- Shape is often simple- rod or polyhedron- Bacterial viruses often have very complex shell- Some animal viruses have a membrane that surrounds the particleHow do viruses replicate?- Obligate intracellular parasites- ONLY replicate INSIDE a CELL- Viruses use many cellular materials: nucleotides, amino acids, protein synthesis machinery, ATP- Virus infected cell makes thousands of viral genomes and thousands of protein shells and these self-assemble to make particles- Once particles have assembled, they emerge from the host cell, often burstingand killing the cellViral Host Range- nearly all living things have viruses- Viruses that infect very different organisms are often similar in many ways- can be classified into SUPER GROUPS according to their replication strategies- Each virus has a particular host rangeo Some have s VERY NARROW host range (ex. Human cold virus infects only cells of the upper respiratory tract of humans)o Others are broader (ex. Influenza virus- infects a number of different species: human, hogs, birds)o Rabies virus infects a number of different mammals including humans, dogs, rodentsBacterial Viruses- bacteriophage2 alternative life cycles are possible lytic cycle and lysogenic cycleLytic Cycle- Bacteriophage enters cell- Produces a protein that destroys host cell DNA- Makes viral parts (genome and shell) and assembles them to make thousandsof viral particles- Makes a protein that destroys the bacterial cell wall- Cell bursts, releasing all the newly made bacteriophageLysogenic Cycle- Bacteriophage co-exists peacefully with host cell- Genome of phage INSERTS into the host genome called PROPHAGE- Prophage replicates along with host DNA and is passed to all daughter cells- Sometimes the prophage EXCISES from host DNA and enters a LYTIC CYCLE- killing the cellAnimal VirusesBIO 101 1st Edition- Diverse group of viruses, some with DNA genomes, some with RNA genomes, some with membranes, wide variation in life cycles- Few of interest that we will mention todayHerpesvirus- dsDNA viruses: causative agent of 1) fever blisters and cold sores and 2) genital herpesCaused by different related viruses- Primary herpesvirus infection is LYTIC- But, like some bacteriophage, can INSERT the viral genome into the host DNA- the inserted viral genome is called a PROVIRUS- The provirus behaves like a lysogenic phage in many wayso It is latent (doesn’t cause symptoms) coexists peacefullyo Sometimes it EXCISES and goes into a LYTIC CYCLE causing a recurrence of the diseaseo Recurrence of disease= OUTBREAKo OUTBREAKS are associated with host stressRetroviruses= “backward” viruses (replication is the REVERSE of transcription in cells)- ssRNA genome of virus dsDNA copy, catalyzed by enzyme REVERSE TRANSCRIPTS- once a dsDNA copy of the viral genome is made, it can insert into the host DNA- 2 examples that behave differentlyHuman immunodeficiency virus (HIV)-causative agent of acquired immunodeficiency syndrome (AIDS)- Virus infects helper T-cells (an integral part of the immune system)- Host cannot eliminate virus because the immune system is compromised- HIV provirus produces virus particles (DOES NOT co-exist peacefully)- The viruses that are made mutate rapidlyHTLV- another retro virus that causes serious disease- HTLV provirus alters gene expression in the host cell (DOES NOT co-exist peacefully)- Alters cell cycle control- causes inappropriate cell division- Causative agent of adult leukemia in humansEmerging VirusesMany examples of “new viruses”Examples: - New strains of influenza virus- HIV made what seemed like a sudden appearance in early 1980sBIO 101 1st Edition- Deadly Ebola virus- 1st recognized in 1976, causes hemorrhage fever- often with massive bleeding, circulatory collapseWhere do the NEW VIRUSES come from?3 processes contribute.1. Mutation of existing viruses- especially RNA viruses because replication of RNA does not have proof-reading steps like DNA replication- Sometimes mutations change the virus in a way that allows it to avoid host defuses- Example is influenza (ssRNA virus)- new strains keep emerging2. Spread of an existing virus from one HOST to ANOTHER- Ex. Hantavirus- common in rodents (especially deer mice)- In 1993 there was a big increase in the deer mouse population in the SW US- Humans acquired hantavirus by inhaling dust with deer mouse urine and fecal contamination (switch in host)3. Virus disease spreads from an isolated population- Ex. AIDS went unnamed and unnoticed for decades until its spread was enabled by technological and social factorso Affordable international travelo Blood transfusion technologyo Drug