Introduction to Viruses
1. Define:
Virion - A virion is a virus particle as it exists outside of its host cell. This is the complete and infective form of the virus.
Prophage - A prophage is a bacteriophage that has entered into a host cell and has become incorporated into the host cell’s chromosome. When it is a prophage, the virus will be replicated along with the host cell’s DNA and transferred to newly formed cells. The prophage is not usually disruptive to host cell function, but can cause lysogenic conversion.
Lysogenic conversion - Lysogenic conversion is a condition that exists when a population of bacteria are expressing viral genes. The bacteria display a new phenotype because they are expressing characteristics determined by the viral genes. For example, many bacteria that produce protein-type toxins (exotoxins) are expressing viral genes. Toxin production is a common viral characteristic.
Retrovirus - A retrovirus (family Retroviridae) is a virus that has an RNA genome and carries the enzyme reverse transcriptase (RNA-dependent DNA polymerase). Such a virus will reverse transcribe its RNA genome into DNA after it has entered a host cell. The newly formed DNA can then become incorporated into the host cell’s chromosome.
Reverse transcriptase - Also known as RNA-dependent DNA polymerase, reverse transcriptase enzymes allow the information contained in RNA molecules to be reverse transcribed into DNA. Reverse transcriptase enzymes were originally found in retroviruses, but are now used in laboratories to form complimentary DNA (c-DNA) from various human m-RNA molecules.
Prion - Prions are infectious protein particles (proteinaceous infectious particles) associated with a variety of diseases including bovine spongiform encephalopathy (mad cow disease), scrapie (in sheep), chronic wasting disease (elk and mule deer), Creutzfeldt-Jakob disease (CJD), Kuru, and fatal familial insomnia in humans. Prion proteins increase in number by causing normal brain proteins with the same amino acid sequence to undergo changes in molecular configuration resulting in prion formation.
Viroid - Viroids are naked RNA molecules (not surrounded by a protein coat) that have been found to cause a variety of infections in plants. These RNA molecules are small, usually from 246 to 375 bases in length with extensive internal base pairing causing them to have a rod-shaped appearance. Some viroids seem to have sequences similar to some introns, but their origin is not fully understood.
2. poison
3. virion/ protein coat called a capsid
4. nucleic acid core/ capsomers
5. nucleic acid (may be DNA or RNA but not both)/ capsid/ tail assembly
6. The viral genome may be double-stranded or single-stranded DNA or double-stranded or single-stranded RNA. It may be linear or a closed loop; it may be composed of one segment or many. The size of the viral genome is quite variable and in different viruses ranges from 4 to over 400 genes.
7. cytolytic bacteriophage/ adsorption
8. temperate/ prophage
9. adsorption/ penetration
10. Avoid host restriction enzymes and increase transcription of viral genes/ proteins of the viral capsid and tail assembly
11. a) Immediate early phage genes code for enzymes involved in evading host resistance, and increasing transcription of viral genes. Some of these enzymes disable the host restriction enzymes or modify viral DNA so it will not be cut by restriction enzymes. One viral enzyme is a highly active viral RNA polymerase with a high affinity for viral DNA (i.e., will transcribe viral DNA more readily than cellular DNA). Other enzymes destroy host sigma factors and thus force cellular RNA polymerase enzymes to use viral sigma factors (with high affinity for viral DNA). chop up the host cell's DNA
b) Delayed early phage genes code for enzymes that chop up the host cell's DNA and replicate viral DNA. These enzymes insure that the nucleotides needed to replicate viral DNA will be readily available.
c) Late phage genes code for proteins of the capsid, tail assembly and enzymes needed to lyse the host cell.
12. Many viruses use host cell RNA polymerase to transcribe their immediate early genes (and may eliminate cellular sigma factors to insure that only viral sigma factors, with high affinity for viral DNA are used). All viruses are dependent upon the host cell's ribosomes to translate their m-RNA into protein sequences. Viruses are also dependent upon the host cell's ability to provide energy (ATP) for both transcription and translation processes.
13. prophage/ lysogenic conversion
14. lysogenic conversion/ protein toxins (exotoxins)
15. envelope
16. Bacteriophage lambda (ql)/ prophage/ repressor
17. HIV is a single-stranded RNA type virus (note error in text) and is called a retrovirus/ reverse transcriptase/ The viral DNA is replicated to form a double-stranded molecule that has cohesive termini at both ends. This is then able to make use of a viral enzyme called integrase to insert itself into the host cell's chromosome and become a provirus. The viral DNA (provirus) will stay within the host cell's chromosome as long as the cell lives, and will be replicated along with the host genome whenever it is reproduced.
18. Viral envelope
19. reverse transcribe
20. Integrase
21. reverse transcriptase or RNA-dependent DNA polymerase/ retroviruses/ the human immunodeficiency virus (HIV)
22. tumors or cancer (They do this by bringing oncogenes into the host cell or by activating oncogenes already present.)
23. Matching letter sequence is - G, D, F, A, B, E, H and C.
Copyright 2002 Sierra College Biological Sciences Department
Last updated: 01/18/02.
