Chapter 19: Viruses
•Viruses are very small infectious particles consisting of nucleic acid enclosed in a protein
coat and, in some cases, a membranous envelope
What is a capsid?
•A capsid is the protein shell that encloses the viral genome
What are capsomeres?
•Capsids are built from protein subunits called capsomeres
All viruses consist of a nucleic acid enclosed in a protein coat.
Some viruses also have a membranous envelope.
What is the role of an envelope in animal viruses?
Viruses are obligate intracellular parasites. What does this mean?
•Viruses are obligate intracellular parasites, which means they can reproduce only
within a host cell
What is meant by host range? Distinguish between a virus with a broad host range and
one with an extremely limited host range, and give an example of each. •Each
virus has a host range, a limited number of host cells that it can infect
Simplified viral reproductive cycle:
1. Transcription: In transcription, an enzyme zips along the DNA, and forms RNA.
RNA nucleotides line up with their complementary DNA partners, transcribing the
information in DNA into RNA.; and
2. Translation: The message in mRNA is translated into a protein in the cytoplasm.
First, a transfer RNA (tRNA) arrives, carrying a specific amino acid. The small
subunit of a ribosome attaches to the mRNA. Now the large subunit of the
ribosome attaches. A second tRNA docks, and brings another amino acid. The
ribosome helps to form a covalent bond between the two amino acids. A new
tRNA brings another amino acid. The ribosome helps to form a new bond, and the
process is repeated. Note one end of a tRNA molecule has a set of three bases,
called an anticodon, that pairs with complementary bases on the mRNA. The
other end of the tRNA carries a specific amino acid. Different types of tRNAs
carry different amino acids. In this way, the message in mRNA is translated into
a specific sequence of amino acids. See diagram below: 3. Infection, 4.
Replication, 5. Self-assembly, and annotate your labels to explain the process of
viral reproduction (•There are two key variables used to classify viruses that
infect animals and reproduce: DNA or RNA; and Single-stranded or doublestranded)
Bacteriophages? Bacteriophages, also called phages, are viruses that infect bacteria.
Distinguish between virulent phages:•A phage that reproduces only by the lytic cycle is called
a virulent phage, and temperate phages: •Phages that use both the lytic and lysogenic cycles
are called temperate phages
What portion of a phage enters the host cell? Capsids and the viral DNA How does it do
this? Through entry and uncoating: •Phages have an elongated capsid head that encloses
their DNA •A protein tailpiece attaches the phage to the host and injects the phage DNA
inside
What are restriction enzymes? They can serve as defenses for bacteria against phages.
What is their role in bacteria? They are able to ‘cut up’ certain phage DNA.
Why don’t restriction enzymes destroy the DNA of the bacterial cells that produce them?
Cutting the DNA is satisfies the defense of the bacteria so they can be eliminated
thorough exocitosis.
Describe the lytic and lysogenic modes of bacteriophage reproduction.
•The lytic cycle is a phage reproductive cycle that culminates in the death of the
host cell
•The lytic cycle produces new phages and digests the host’s cell wall, releasing the
progeny viruses
•A phage that reproduces only by the lytic cycle is called a virulent phage
•Bacteria have defenses against phages, including restriction enzymes that recognize
and cut up certain phage DNA
What is a prophage?
•The lysogenic cycle replicates the phage genome without destroying the host
•The viral DNA molecule is incorporated into the host cell’s chromosome
•This integrated viral DNA is known as a prophage
•Every time the host divides, it copies the phage DNA and passes the copies to
daughter cells
Retrovirus?
How do retroviruses, such as HIV, replicate their genome?
•The broadest variety of RNA genomes is found in viruses that infect animals
•Retroviruses use reverse transcriptase to copy their RNA genome into DNA
•HIV (human immunodeficiency virus) is the retrovirus that causes AIDS (acquired
immunodeficiency syndrome)
Here is a sketch of HIV. Label these parts: envelope, reverse transcriptase, RNA, and
capsid.
Provirus:•The viral DNA that is integrated into the host genome is called a provirus
•Unlike a prophage, a provirus remains a permanent resident of the host cell
Thi
s sketch shows the infection of a cell by HIV
The envelope glycoproteins enable the virus to bind to specific receptors on certain
white blood cells;
1. The virus refuses with the cell’s plasma membrane. The capsid proteins are
removed, releasing the viral proteins and RNA;
2. Reverse transcriptase catalyzes the synthesis of a DNA stand
complementary to the viral RNA;
3. Reverse transcriptase catalyzes the synthesis of a second DNA strand
complementary to the first;
4. The double-stranded DNA is incorporated as a provirus into the cell’s DNA;
5. Proviral genes are transcribed into RNA molecules, which serve as genomes
for the next viral generation as mRNAs for translation into viral protein;
6. The viral proteins include capsid proteins and reverse transcriptase (made in
the cytosol) and envelope glycoproteins (made in the ER);
7. Vesicles transport the glycoproteins to the cell’s plasma membrane;
8. Capsids are assembled around viral genomes and reverse
molecules; and
9. New viruses bud off from the host cell.
transcriptase
viroids, and prions are formidable pathogens in animals and plants
What is a viroid?
•Viroids are circular RNA molecules that infect plants and disrupt their growth
•Prions are slow-acting, virtually indestructible infectious misfolded proteins that cause
brain diseases in mammals
•Prions propagate by converting normal proteins into the prion version
1.
2.
3.
4.
Name four diseases caused by prions..
Scrapie in sheep;
Mad cow disease;
Creutzfeldt-Jakob disease (humans); and
Kuru in humans are all caused by prions
What are two alarming characteristics of prions?
1. Slow-acting (ten year incubation); and
2. Virtually indestructible