Chapter 19 Flashcards
What are the four forms of viral genomes?
Double-stranded DNA
Single-stranded DNA
Double-stranded RNA
Single-stranded RNA
What is a capsid? What are capsomeres? What different shapes may capsids have?
The protein shell enclosing the viral genome is called a capsid. Depending on the type of virus, the capsid may be different in shape. Capsids are built from a large number of protein subunits called capsomeres.
All viruses contain
A nucleic acid enclosed in a protein coat
Some have a membranous envelope
What is the role of an envelope in animal viruses?
The envelopes help the virus infect their host.
What property of a virus determines its attachment to a host cell membrane?
Viruses usually identify host cells by a “lock-and-key” fit between viral surface proteins and specific receptor molecules on the outside of cells.
Viruses are obligate intracellular parasites. What does this mean?
Viruses can replicate only within a host cell.
What is meant by host range?
Each particular virus can infect cells of only a limited number of host species, called the host range of the virus.
What components of the host cell does a virus use to reproduce itself?
The host cell 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.
How does a DNA virus reproduce its genome?
Many DNA viruses use the DNA polymerase of the host cell to synthesize new genomes along the templates provided by the viral DNA.
How do most RNA viruses replicate their genome?
To replicate their genomes, RNA viruses use virally encoded RNA polymerase that can use RNA as a template.
What are bacteriophages? Distinguish between virulent and temperate phages.
A bacteriophage is a virus that infects bacteria; also called a phage. A phage that replicates only by a lytic cycle which destroys the host cell is a virulent phage. Temperate phages can undergo a lysogenic cycle, which allows the phage genome to be replicated without destroying the host.
What portion of a phage enters the host cell? How does it do this?
The phage DNA enters the host cell. After the bacteriophage binds to a specific receptor on the outer surface of the host cell, the sheath of the tail contracts, injecting the DNA into the cell and leaving an empty capsid outside.
What are restriction enzymes? How do they help prevent viral infection of bacteria?
A restriction enzyme is an endonuclease (type of enzyme) that recognizes and cuts DNA molecules foreign to a bacterium (such as phage genomes). The enzyme cuts at specific nucleotide sequences (restriction sites). Restriction enzymes identify and cut up viral DNA that is detected as foreign.
Why don’t restriction enzymes destroy the DNA of the bacterial cells that produce them?
The bacterial cell’s own DNA is methylated in a way that prevents attack by its own restriction enzymes.
What are three ways bacteria may win the battle against the phages?
Natural selection favors bacterial mutants with receptors that are no longer recognized by a particular kind of phage.
When phage DNA successfully enters a bacterium, the DNA is often identified as foreign and cut up by restriction enzymes.
Instead of lysing their host cells, many phages coexist with them in a state called lysogeny.
