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