The Evolution of Viruses and the RNA to DNA Transition Quiz

The four major steps involved in virus entry into host cells are: 1) attachment, 2) entry, 3) transport, and 4) uncoating.

Enveloped viruses can enter by fusion and fusion of the envelope with the plasma membrane, or by receptor-mediated endocytosis followed by fusion/fission with an endosome. Non-enveloped viruses do not use fusion for entry; they can form channels or rupture membranes for entry.

Viruses can be transmitted cell to cell through mechanisms such as cell-cell contact via actin-based extensions called filopodia, virus assembly and transmission at virological synapses, formation of syncytia, and intracellular channels such as plasmodesmata.

<p>Attachment factors are cell surface components involved in binding of virions to cells but not uptake, while entry receptors play an active role in conformational changes, cell signaling, endocytosis, etc. They interact with surface components of a virion, leading to changes that facilitate entry via various mechanisms.</p> Signup and view all the answers

<p>Receptor-mediated endocytosis allows viruses to enter the cell by carrying the nucleocapsid into cytosolic vacuoles/vesicles and then into the cytosol. This process may occur at clathrin-coated pits, caveolae, or lipid rafts, and it is significant in trapping the virus at the plasma membrane and delivering it to the cytoplasm through fusion, lysis, or permeabilization.</p> Signup and view all the answers

<ol> <li>Viroids and RNA viruses may have originated in the RNA world: a) Catalyze some steps in replication without the host protein, b) Depend on RNA polymerase, either from host or helper viruses.</li> </ol> Signup and view all the answers

<p>Small and medium-sized DNA viruses could have arisen as independently replicating genetic elements in cells with a limited number of viral genes that encode for: a) Structural proteins, b) Proteins that stimulate DNA replication enzymes in the host, c) Proteins that recognize and bind to viral DNA and assemble cellular replication machinery.</p> Signup and view all the answers

<p>Large DNA viruses could have evolved from cellular forms that become obligatory intracellular parasites. Genes that overlapped with cellular genes were likely lost due to similar function(s).</p> Signup and view all the answers

<p>The mission of viruses is to transport the viral genome from an infected cell to an uninfected cell or from organism to organism. They have to 'evade' host cell defense barriers and the host immune system.</p> Signup and view all the answers

<p>Viruses of bacteria, archaea, algae, and plants use different entry mechanisms than animal viruses. For example, bacteriophages may puncture through the cell wall, while plant viruses may enter through mechanical means and then spread through plasmodesmata.</p> Signup and view all the answers