BIOL 2112 Chapter Notes - Chapter 19: Herpes Labialis, Natural Selection, Crispr
10 May 2018
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Viruses
What are viruses and why are they not considered to be an independent life form? Viruses
cannot reproduce or carryout metabolic activities outside of a host cell. Most biologists studying
viruses today would agree that they are not alive but exist in a shady area between life-forms and
chemicals. Examining the structure of a virus more closely reveals that it is an infectious particle
consisting of nucleic acid enclosed in a protein coat and, for some viruses, surrounded by a
membranous envelope. Animal viruses are classified by genome type. What types of genomes
have been identified in viruses? Their genomes may consist of double stranded DNA, single
stranded DNA, double stranded RNA, or single stranded RNA, depending on the type of virus. A
virus is called a DNA virus or an RNA virus based on the kind of nucleic acid that makes up its
genome. The genome is usually organized as a single linear or circular molecule of nucleic acid,
although the genomes of some viruses consist of multiple molecules of nucleic acid. What is a
capsid? The protein shell enclosing the viral genome is called a capsid. The capsid may be rod-
shaped, polyhedral, or more complex in shape. Capsids are built from a large number of protein
subunits calls capsomeres, but the number of different kinds of proteins in a capsid is usually
small. What is the source of the viral envelope and what is its function? Some viruses have
accessory structures that help them infect their hosts. For instance, a membranous envelope
surrounds the capsids of influenza viruses and many other viruses found in animals. These viral
envelopes which are derived from the membranes of the host cell contain host cell phospholipids
and membrane proteins. They also contain proteins and glycoproteins of viral origin. Some viruses
carry a few viral enzyme molecules within their capsids. Why do viruses exhibit a host range?
Viruses in isolation are merely packaged sets of genes in transit from one host cell to another.
What components of the host cell are required for viral replication? The host provides the
nucleotides for making viral nucleic acids, as well as enzymes, ribosomes, tRNAs, amino acids,
ATP, and other components needed for making the viral proteins. Many DNA viruses use the DNA
polymerases of the host cell to synthesize new genomes along the templates provided by the viral
DNA. In contrast, to replicate their genomes, RNA viruses use virally encoded RNA polymerases
that can use RNA as a template. What is the difference between a virulent and a temperate
bacteriophage? The lytic cycle is a phage replicative cycle that culminates in death of the host
cell. This term refers to the last stage of infection in which the bacterium lyses (breaks open) and
releases the phages that were produced within the cell. Each of these phages can then infect a
healthy cell. A phage that replicates only by a lytic cycle is a virulent phage. Instead of lysing their
host cells, many phages coexist with them in a state called lysogeny. The lysogenic cycle allows
replication of the phage genome without destroying the host. Phages capable of using both
modes of replication within a bacterium are called temperate phages. Describe three
mechanisms used by bacteria as defenses against bacteriophage infection. Natural selection
favors bacterial mutants with surface proteins that are no longer recognized as receptors by a
particular type of phage. Second, when the phage DNA does enter a bacterium, the DNA often is
identified as foreign and cut up by cellular enzymes called restriction enzymes. Third, a system
present in both bacteria and archaea called the CRISPR-Cas system. How can a bacterial cell
recognize the genome of an invading bacteriophage and eliminate the phage? Infection by a
phage triggers transcription of the CRISPR region of the bacterial DNA. This region consists of
DNA from phages that previously infected the cell, separated by repeats. RNA transcript is
processed into short RNA strands. Focus on RNA complementary to the DNA of the invading
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Document Summary
Viruses cannot reproduce or carryout metabolic activities outside of a host cell. Most biologists studying viruses today would agree that they are not alive but exist in a shady area between life-forms and chemicals. Examining the structure of a virus more closely reveals that it is an infectious particle consisting of nucleic acid enclosed in a protein coat and, for some viruses, surrounded by a membranous envelope. Their genomes may consist of double stranded dna, single stranded dna, double stranded rna, or single stranded rna, depending on the type of virus. A virus is called a dna virus or an rna virus based on the kind of nucleic acid that makes up its genome. The genome is usually organized as a single linear or circular molecule of nucleic acid, although the genomes of some viruses consist of multiple molecules of nucleic acid. The protein shell enclosing the viral genome is called a capsid.
