11. SARS-CoV-2 is an RNA virus with a single stranded nucleocapsid and a 30 kb long genome.
12. Four viral encoded proteins are inserted into the phospholipid bilayer of the SARS-CoV-2 viral envelope. The largest is the spike protein made of three 1,300 amino acids of which 2 anchor it in the envelope of the virus.
13. The envelope protein is tightly associated with the membrane and has three transmembrane domains and is thought to give great structural stability to the envelope.
14. The membrane for the SARS-CoV-2 virus is taken from the endoplasmic reticulum and Golgi membranes.
15. Influenza viruses have segmented genomes consisting of seven to eight different RNA molecules together making up the full viral genome. Each segment is inserted in its own flexible helical nucleocapsid.
16. The influenza virus has two types of envelope spikes hemagglutinin (HA) and neuraminidase (NA) of which there are 18 and 9 subtypes respectively giving rise to the 18 HA/NA subtypes. The HA spike-type are for binding to host cells and the NA type are for releasing newly formed virions.
17. Genomes of the influenza virus change frequently due to point mutations and reassortment of the RNA segments and because of this, the sudden appearance of new influenza viruses can cause pandemics. Point mutations give rise to small changes called antigenic drift. Reassortment produces large antigenic changes called antigenic shift. Antigenic variation occurs almost yearly with influenza A virus.
18. The animal reservoirs critical to the epidemiology of influenza are found in many parts of rural China where poultry, pigs, and humans live in close, crowded conditions. Influenza is widespread in birds and can transfer to pigs but usually not to humans. Pigs can transfer it to humans and humans back to pigs. Reassortment between human and avian strains thus occurs in pigs leading to novel HA/NA combinations causing major antigenic shifts.
19. HIV-1 virions are approximately 110 nm in diameter, have a conical capsid which contains two copies of its RNA genome and several viral enzymes.
20. All HIV-1 strains require CD4 as the receptor on their target cells. A second receptor is also required. CCR5, a beta chemokine receptor is the second receptor in the early part of the infection. CXCR4 (fusin) serves as the second receptor in the later part of the infection cycle.
21. Each of the two copies of RNA of HIV is copied into a single strand of DNA by the reverse transcriptase activity of HIV reverse transcriptase. The RNA is degraded by HIV ribonuclease H, and the DNA strand is duplicated to form a double-stranded DNA copy of the original RNA genome. A complex of the double-stranded DNA (preintegration complex) and HIV integrase enzyme moves into the nucleus. Then proviral DNA is integrated into the cell's DNA. The integrated provirus can remain latent, giving no clinical sign of its presence.
22. When HIV reproduces in T cells they are killed. Healthy persons have about 800-1200 CD4+ T cells per microliter of blood. A count of 200 or less of CD4+ T cells per microliter of blood is used as the defining point for a diagnosis of AIDS. It represents the threshold number of CD4+ cells to defend against opportunistic infections.