Lecture 26 Viruses

Define the differences between viruses and other living organisms

• Viruses straddle the definition of “life”.

• Viruses do not have the biochemical or genetic potential to generate the energy necessary for driving biological processes.

• They require a host to replicate.

• Viruses are infectious particles consisting of nucleic acid encased in a protein coat, and, in some cases, a membranous envelope.

• They lack enzymes for metabolism or ribosomes for protein synthesis.

• Viruses are obligate intracellular parasites.

• They can reproduce only within a host cell.

• An isolated virus is unable to reproduce or do anything else, except infect an appropriate host.

• An isolated virus is merely a packaged set of genes in transit from one host cell to another.

Understand the difference between lytic and lysogenic reproductive cycles

• Viruses can reproduce using lytic or lysogenic cycles. These cycles are best understood in bacteria viruses (phage).

• In the lytic cycle, virus reproduction leads to host death. This cycle ends with cell lysis (the cell breaks open) and the release of many viruses produced within the cell. These released viruses can then infect other nearby cells.

• In the lysogenic cycle, the viral genome integrates into the host DNA and gets replicated along with the host’s genome. This cycle occurs without destroying the host cell. Herpes virus is mentioned as an example that does this in humans.

Be able to describe the typical replicative cycle of animal virus

• Animal viruses are diverse in their modes of infection and replication, differing in nucleic acid type and presence/absence of an envelope.

• Viral infection requires a highly specific interaction between viral surface glycoprotein and a specific receptor molecule on the surface of the host’s target cell.

• For enveloped animal viruses:

  • Viral glycoproteins in the envelope bind to specific receptors on the host’s membrane.

  • The envelope fuses with the host membrane, delivering the capsid and viral genome inside the cell.

  • Once inside, the viral genome commandeers the host, reprogramming the cell to copy viral nucleic acid and manufacture proteins.

  • The viral genome (RNA or DNA) duplicates and directs the host machinery to synthesize viral capsid proteins (using free ribosomes) and glycoproteins (on ER-bound ribosomes, secreted via vesicular trafficking to the cell surface).

  • After the capsid and viral genome self-assemble, they bud from the host cell, covered with an envelope derived from the host plasma membrane, including viral glycoproteins.

  • One infected cell may release many new viruses that can go on to infect new cells.

• The reproductive cycle of HIV (a retrovirus) involves reverse transcriptase synthesizing double-stranded DNA from viral RNA after entering the host. Host RNA polymerase then transcribes this viral DNA into more RNA molecules, which function as both mRNA for protein synthesis and genomes for new particles. Viral proteins self-assemble into particles and leave the host.

Understand the origin of an animal viral envelope vs coat

• A virus consists of nucleic acid encased in a protein coat. This protein coat is called a capsid. All viruses wrap their genome in a capsid, which is built of protein subunits called capsomeres.

• Most animal viruses also have envelopes with lipid membranes cloaking their capsids.

• These envelopes are derived from the host cell membrane.

• The envelopes also usually have embedded viral glycoproteins. When an enveloped virus buds from the host cell, it takes a piece of the host plasma membrane, which becomes its envelope, enriched with these viral proteins.

Be able to describe ways that viruses have pirated host machinery for viral replication

• Viruses require a host cell to reproduce.

• Once inside a host cell, the viral genome commandeers the host, reprogramming it to copy viral nucleic acid and manufacture viral proteins.

• Viruses lack enzymes for metabolism or ribosomes for protein synthesis, thus they pirate the host's ribosomes for making viral proteins.

• They direct the host machinery to synthesize viral capsid proteins (using free ribosomes) and glycoproteins (using ER-bound ribosomes).

• They utilize the host's endoplasmic reticulum (ER) and vesicular trafficking for processing and secreting viral glycoproteins to the cell surface.

• Enveloped viruses acquire their membranous envelope from the host cell's plasma membrane during budding.

• For retroviruses, the viral DNA integrates into the host chromosome, and the host RNA polymerase transcribes this viral DNA into viral RNA molecules.

Understand the different ways viruses with RNA genomes replicate

• Viral RNA genomes are used in different ways.

• Single-stranded RNA genomes can act directly as messenger RNA (mRNA) and be translated immediately by host ribosomes into viral proteins.

• Other single-stranded RNA genomes can serve as a template for mRNA, and also serve as a template for complementary RNA which then serves as a template for the synthesis of additional genomic copies.

• Single-stranded RNA genomes of retroviruses serve as a template for DNA synthesis. These viruses carry a unique enzyme called reverse transcriptase (RT) for this RNA to DNA synthesis. The RT enzyme is packaged with the genome inside the capsid. The newly made viral DNA can then integrate into the host chromosome or serve as a template for mRNA.

• The source notes that Coronavirus is a single (+) stranded RNA virus. The antiviral drug Remdesivir inhibits viral RNA polymerase for corona virus, suggesting this type of RNA virus uses a viral RNA polymerase for replication or transcription from its RNA genome.