HIGH SPOTS FOR VIRUS CHAPTER—HOPE THIS HELPS

Here are some of the most important things to remember:

Bacteriophages: Lytic cycle (most important points) in 5 steps:

a. Attachment (adsorption)—virus has ATTACHMENT site, susceptible cell has RECEPTOR site—these must be complementary to each other so virus can attach; otherwise cell is immune

b. Penetration—phage injects only nucleic acid into cell, rest of phage remains on outside

c. Biosynthesis

1) First step is destruction or inactivation of host cell DNA

2) Many copies of phage DNA are made, using mostly enzymes of host cell and host cell nucleotides

3) Often only enzymes of host cell are needed; if any phage enzymes are needed they are synthesized from mRNA made from phage DNA; if made, these are called early proteins

4) Once many copies of phage DNA are completed, viral proteins (other than early enzymes, if any) will be synthesized—these are mostly capsid proteins and are called late proteins

d. Maturation--Once all parts of new virions are available, virions assemble spontaneously

e. Release (lysis)—plasma membrane bursts open, due to a phage enzyme (phage lysozyme) and new virions are released; usually 50 – 200

Phases of phage growth:

a. Latent period—phages have invaded cells but no new ones have been released

b. Eclipse period—no new virions have been completed

c. Rise period—new virions are released & plate count of plaques increases greatly

d. Burst size—number of new phages released per cell

Bacteriophages: Lysogenic cycle: phages that do this are called avirulent or temperate phages

a. Adsorption & penetration same as lytic cycle

b. Linear phage DNA forms a circle

c. Now 2 possibilities:

1) Phage DNA replicates & lytic cycle begins

2) Phage DNA gets spliced into the bacterial chromosome, now known as a prophage

a) 2 phage genes are transcribed immediately & the repressor proteins produced from them prevent lytic cycle

b) Host cell may now produce toxins from phage genes that otherwise the cell could not produce—diphtheria, botulism, scarlet fever, etc.

3) At any time, prophage may change & enter lytic cycle

Animal viruses

a. Attachment—ATTACHMENT site of virus must fit RECEPTOR site on host cell; otherwise cell is immune

b. Penetration—entire virus enters cell, capsid and all (different from phages)

1) Non-enveloped viruses induce the cell to take them in by endocytosis

2) Enveloped viruses may fuse their envelope with the plasma membrane so that capsid is released into the cytoplasm

c. Uncoating—capsid is removed from nucleic acid by enzymes of lysosomes, enzymes free in cytoplasm of host cell, or enzymes produced from viral genes

d. Host cell DNA is destroyed & virus takes control of cell

e. Genes for any enzymes not present in host cell that are needed for multiplication of viral nucleic acid are transcribed and/or translated and multiple copies of the nucleic acid are made

f. mRNA is made for viral proteins such as capsid proteins and these are synthesized

g. New virions assemble spontaneously—several thousand to a million

DNA viruses

a. Usually viral DNA enters nucleus of host cell and is replicated there, using viral enzymes

b. mRNA is easily made from viral DNA (all cells make mRNA using DNA the template)

c. Viral proteins are made in the cytoplasm, using host cell enzymes

d. New virions spontaneously assemble & are released, either by lysis of the cell (if non-envel;oped) or by budding (if enveloped)

e. Pox viruses are different—they carry in enzymes for replication of their DNA in the cytoplasm of the host cell

RNA viruses

a. Cells are not set up to use RNA as the template for making more RNA, but RNA is the only nucleic acid these viruses have

b. In some of these viruses, the RNA of the virus can be directly used as mRNA

c. In others, mRNA must be complementary to the viral RNA that comes in—this will require a viral enzyme (RNA-dependent RNA polymerase) since cells do not do this

d. To make copies of the nucleic acid (RNA), the viral enzyme is also required, since making copies of RNA with RNA as the template is not normally done by cells (cellular RNA polymerase is DNA-dependent)

Retroviridae

These are RNA viruses but they behave differently from the rest. AIDS virus (HIV) and some cancer-causing viruses belong to this family.

a. These viruses bring with them their own polymerase, called reverse transcriptase, which can direct the synthesis of DNA with RNA as the template

b. A strand of DNA complementary to the viral RNA is synthesized

c. Now a strand of DNA complementary to the first is synthesized, so now regular double-stranded DNA has been made from the RNA of the virus

d. The original viral RNA is destroyed and the DNA is spliced into the chromosome of the host cell, where it is called a provirus

e. The provirus may remain inactive for a long time, and it is replicated and passed on to daughter cells with each division

f. At some point, the genes of the provirus may begin to be transcribed, and production of new viruses may begin

g. Another possibility is that the provirus may change the host cell to a cancer cell