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Viral Replication steps

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- Attachment
- Penetration
- Uncoating
- Biosynthesis
- Maturation

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Attachment:

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Binding of viruses to receptor sites on the host cells. Different viruses use different receptors.

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. Penetration

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: This occurs by endocytosis or fusion of viral envelope with cell membrane

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Uncoating:

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This is the separation of the viral nucleic acid from its protein coat

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. Biosynthesis

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A complex process that involves

-Transcription of DNA into RNA (or vice versa) for genetic information.

-Translation of RNA into proteins that are required for DNA replication and capsid assembly.

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Transcription is mostly first for

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regulatory genes

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. Maturation

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: Newly synthesized viral genome and capsid proteins assemble together to form progeny viruses. Examples are:

- Multiplication of bacteriaphage
- multiplication of DNA

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Cell culture

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When viruses are grown in a laboratory, living cells are used.
- primary cell lines
- continuous cell lines

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Primary cell line:

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you can cutlivate it for a couple of generations and then the cell dies

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One step growth cycles of viruses

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Infection of the culture such that the viral growth in each cell is synchronous

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Phases of viral growth

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Eclipse period
Latent period:
Rise period
Yield

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Eclipse period

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loss of the detectable input viruses due to uncoating of infecting virions. Viral gene expression and DNA replication begin during this period

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Latent period

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measured from time of adsorption until the detection of the first extracellular progeny virions

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Rise period

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: period of accumulation of progeny virions, both intra- and extracellular

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Yield

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the final number of progeny produced by the infection cycle

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The genomes of most DNA viruses are

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double-stranded, but a few are single-stranded.

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General principles of DNA replication

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Template-dependent, Polymerization occurs in one direction: 5’ to 3’, using dNTPs as substrate
Semi-conservative replication
Replication origins and termini
DNA-dependent DNA polymerase and many accessory proteins
All require a primer

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Many DNA viruses express their genes and replicate in the nucleus
e.g

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, adenoviruses and herpesviruses

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Some DNA viruses, such as pox virus, encode all the functions necessary for replication in the cytoplasm of the host cell. They need only

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the host’s energy production systems and protein synthetic functions

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Herpes virus has two phases

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Lytic Infection
Latent Infection

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E.g of herpes phases

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Latent infection stays in the neuronal cells, lytic is like blister in lip

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RNA viruses are different from DNA viruses in two basic ways

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They usually replicate completely in the cytoplasm.

(2) They replicate their genomes via mechanisms different from cellular mechanisms in that they transcribe RNA from RNA. Thus they must encode their own enzymes for this purpose

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Compared to DNA viruses, RNA genomes are more diverse in nature:

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Unimolecular (hepatitis C virus, or HCV) or segmented (influenza virus)
Single-stranded, either (+) polarity, e.g., HCV and HIV,
or (-) polarity, e.g., influenza virus

Or double-stranded, e.g., reovirus

Each uses a different scheme for gene expression.

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Compared to DNA viruses, much higher mutation rates are found in RNA viruses because

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the RNA-dependent RNA polymerases do not have proof-reading function

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RNA viruses with (-) strand and double-stranded RNA genomes must carry the RNA polymerase in the viral particle because

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because the incoming viral RNA cannot be translated or copied by the cellular machinery. Thus, deproteinized RNA genomes of these viruses are non-infectious

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RNA , (+) strand viral particles lack a virion polymerase. Thus

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deproteinized RNA genomes of these viruses are infectious

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Genetic information essential to retroviruses. Genome consists of

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of two copies (diploid) of viral RNA 8-10 kb each.

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Essential genes for all retroviruses include:

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gag (group specific antigen): viral nucleocapsid and core proteins.
pol (polymerase): reverse transcriptase has 4 activities: RNA to ssDNA, ssDNA to dsDNA, RNase H, integrase.
env (envelope): envelope glycoproteins provide the ligand to bind host cell receptors for the virus and are responsible for viral entry.

The translation products of each of these genes are cleaved by a viral encoded protease to smaller peptides.

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Reverse transcription requires

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a primer to begin copying the viral RNA. This primer is a captured cellular tRNA.

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RNA-dependent DNA polymerase activity copies

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the viral RNA producing an RNA:DNA hybrid.

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The RNA strand of the hybrid is partially digested by

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the RNase H activity. This partially digested genome RNA now acts as primer for plus strand DNA synthesis by the DNA-dependent polymerase.

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LTR (long-terminal repeat) sequences are duplicated at

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each end by continuing to copy around the circle.

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Integration of provirus (the reverse transcription product)

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Integrase enzyme. Inserts linear provirus into cell DNA using a staggered cut of host DNA.
Specificity of integration. No absolute specificity but there is a range of preferred sites.

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Transcription serves to produce

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produce (1) mRNAs which encode viral proteins, (2) full length transcripts for progeny virus genomes.

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The LTR is a cis-acting element required for

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efficient transcription (enhancer and promoter).

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Role of host transcription factors

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Viral regulators of transcription and viral RNA processing. These are virus encoded regulatory proteins which can provide more control over replication and result in bursts of high levels of virus production often accompanied by host cell death.