Viruses Viruses Nonliving particles p Very small 12

Viruses

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

T 4 bacteriophage infecting an E. coli cell 0. 5 m

Comparing the size of a virus, a bacterium, and an animal cell Virus Bacterium Animal cell nucleus 0. 25 m

Viral Structure p 2 main parts: p inner core of nucleic acid (DNA or RNA) § instructions for making copies of the virus p 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 Capsomere of capsid RNA Capsomere Membranous envelope DNA Head Capsid Tail sheath RNA DNA Tail fiber Glycoprotein 18 250 mm 20 nm (a) Tobacco mosaic virus Glycoprotein 70– 90 nm (diameter) 80– 200 nm (diameter) 50 nm (b) Adenoviruses (c) Influenza viruses 80 225 nm 50 nm (d) Bacteriophage T 4

Infection by tobacco mosaic virus (TMV)

Attachment to Host Cell p Order of events: pvirus recognizes host cell pvirus attaches to receptor site on membrane of host cell § Receptor site on host matches with viral proteins pvirus enters host cell pvirus replicates inside host cell

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

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

Lytic cycle of phage T 4, a virulent phage 1 Attachment. The T 4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 5 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. 2 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. Phage assembly 4 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. Head Tails Tail fibers 3 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.

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

AIDS Glycoprotein Viral envelope Capsid Reverse transcriptase RNA (two identical strands)

The reproductive cycle of HIV, a retrovirus HIV Membrane of white blood cell 1 The virus fuses 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 strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. Reverse transcriptase Viral RNA-DNA hybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0. 25 µm HIV entering a cell DNA NUCLEUS Chromosomal DNA RNA genome for the next viral generation Provirus 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as m. RNAs for translation into viral proteins. m. RNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). New HIV leaving a cell 9 New viruses bud off from the host cell. 8 Capsids are assembled around viral genomes and reverse transcriptase molecules. 7 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane.

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