BIO 101 1st Edition Lecture 22 Outline of Last Lecture I Transcription II Translation a Initiation b Elongation c Termination III Genetic Code a Messenger RNA b Transfer RNA c Ribsosome IV Mutations a Base pair substitutions b Base pair deletions or insertions V Split Genes and RNA Processing VI MicroRNAs and Gene Expression VII Molecular Biology of Cancer Outline of Current Lecture I Virus Background a Virus particle i Viral genome ii Viral shell II Replication III Host Range IV Bacterial Viruses a Lytic cycle b Lysogenic cycle V Animal Viruses VI Emerging Viruses Genetics of Viruses Virus 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 cell to make lots of virus particles Virus particle 2 basic parts A 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 dsRNA B 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 particle How 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 bursting and killing the cell Viral Host Range nearly all living things have viruses Viruses that infect very different organisms are often similar in many wayscan be classified into SUPER GROUPS according to their replication strategies Each virus has a particular host range o 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 rodents Bacterial Viruses bacteriophage 2 alternative life cycles are possible lytic cycle and lysogenic cycle Lytic Cycle Bacteriophage enters cell Produces a protein that destroys host cell DNA Makes viral parts genome and shell and assembles them to make thousands of viral particles Makes a protein that destroys the bacterial cell wall Cell bursts releasing all the newly made bacteriophage Lysogenic 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 CYCLEkilling the cell Animal Viruses BIO 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 today Herpesvirus dsDNA viruses causative agent of 1 fever blisters and cold sores and 2 genital herpes Caused 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 ways o It is latent doesn t cause symptoms coexists peacefully o Sometimes it EXCISES and goes into a LYTIC CYCLE causing a recurrence of the disease o Recurrence of disease OUTBREAK o OUTBREAKS are associated with host stress Retroviruses 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 differently Human 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 rapidly HTLV 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 humans Emerging Viruses Many examples of new viruses Examples New strains of influenza virus HIV made what seemed like a sudden appearance in early 1980s BIO 101 1st Edition Deadly Ebola virus 1st recognized in 1976 causes hemorrhage fever often with massive bleeding circulatory collapse Where 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 emerging 2 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 factors o Affordable international travel o Blood transfusion technology o Drug abuse