VirusesChapter 19
Bacteria vs. VirusesBacteria Virus
� Prokaryotic cell
� Most are free-living (some parasitic)
� Relatively large size� Antibiotics used to kill
bacteria
� Not a living cell (genes packaged in protein shell)
� Intracellular parasite� 1/1000 size of bacteria� Vaccines used to prevent
viral infection
� Antiviral treatment
Viruses� Very small (<ribosomes)� Components = nucleic acid + capsid◦ Nucleic acid: DNA or RNA (double or
single-stranded)◦ Capsid: protein shell◦ Some viruses also have viral envelopes
that surround capsid
� Viruses are not cells
Viruses
� Viruses are obligate intracellular parasites, which means they can reproduce only within a host cell
� Each virus has a host range, a limited number of host cells that it can infect
� Reproduce quickly within host cells� Can mutate easily◦ RNA viruses: no error-checking mechanisms
Viral Genomes� Viral genomes may consist of either◦ Double- or single-stranded DNA, or◦ Double- or single-stranded RNA
• Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
General Features of Viral Reproductive Cycles� Once a viral genome has entered a cell, the
cell begins to manufacture viral proteins� The virus makes use of host enzymes,
ribosomes, tRNAs, amino acids, ATP, and other molecules
� Viral nucleic acid molecules and capsomeres spontaneously self-assemble into new viruses
Simplified Viral Replicative Cycle
Reproductive Cycles of Phages� Phages are the best understood of all viruses� Phages have two reproductive mechanisms:
the lytic cycle and the lysogenic cycle
Video: t4 Phage infection
� 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
The Lytic Cycle
The Lysogenic Cycle� 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
� An environmental signal can trigger the virus genome to exit the bacterial chromosome and switch to the lytic mode
� Phages that use both the lytic and lysogenic cycles are called temperate phages
Viral Reproduction� Lytic Cycle:◦ Use host machinery to replicate, assemble, and
release copies of virus◦ Virulent phages: Cells die through lysis or apoptosis
� Lysogenic Cycle:◦ DNA incorporated into host DNA and replicated
along with it◦ Bacteriophage DNA = prophage◦ Animal virus DNA = provirus
◦ Can switch to lytic cycle◦ Temperate Phage: uses both methods of replication
Lytic Cycle of T4 Phage
Lytic Cycle vs. Lysogenic Cycle
Animal viruses have a membranous envelope
� Host membrane forms around exiting virus
� Difficult for host immune system to detect virus
Video: How Dengue Virus enters a cell
Retrovirus� RNA virus that uses reverse transcriptase
(RNA à DNA)� Newly made viral DNA inserted into
chromosome of host (provirus)� Host transcribes provirus to make new virus
parts� Example: HIV (Human Immunodeficiency Virus)
Video: HIV Life Cycle
HIV = Retrovirus
Retrovirus
� 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
� The host’s RNA polymerase transcribes the proviral DNA into RNA molecules
� The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus particles released from the cell
Evolution of Viruses� Viruses do not fit our definition of living
organisms� Since viruses can reproduce only within cells,
they probably evolved as bits of cellular nucleic acid
� Candidates for the source of viral genomes are plasmids, circular DNA in bacteria and yeasts, and transposons, small mobile DNA segments
� Plasmids, transposons, and viruses are all mobile genetic elements
Emerging Viruses = mutation of existing virusesPandemic: global epidemic
Drugs for Prevention/Treatment� Vaccine: weakened virus or part of pathogen that
triggers immune system response to prevent infection◦ Ex. HPV, MMR, HepA, Flu shot
� Antiviral Drugs: block viral replication after infection◦ Ex. Tamiflu (influenza), AZT (HIV)
Prions� Misfolded, infectious proteins that cause
misfolding of normal proteins� Eg. scrapie (sheep), mad cow disease (BSE),
Creutzfeldt-Jakob disease (humans), kuru (humans – New Guinea)
Diseases caused by prions� Prions act slowly – incubation period of at least 10
years before symptoms develop� Prions are virtually indestructible (cannot be
denatured by heating)� No known cure for prion diseases
Kuru in New Guinea
AMOEBA SISTERS: VIRUSES: VIRUS REPLICATION AND THE MYSTERIOUS COMMON COLD
TED-ED: HOW WE CONQUERED THE DEADLY SMALLPOX VIRUS