Lecture 36_Virology I_2024 PDF

Outline 1. What is a virus 2. Viral Structure 3. Viral ‘Life’ Cycle 4. Viral Genomes 5. Viral Diversity & Ecology 1. What is a virus? Viruses are not cells No cytoplasmic membrane No cytoplasm No ribosomes Generally smaller than cells...

Outline 1. What is a virus 2. Viral Structure 3. Viral ‘Life’ Cycle 4. Viral Genomes 5. Viral Diversity & Ecology 1. What is a virus? Viruses are not cells No cytoplasmic membrane No cytoplasm No ribosomes Generally smaller than cells λ phage E.coli 1. What is a virus? What is a virus? Obligate intracellular parasites Small 10-300 nm, typically Diverse genome types -small genomes: 2-300 genes, typically (bacteria have ~500 - 12k) -RNA or DNA; ss or ds; linear or circular; “+” or “-” sense Diverse shapes spherical, helical, polyhedral, complex Discrete host range generally have narrow host range 1. What is a virus? Are viruses alive? A.Yes B.No C.Sometimes D.Maybe E. I don’t care, just give me the right answer 1. What is a virus? Are viruses alive? Cannot live without host No metabolic machinery, cannot generate energy Do not grow in size No ribosomes Incapable of replicating its own DNA, transcribing its own genes, or making its own proteins Have only one type of nucleic acid But... they live when inside of the host, and they evolve... 2. Viral structure Viral Structures Present in all viruses Nucleic acid genome: DNA or RNA (never both), genome can be single stranded (ss) or double stranded (ds), linear or circular Capsid: protective protein coat, polyhedral or helical Present in certain viruses Envelope: lipid layer (not a cell membrane) Enzymes Other: complex viruses can have diverse protein structures 2. Viral structure 18 nm Structural subunits Capsid (capsomeres) Virus RNA Capsid proteins make up capsid Helical Virus Capsid proteins self assemble into capsid Capsids have symmetry Capsids are either icosohedral or helical TM V TM 2. Viral structure Icosahedral Capsids Capsid proteins self assemble to form capsid Helical Virus Icosahedral Virus Naked 2. Viral structure Enveloped Complex Virus  Is SARS-COV-2 an icosahedral or a helical virus? https:// www.scientificamerican.co m/interactive/inside-the- coronavirus/  Smallpox virus Rhabdovirus Paramyxovirus Orf virus T - Bacteriophages Herpes virus Adenovirus Influenza virus Filamentous flexuous virus Alflafa mosaic virus Bacteriophage φX174 Tobacco mosaic virus Papilloma virus poliovirus. Viral replication cycle Replication Cycle of a Lytic Bacteriophage t Penetration Synthesis Assembly Phage: a virus that infects bacteria (e.g. T4, M13.. Lytic: a process that causes cell lysis. Viral replication cycle 1) Attachment of phage to specific molecules on host surface  Is the rate of attachment the same for all viruses? Superbig hint: how many receptors per cell? https://link.springer.com/protocol/10.1007%2F978-1-60327- 164-6_15. Viral replication cycle 2) Penetration Phage T4 attaches to LPS Has lysozyme-like enzyme Penetration of M13 phage into bacterial cell Some filamentous viruses like M13 enter bacteria by hijacking the pilus retraction mechanism. The virus binds the tip of the bacteria pilus and upon pilus retraction the virion is brought to the inner membrane where the capsid disassembles to release the viral genomic DNA into the cytoplasm MS2 attachment. Viral replication cycle 3) Synthesis Transcribe viral genes to make viral mRNA Translate viral mRNA Synthesis of viral proteins Replication of viral genome Details of synthesis vary widely depending on the properties of each viral genome. dsDNA virus example: e.g. T bacteriophage viruses dsDNA transcribed in host cytoplasm to make viral mRNA viral proteins translated from viral mRNA viral proteins direct viral DNA replication From Lecture 8, Gene Structure (in DNA) we learned that… mRNA During transcription, RNA polymerase will interact with and make a complementary copy of the Template strand. 4. Viral genomes Viral Genomes dsDNA : requires transcription, translation, DNA replication ssDNA (+) : sense strand (coding strand), cannot make mRNA directly ssRNA (+) : sense strand, same as mRNA, can be translated directly ssRNA (-) : anti-sense (template strand), cannot be translated (+) (-) (+) 4. Viral genomes ssRNA (+) virus example: MS2 Which of the following must be true of an ssRNA (+) virus? A. The (+) ssRNA is translated directly into viral protein B. The virus must have an origin of replication C. The viral genome must be converted into DNA prior to replication D. The virus must encode an RNA dependent RNA polymerase E. Both A and D are likely true 4. Viral genomes (+) ssDNA virus example: φX174 Which of the following is likely true of an ssDNA (+) virus? A. The viral genome is transcribed directly into mRNA B. The virus is likely to have an origin of replication C. The host ribosomes bind to and translate the ssDNA D. The virus likely has an RNA dependent RNA polymerase E. The virus must integrate into host DNA 4. Viral genomes viruses need RNA Replicase (RNA dependent RNA polymerase) viruses need Reverse Transcriptase (RNA dependent DNA poly