A 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
The lytic cycle of phage T4, a virulent phage 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. 1 2 4 3 5 Phage assembly Head Tails Tail fibers
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. 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 Bacterial chromosome Phage DNA
12 Bacterial Defense What defense do bacteria have against phage infection? Restriction enzymes What do restirciton enzymes do? They cut up DNA The bacterial DNA is modified to protect it from the restriction endonucleases.
14 Herpesvirus Consists of double stranded DNA Envelope derived from host cell nuclear envelope not from plasma membrane It, therefore, reproduces within the nucleus May integrate its DNA as a provirus Tends to recur throughout lifetime of infected individual. Why do you think your immune system never removes this virus from your body?
SARS (severe acute respiratory syndrome), a recently emerging viral disease (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. (b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope.
The reproductive cycle of an enveloped RNA virus Capsid and viral genome enter cell 2 The viral genome (red) functions as a template for synthesis of complementary RNA strands (pink) by a viral enzyme. 3 New virus 8 RNA Capsid Envelope (with glycoproteins) Glycoproteins on the viral envelope bind to specific receptor molecules (not shown) on the host cell, promoting viral entry into the cell. 1 New copies of viral genome RNA are made using complementary RNA strands as templates. 4 Vesicles transport envelope glycoproteins to the plasma membrane. 6 A capsid assembles around each viral genome molecule. 7 Complementary RNA strands also function as mRNA, which is translated into both capsid proteins (in the cytosol) and glycoproteins for the viral envelope (in the ER). 5 HOST CELL Viral genome (RNA) Template Capsid proteins Glyco- proteins mRNA Copy of genome (RNA) ER
19 RNA Viruses Different classes of RNA viruses: single stranded range from class IV to class VI Class IV: invades as mRNA, is ready for translation Clas V: RNA serves as template for mRNA synthesis RNA is transcribed and serves as template for mRNA and genomic RNA Viral replicase enzyme is packages within viral capsid.
The structure of HIV, the retrovirus that causes AIDS Reverse transcriptase Viral envelope Capsid Glycoprotein RNA (two identical strands) Why are RNA Viruses More deadly than DNA viruses?
The reproductive cycle of HIV, a retrovirus 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
24 Vaccines / Drugs What are vaccines and how do they work? Introduce body to harmless or weakened strain of the virus, so that your immune system learns to recognize the virus prior to invasion Few drugs around to fight viruses, most interfere with DNA, RNA or protein synthesis
25 Viroids and Prions Viroids are naked circular RNA that infect plants Prions are proteins that infect cells Examples of prions seen in scrapies in sheep, mad-cow disease, and Creutzfeldt-Jakob disease in humans How can a prion spread infection? Altered versions of proteins that can alter other proteins
Model for how prions propagate Prion Normal protein Original prion New prion Many prions
1. Viruses are not classified as prokaryotes because: A. They contain membrane bound organelles B. They are multicellular C. They are unicellular D. They are acellular E. They lack genetic material
2. Which of the following can be found in all viruses: A. Ribosomes B. RNA C. DNA D. A Protein Coat E. Cell membrane
3. The virus HIV replicates using reverse transcriptase. Thus it can be inferred that the virus A. Uses only DNA B. Uses only RNA C. Uses RNA as a template for DNA D. Uses DNA as a template for RNA E. Replicates continuously