Virus Structure and Replication

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What are viruses?:

What are viruses? Small obligate intracellular parasites Virion Complete virus particle : nucleic acid + protein coat, which may be surrounded by an envelope It is the form in which the virus moves between cells or hosts Viral Genome EITHER RNA or DNA genome surrounded by a protective virus-coded protein coat (Capsid) Propagation depends on specialized host cells supplying the machinery for replication, metabolism and biosynthesis Dr.T.V.Rao MD 2

Configuration of Virus:

Configuration of Virus The DNA or RNA genome may be : ss – single stranded or ds – double stranded Genomes may be either: (+) sense: Positive-sense viral RNA is identical to viral mRNA and thus can be immediately translated into protein by the host cell. OR (-) sense: Negative-sense viral RNA is complementary to mRNA and thus must be converted to positive-sense RNA by an RNA polymerase before translation. Retroviruses? Dr.T.V.Rao MD 3

Viral Structure - Overview:

Viral Structure - Overview Fig 1. Schematic overview of the structure of animal viruses ** does not exist in all viruses Nucleic acid Capsid Nucleocapsid Envelope protein Membrane protein Viral envelope** Spike protein Dr.T.V.Rao MD 4

Bacteriophages :

Bacteriophages Dr.T.V.Rao MD 5

Viroids & Prions:

Viroids & Prions Viroids ss RNA genome and the smallest known pathogens. Affects plants Prions Infectious particles that are entirely protein. No nucleic acid Highly heat resistant Animal disease that affects nervous tissue Affects nervous tissue and results in Bovine spongiform encephalitis (BSE) “mad cow disease”, Scrapie in sheep kuru & Creutzfeldt-Jakob Disease (CJD) in humans Dr.T.V.Rao MD 6

Viral Structure:

Viral Structure Varies in size, shape and symmetry VIP for classification 3 types of capsid symmetry: Cubic (icosahedral) Has 20 faces, each an equilateral triangle. Eg. adenovirus Helical Protein binds around DNA/RNA in a helical fashion eg. Coronavirus Complex Is neither cubic nor helical eg. poxvirus Dr.T.V.Rao MD 7

Viral Structure:

Viral Structure Figure 1 An array of viruses. (a) The helical virus of rabies. (b) The segmented helical virus of influenza. (c) A bacteriophage with an icosahedral head and helical tail. (d) An enveloped icosahedral herpes simplex virus. (e) The unenveloped polio virus. (f) The icosahedral HIV with spikes on its envelope . Dr.T.V.Rao MD 8

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Definitions Bacteriophage Virus that infects prokaryotic (bacterial) cells. Nucleocapsid: viral nucleic acid + the protein coat that encloses it. Represents the packaged form of the viral genome. Dr.T.V.Rao MD 10

Viral Replication:

Viral Replication Viruses are intracellular obligate parasites which means that they cannot replicate or express their genes without the help of a living cell. A single virus particle (Virion) is in and of itself essentially inert. It lacks needed components that cells have to reproduce. When a virus infects a cell, it marshals the cell's ribosomes, enzymes and much of the cellular machinery to replicate. Dr.T.V.Rao MD 11

Progress of Viral Multiplication:

Progress of Viral Multiplication Once a virus infects its host and the viral progeny components are produced by the host's cellular machinery, the assembly of the viral capsid is a non-enzymatic process. It is usually spontaneous. Viruses typically can only infect a limited number of hosts (also known as host range). The "lock and key" mechanism is the most common explanation for this range. Certain proteins on the virus particle must fit certain receptor sites on the particular host's cell surface. Dr.T.V.Rao MD 12

Viral Replication:

When a virus infects a cell, nucleic acid must be uncoated and gain access to metabolic machinery of cell. Virus life cycle is characterized by: attachment penetration , with entry of nucleic acid into cell early expression of virus genes (either directly by translation, if virus contains "+" RNA, or indirectly after transcription and then translation) replication of virus nucleic acid synthesis of new Virion components packaging and assembly of new virions exit from cell Viral Replication Dr.T.V.Rao MD 13

Pricipliple of Replication :

Pricipliple of Replication Replication of the genome is governed by the principle of complementarity  requires a strand with complementary base sequence be synthesized  serve as template  synthesis of actual viral genome Late synthesis  viral structural proteins for the capsid are also synthesized almost simultaneously Dr.T.V.Rao MD 14


IMPORTANT FACTS ABOUT VIRUS REPLICATION Viruses multiply only in living cells Host cell provides the energy & machinery for the synthesis of viral proteins & nucleic acids Viral genome must be able to produce mRNA  host cell protein-synthesizing machinery may be able to synthesize viral proteins Dr.T.V.Rao MD 15

Stages in Multiplication :

Stages in Multiplication Soon after interaction with the host cell  Virion is disrupted  infectivity is lost  ECLIPSE PERIOD The yield of infectious virus per cell ranges from moderate numbers to > 100,000 particles The duration of replication cycle varies from 6-8 hrs to > 40 hrs Dr.T.V.Rao MD 16

All infections are not Productive:

All infections are not Productive PRODUCTIVE INFECTIONS > occur in permissive cells  infectious virus ABORTIVE INFECTIONS > no infectious virus produce because : a. cell is non-permissive b. virus may be defective DEFECTIVE VIRUS – lack certain genes for replication & requires the help of another virus (dependo or helper virus) Dr.T.V.Rao MD 17


LATENT INFECTIONS LATENT INFECTIONS a. viral genome persists within the host cell b. expression of no or few viral genes c. survival of the infected cell Dr.T.V.Rao MD 18

