Chapter 25 · 2014. 1. 7. · 3 Virus Classification • Classification based on numerous...
Transcript of Chapter 25 · 2014. 1. 7. · 3 Virus Classification • Classification based on numerous...
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Chapter 25
바이러스
(The Viruses)
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Generalized Structure of Viruses
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Virus Classification• Classification based on numerous characteristics– Nucleic acid type– Presence or absence of envelope– Capsid symmetry– Dimensions of virion and capsid
• 3목(order), 73과(family), 9아과(subfamily), 287속(genera), 2000종(species)
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생물학의중심명제(Central dogma)
복제
전사
번역
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Alternative Classification Scheme
• David Baltimore
• Focuses on viral genome replication and process used to synthesize viral mRNA
• 7 life cycle groups
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볼티모어시스템바이러스분류
• dsDNA virus• ssDNA virus• dsRNA virus• ss (+)RNA virus• ss (‐)RNA virus• ssRNA retro virus• Gapped dsDNA virus
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Viruses with dsDNA Genomes (Group I)
• Largest group of known viruses• Most bacteriophages, archael viruses, herpes viruses, nucleo‐cytoplasmic large DNA viruses
• Some dsDNA viruses rely on host’s DNA and RNA polymerases
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Bacteriophage T4
• Some of bacteriophage enzymes synthesize hydroxymethyl cytosine (HMC), a modified nucleotide that replaces cytosine in T4 DNA
• HMC is then chemically modified by glucosylation• Protects T4 phage DNA from E. coli restriction enzymes (제한효소)
• Restriction is a bacterial defense mechanism used against bacteriophage infection
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Bacteriophage T4 DNA Is Terminally Redundant
• Telomere problem• Base sequence repeated at both ends• 상동재조합 (homologous recombination)• Allows for formation of concatamers (연쇄체)– Long strands of DNA consisting of several units linked together
– Structure allows for cleaving of genome for viral progeny packaging
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Herpesviruses• Alpha ()
– Herpes simplex virus type I (HSV‐1, 입술포진)– Herpes simplex virus type II (HSV‐2, 생식기포진)– Varicella zoster virus (VZV) – chicken pox (수두), shingles (대상포진)
• Beta ()– Cytomegalovirus (CMV, 거대세포바이러스)
• Gamma () – Epstein‐Barr virus (EBV) ‐ infectious mononucleosis (감염성단핵구증)
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Herpesvirus Virons
• Icosahedral(정이십면체), 120–200 nm, pleomorphic (다형성), enveloped, surface spikes
• Envelop surrounds tegument (layer of proteins, 표피단백질층) which surrounds nucleocapsid
• Linear genomes, 50–100 genes
• Target cells are epithelial or nerves
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Herpesvirus Infections• Productive (primary) infections (증식성감염)– 50,000–200,000 virons produced/cell– Cell dies due to degraded host DNA
• Latent infections (잠복성감염)– Occurs in neuronal cells– Infectious virus not detected– Can be reactivated in neurons– Productive infection recurs
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Herpesvirus Life Cycle• Circularization of genome and transcription of
immediate‐early genes (전‐전기유전자)
• Immediate‐early proteins stimulate transcription of early genes (전기유전자)
• Early proteins function in DNA replication, yielding concatemeric DNA.
• Late genes (후기유전자) are transcribed
• Late proteins participate in virion assembly
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Herpesvirus Life Cycle
DNA replication – concatemer formation
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Herpesvirus Life Cycle
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Nucleo‐Cytoplasmic Large DNA (NCLD) Viruses
• 핵세포질성대형 DNA 바이러스• NCLD group thought to have arisen from common ancestor
• Similar life cycle, most in cytoplasm• Enveloped, icosahedral capsids(정이십면체)• Virion and DNA are large• Poxviridae
– Variola virus (Small pox virus, 천연두)– Vaccinia virus
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Viruses with Single‐Stranded DNA Genomes (Group II)
• Bacteriophages φX174 and fd• Parvoviruses
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Rolling Circle Mechanism of Replication (회전환복제)
• Nicking enzyme• Lagging strand synthesis
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Viruses with Double‐Stranded RNA Genomes (Group III)
• Bacteriophage 6 and rotavirus (설사, 탈수, 사망)• RNA genomes cannot rely on host cell enzymes for genome replication or mRNA synthesis• Use RNA‐dependent RNA polymerase
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Virus with Plus‐Strand RNA Genomes (Group IV)
• Bacteriophages MS2 and Qβ, poliovirus, hepatitis C virus (HCV), and tobacco mosaic virus• Non‐segmented plus‐strand RNA genomes (=mRNA)• Replicate in cytoplasm and synthesize RNA‐dependent RNA polymerase•Replication complex for replication & assembly
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Poliovirus• 소아마비바이러스• Attaches to human PV receptor• Viral genome acts as mRNA• Virus uses internal ribosome entry site (IRES, 내부리보솜결합부위) instead of 5’ cap
• Polyprotein translated, cleaves itself into smaller proteins
• Genomic RNA synthesized• Assembly and lysis
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Cap‐dependent translation
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mRNAmG AAAAA
Ribosome
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Internal ribosome entry site (IRES)
• No 5’ cap found on eukaryotic mRNAs important in ribosome binding
• Translate its capless RNA using a 5’ region on the RNA called “internal ribosome entry site” (IRES)
• Cap‐independent translation• Numerous elements and extensive secondary structures
• Important for recognition of the RNA by the ribosome
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Cleavage of Poliovirus Polyprotein
• The polyprotein has a protease activity and cleaves itself into three smaller proteins
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Viruses with Minus‐Strand RNA Genomes (Group V)
• Enveloped virions, pleomorphic shape• Cannot serve as mRNA• Must bring a RNA‐dependent RNA polymerase• Segmented or nonsegmented genomes• Rabies (광견병), Ebola, measles (홍역) ,hantaviruses, and
influenza virus
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Influenza Virus
• Causative agent of the flu (독감)– Transmitted by inhalation or ingestion
• Three types of viruses – A, B, C• Seven to eight segments of linear RNA
– Hemagglutin (HA) binds host receptors– Neuraminidase (NA) hydrolyzes mucus (점액질), cleaves virus from host receptor
HA
NA