MCB 150 The Molecular and Cellular Basis of Life Mobile Genetic Elements Genetic Technologies Handouts for Lectures 35 40 MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Mobile Genetic Elements viruses and plasmids Viruses Blessing to the research community Curse much of the time to the medical community Have been found in all 3 domains viruses that infect bacteria are speci cally called bacteriophage or phage MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 1 2 Viruses Are Can persist on their own under permissive conditions but Can reproduce only within a host cell using tools from host Genomes may code for a few enzymes or proteins but they must use ATP AAs tRNA nucleotides ribosomes etc from host to replicate Simple structures consisting of nucleic acid protein coat Capsid and sometimes a few enzymes Some animal viruses are surrounded by a biological membrane acquired during release from host cell called an MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Viruses exist in a few basic shapes MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 3 4 Viruses have nucleic acid genomes Typically a few dozen to a few hundred genes Smallest known viral genome is only 1 700 bases and a few genes Largest known at this time has a genome over 2 million bp Nucleic acid in viral genomes varies widely Can be either DNA or RNA Can be either single or double stranded Can be either linear or circular Can be single copy or multiple copies Can be segmented or non segmented Most common type is MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Viral replicative cycles life cycles consist of 3 tasks that fall into most or all of the following 7 basic steps MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 5 6 Viral replicative cycles 1 to speci c host cell surface receptors Speci c interaction of capsid protein or glycoprotein of enveloped virus with a receptor on host usually a glycoprotein with its own natural function 2 transfer of viral nucleic acid into host cell Sometimes just the nucleic acid enters the cell sometimes the entire viral particle enters the cell Phage usually have to inject their nucleic acid through a cell wall leaving the capsid behind Animal viruses typically enter by fusion or endocytosis MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Viral replicative cycles 3 production of enzymes needed to replicate viral nucleic acid and sometimes shut down transcription replication of host cell s DNA For some viruses enzymes for replicating viral nucleic acids aren t made by the host cell so virus has to bring them along Not all virus types have a de ned early gene phase 4 of viral nucleic acid New virions must package the original type of nucleic acid 5 production of capsid proteins needed for assembly of viral particles and enzymes needed for release of virus from host cell MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 7 8 Viral replicative cycles 6 of new virus particles from replicated nucleic acid and newly synthesized capsid proteins 7 of new virus particles from host cell Non enveloped viruses and most phage burst cell when released enzyme made by phage lyses cell wall Enveloped viruses e g most animal viruses are released by budding released at regions of membrane modi ed by viral proteins surrounds the virus with host membrane why is this advantageous for the virus MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Fusion of enveloped animal virus with cell membrane Viral nucleic acid Envelope Capsid protein Receptor binding spikes Host cell plasma membrane Viral nucleic acid Viral capsid MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 9 10 Entry by endocytosis can be enveloped or naked Viral nucleic acid Viral RNA polymerase replicase MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Once viral nucleic acid is released in a host cell the remaining steps in the replicative cycle depend on the nature of the genetic material DNA RNA DNA Viruses Most common DNA viruses are ds DNA viruses Best studied are phage T4 and lambda or Virulent phage follow a lytic pathway resulting in production of new viral particles and release from the cell Temperate phage can follow either a lytic pathway or a lysogenic pathway which results in a period of dormancy that they can exit from at a later time MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 11 12 The replicative cycle of the virulent phage T4 Release and host cell lysis 6 Viral DNA Transcription by host RNA polymerase 1 Replication of viral DNA 3 5 Virus assembly Viral capsid Early viral mRNA Transcription by modi ed host RNA polymerase Translation Synthesis of early viral proteins 2 Late viral mRNA Modi cation of host RNA polymerase 4 Translation Synthesis of late viral proteins Viral tail proteins Host genome Viral proteins MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Degradation of host genome 13 T4 is a virulent phage but some phage are Bacteriophage is a good example of a temperate virus Temperate phage like make molecular decision to enter or cycle Lytic cycle Viral DNA is transcribed and replicated new virus particles produced cell lysed Lysogenic cycle Viral DNA is integrated into host genome no new virus particles produced most viral genes turned o MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees 14 If lysogeny is established Cell is immune to infection by same type of phage Phage in lysogeny sometimes called a Active genes within prophage can confer properties to host cell e g non pathogenic bacteria can make toxins MCB 150 The Molecular and Cellular Basis of Life University of Illinois Board of Trustees Question If the main goal of a virus is to replicate its genetic material and package more virus particles how would an RNA virus make more RNA What are the nucleic acid synthesizing enzymes we ve discussed this term DNA polymerase RNA polymerase and primase What does each of these use as a template DNA polymerase makes DNA from a RNA polymerase and primase make