Pricipliple of Replication :

Pricipliple of Replication Replication of the genome is governed by the principle of complementarity  requires a strand with complementary base sequence be synthesized  serve as template  synthesis of actual viral genome Late synthesis  viral structural proteins for the capsid are also synthesized almost simultaneously Dr.T.V.Rao MD 19


STEPS IN VIRAL REPLICATION Attachment & adsorption Penetration Uncoating Early viral mRNA synthesis Early viral protein synthesis Dr.T.V.Rao MD 20


STEPS IN VIRAL REPLICATION 6. Viral genome replication 7. Late viral mRNA synthesis 8. Late viral protein synthesis 9. Assembly 10. Release Dr.T.V.Rao MD 21

Early steps in Multiplication:

Early steps in Multiplication Attachment specific binding of a Virion protein (the anti-receptor) to a constituent of the cell surface (the receptor) e.g . hemagglutinin of influenza virus some complex viruses (HSV) may have more than one species of anti-receptor molecule Penetration energy-dependent step occurs almost instantaneously after attachment Dr.T.V.Rao MD 22


ASSEMBLY & RELEASE It is thru budding that the virus acquire its envelope Budding process begin when virus-specific proteins enter the cell membrane at specific sites Herpesviruses  nuclear membrane Poxviruses  inclusion bodies Coronaviruses  endoplasmic reticulum Dr.T.V.Rao MD 23


ASSEMBLY & RELEASE Viral nucleocapsid then interacts with the specific membrane site mediated by matrix protein Cell membrane evaginates at that site  an enveloped particle bud off from the membrane Dr.T.V.Rao MD 24


ATTACHMENT & ADSORPTION Reversible step Does not require energy 2 requirements: 1. recognition & attachment to specific host receptor  determines the host range of the virus Dr.T.V.Rao MD 25

Penetration and Fusion :

Penetration and Fusion Dr.T.V.Rao MD 26

Mechanism differ the various types :

Mechanism differ the various types > Rhinovirus – ICAM-1 > HIV – CD4 + T cells > Rabies virus – acetylcholine receptors > HSV-1 – fibroblast growth factor receptor 2. appropriate ph & ionic concentration  both the host cell & viral particle are negatively charged at ph 7  require counter-ion  Magnesium ion Dr.T.V.Rao MD 27


PENETRATION Refers to the entry of the viral particle into the cytoplasm of the host cell Temperature-dependent step (37 C) Carried out through: a. receptor-mediated endocytosis b. direct penetration (viropexis) c. cell fusion Dr.T.V.Rao MD 28


UNCOATING Refers to the physical separation of the viral capsid from the viral genome Considered an obligatory step in viral replication  makes the viral genome accessible for transcription Favored by low pH Dr.T.V.Rao MD 29

Assembly and Release:

Assembly and Release Components of capsid synthesis directed by late genes Assembly of enveloped viruses needs interaction with plasma membrane which has been modified Final stage of infection Enveloped viruses released gradually by budding or exocytosis Naked viruses accumulate in cytoplasm and released during lysis Dr.T.V.Rao MD 30


INFECTIOUS NUCLEIC ACID Purified viral nucleic acid (without any protein) that can carry out the entire viral growth cycle  complete viral particles Can bypass the host range specificity provided by the viral protein-host cell receptor interaction e.g. Poliovirus Dr.T.V.Rao MD 31


GENE EXPRESSION & GENOME REPLICATION 1 st step in gene expression  mRNA synthesis DNA viruses Except Poxviruses replicate in the nucleus  use host cell DNA-dependent RNA polymerase  synthesize mRNA Dr.T.V.Rao MD 32

Multiply in :

Multiply in Poxviruses replicate in the cytoplasm no access to host cell RNA polymerase  carry their own RNA polymerase RNA viruses replicate in the cytoplasm EXCEPT Influenza virus & Retrovirus Dr.T.V.Rao MD 33


ASSEMBLY & RELEASE Progeny particles assembled by packaging the viral nucleic acid within the capsid proteins 2 processes of Release: 1. Rupture or lysis of cell membrane 2. Budding through the outer membrane Dr.T.V.Rao MD 34

Differences in Single Stranded and Double Stranded Virus :

Differences in Single Stranded and Double Stranded Virus Double-stranded RNA  cell has no enzyme to transcribe RNA into mRNA  virus carries its own polymerase Single-stranded RNA (+ polarity) e.g. Retrovirus  carries RNA-dependent DNA polymerase  ds DNA  transcribed to mRNA by host cell polymerase Dr.T.V.Rao MD 35

Synthesis of RNA:

Synthesis of RNA Once m RNA is synthesized  translated into viral proteins (enzymes & early proteins for the genome) by host ribosomes The most important of the early proteins is the Polymerase  synthesize many copies of viral genetic material for the progeny virus particle Dr.T.V.Rao MD 36

Synthesis of PapovaVirus :

Synthesis of PapovaVirus Dr.T.V.Rao MD 37

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RNA Genome Polarity Virion Polyme- rase Source Of mRNA Infectivity Of Genome Prototype SS, non segmented + No Genome Yes Poliovirus SS Nonseg- mented - Yes Transcrip- tion No Measles Rabies segmented - Yes Transcrip- tion No Influenza DS segmented +/- Yes Transcrip- tion No Reovirus SS diploid + Yes Transcrip- tion No HLTV HIV Dr.T.V.Rao MD 38

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