Viruses

Nonliving particles Very small (1/2 to 1/100 of a bacterial cell) Do not perform respiration, grow, or develop Are able to replicate (only with the help of living cells) Host cell—a cell where a virus replicates Bacteriophage (phage)—virus that infects a bacterium

T4 bacteriophage infecting an E. coli cell 0.5  m

Comparing the size of a virus, a bacterium, and an animal cell 0.25  m Virus Animal cell Bacterium Animal cell nucleus

Viral Structure 2 main parts: inner core of nucleic acid (DNA or RNA) –instructions for making copies of the virus outer coat of protein (capsid) –determines shape of virus (which cells & how cells are infected) »polyhedral »helical »envelope with projections »classic phage shape

Viral structure 18  250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm 20 nm50 nm (a) Tobacco mosaic virus(b) Adenoviruses(c) Influenza viruses (d) Bacteriophage T4 RNA Capsomere of capsid DNA Capsomere Glycoprotein Membranous envelope Capsid DNA Head Tail fiber Tail sheath

Infection by tobacco mosaic virus (TMV)

Attachment to Host Cell Order of events: virus recognizes host cell virus attaches to receptor site on membrane of host cell –Receptor site on host matches with viral proteins (like a puzzle) virus enters host cell virus replicates inside host cell

Attachment is Specific viruses have specifically shaped attachment proteins each virus infects only certain types of cells –most are species specific Smallpox, polio, measles—affects only humans –although some are not West Nile virus—mosquitoes, birds, humans, horses –some are cell-type specific polio—affects intestine & nerve cells

Simplified viral reproductive cycle VIRUS Capsid proteins mRNA Viral DNA HOST CELL Viral DNA Entry into cell and uncoating of DNA Replication Transcription DNA Capsid Self-assembly of new virus particles and their exit from cell

Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. Entry of phage DNA and degradation of host DNA. The sheath of the tail contracts, injecting the phage DNA into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles Phage assembly Head Tails Tail fibers Lytic cycle of phage T4, a virulent phage

Lytic vs Lysogenic Lytic cycle (virulent phage) –Release of virus bursts and kills host cell (lysis) Lysogenic cycle (temperate phage) –Viral DNA integrates into host genome (provirus) –Can be transmitted to daughter cells –Can initiate lytic cycle in response to environmental signal (stress)

The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage (provirus). New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage/Provirus Bacterial chromosome Phage DNA

The structure of HIV, the retrovirus that causes AIDS Reverse transcriptase Viral envelope Capsid Glycoprotein RNA (two identical strands)

Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 7 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 6 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 4 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 5 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. 3 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. 2 New viruses bud off from the host cell. 9 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 mRNA RNA genome for the next viral generation Viral RNA RNA-DNA hybrid DNA Chromosomal DNA NUCLEUS Provirus HOST CELL Reverse transcriptase New HIV leaving a cell HIV entering a cell 0.25 µm HIV Membrane of white blood cell The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 1 The reproductive cycle of HIV, a retrovirus

Lytic Cycle Retrovirus Cycle Lysogenic Cycle Complete the Following Venn Diagram. Describe in detail similarities and differences, give examples.

Viral Diseases Viruses are pathogenic and therefore disease causing. Some viral diseases: –HIV –Influenza –Colds –Polio –Rabies –Smallpox –Hepatitis

HIV HIV is a retrovirus and can be spread by sexual contact, contaminated blood, or contaminated needles. HIV affects the immune system by destroying helper T cells. Eventually HIV will lead to AIDS, which is a disease in which a person’s immune system is unable to defend against infections.

Viral Diseases Some viral diseases can also cause cancer. –Hepatitis B = liver cancer –Epstein-Barr virus = Burkitt’s lymphoma –Human papilloma virus (HPV) = cervial cancer