Chapter 19 Flashcards
What are viruses?
Small, non-cellular infectious particle consisting of genes packaged in a protein coat and, sometimes, a membrane
Why aren’t viruses considered to be alive?
Do not consist of cells, cannot reproduce or metabolize outside of a host cell
What are the molecular components of a virus?
A nucleic acid surrounded by a protein coat
Describe viral genomes types.
Viral genomes consist of either double- or single-stranded DNA or double- or single-stranded RNA; Their genome has 3 to 2000 genes and may be a single linear or circular molecule of nucleic acid
What are capsids?
the protein shell that encloses the viral genome
What is the function of the capsid?
to hold the viral nucleic acid
What are capsids composed of?
protein subunits called capsomeres
How do capsids vary between viruses?
they vary in shape and size; may be spherical, helical, icosahedral, etc.
What is a viral envelope?
A membrane that surrounds the capsids of influenza viruses and many other viruses found in animals; they contain a combination of viral and host cell molecules
What are viral envelopes derived from?
From membranes of host cells
Give an example of a virus that has an envelope.
influenza
What is a bacteriophage?
Viruses that infect bacteria
Describe the appearance of a phage
Have an elongated capsid head that encloses their DNA and attached to the protein tailpiece
What does it mean that viruses are obligate intracellular parasites?
They can only replicate within a host cell
Describe the general life cycle of a virus.
The virus enters the host cell and the nucleic acid and capsids separate. Once a viral genome has entered a cell, the cell begins to manufacture viral proteins, using host enzymes, ribosomes, tRNAs, amino acids, ATP, etc. The nucleic acid is replicated to produce copies of the viral genome and transcribed/translated to produce more capsomeres. These products self-assemble into new virus particles which then exit the cell.
Lytic cycle
results in the death of the host cell; Produces new phages and lyses (breaks open) the host’s cell wall, releasing the progeny viruses
Lysogenic cycle
The viral DNA molecule is incorporated into the host cell’s chromosome without destroying the cell as a prophage; Every time the host divides, it copies the phage DNA and passes a copy to daughter cells
What is a virulent phage?
A phage that reproduces only by the lytic cycle
What is a temperate phage?
Phages that use both the lytic and lysogenic cycles
What is a prophage?
the viral DNA molecule is incorporated into the host cell’s chromosome without destroying the cell
How are viral envelopes generated by animal viruses?
They are made from the host’s plasma membrane as viral capsids exit
Describe the process of production of viral DNA, RNA, and proteins in a cell.
In DNA viruses, the process of viral DNA transcription/translation is similar to normal gene expression. The viral DNA is transcribed into viral RNA which is translated into viral capsomeres
and/or glycoproteins. Additionally, replication is similar to regular replication. In non-retroviral RNA viruses, the viral RNA can be immediately translated into viral proteins such as capsomeres and/or glycoproteins. The viral RNA is replicated from the viral RNA template rather than from DNA.
How is a retrovirus different from a typical animal virus?
It becomes a permanent resident of the host cell because the viral DNA is integrated into the host genome. After this integration, the DNA is expressed in the same manner as normal gene expression
What is a provirus?
The viral DNA that is integrated into the host genome
