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CHAPTER 10
Conceptual Questions
C1.
Answer: Viruses do not possess many of the characteristics that are associated
with living organisms. By themselves, they do not carry out metabolism, reproduce, or
utilize energy.
C2.
Answer: All viruses have a nucleic acid genome and a capsid composed of
protein. Some eukaryotic viruses are surrounded by an envelop that is composed of a
membrane with embedded proteins.
C3.
Answer: All viral genomes are composed of nucleic acid and carry genes. They
differ in that some genomes are DNA whereas others are RNA. Some are single stranded
and others are double stranded. Also, viral genomes differ greatly in their relative sizes.
C4.
Answer: A viral envelop is composed of a membrane with embedded proteins. It
is made when the virus buds from the host cell, taking with it a portion of the host cell’s
plasma membrane.
C5.
Answer: With regard to viruses, a host cell is a cell that can be infected by a virus
and allows the virus to replicate. Host range refers the types of cells that any particular
virus may infect.
C6.
Answer: The attachment step usually involves the binding of the virus to a
specific protein on the surface of the host cell. Only certain cell types will make that
specific protein.
C7.
Answer: For phage  the virus attaches to the surface of the host cell and then
injects its genetic material into the cytoplasm. The entire virus does not enter. By
comparison, HIV binds to the surface of its host cell and the entire virus is taken into the
cell via endocytosis.
C8.
Answer: Reverse transcriptase is used to copy the viral RNA into DNA so it can
be integrated into a chromosome of the host cell.
C9.
Answer: Lytic bacteriophages encode an enzyme called lysozyme that digests the
bacterial cell wall. This causes the bacteria to burst, thereby releasing the newly made
phages.
C10. Answer: A temperate phage can follow either the lytic or lysogenic cycle,
whereas a virulent phage can follow only the lytic cycle.
C11. Answer: A prophage or provirus refers to viral DNA that has integrated into a
chromosome of a host cell. An episome is segment of DNA that exists independently of
the host cell’s chromosomes. All of them encode the information to make new viruses.
C12. Answer: The term complementation refers to the outcome when two mutations
are combined in a single individual. If two defective mutations combined in a single
individual result in a nondefective phenotype, complementation has occurred. This is
because the two mutations are in different genes, and the corresponding wild-type
versions are dominant. If complementation does not occur, this means that the defective
mutations are in the same gene. In this case, when combined in the same individual,
complementation cannot occur because the individual does not have a wild-type
(functional) copy of the gene.
C13. Answer: The term allele means alternative forms of the same gene. Therefore,
mutations in the same gene among different phages are alleles of each other; the
mutations may be at different positions within the same gene. When we map the distance
between mutations in the same gene, we are mapping the distance between the mutations
that create different alleles of the same gene. An intragenic map describes the locations of
mutations within the same gene.
C14. Answer: Perhaps the mutations are at exactly the same spot (i.e., the same
nucleotide) in the gene. For example, if a gene is composed of 500 nucleotides, perhaps
two different strains both have a mutation at nucleotide number 377 (from one end of the
gene).