0.5 m T4 bacteriophage infecting an E. coli cell
Science as a Process • Research into TMV led to the conclusion that the pathogen was smaller than a bacterial cell • The pathogen was named virus
Virus Bacterium Animalcell Animal cell nucleus 0.25 m Comparing the size of a virus, a bacterium, and an animal cell
Capsomereof capsid Membranousenvelope RNA Capsomere DNA Head Capsid Tail sheath DNA RNA Tail fiber Glycoprotein Glycoprotein 80 225 nm 18 250 mm 80–200 nm (diameter) 70–90 nm (diameter) 50 nm 20 nm 50 nm 50 nm (d) Bacteriophage T4 (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses Figure 18.4 Viral Structure
A virus has either DNA or RNA for its genome, but not both and has no mechanism for protein synthesis (ribosomes) or metabolism (mitochondria) It also has a protein coat called a capsid. Some have a coat made of glycoproteins. A virus can reproduce only within a host and not on its own.
Capsids and Envelopes • Capsid = protein coat that surrounds the viral genome • viral envelope = derived from host cell or nuclear membranes; it helps the virus invade the host cell
Viral Genome • Double stranded DNA • Single Stranded DNA • Double stranded RNA • Single stranded RNA • A virus has only one of these types of nucleic acids
Viral Replication • What are the possible patterns of viral replication? • DNA --> DNA • RNA --> RNA, where viral genes code for RNA replicase • RNA --> DNA --> RNA; where viral gene uses reverse transcriptase
Bacterial Viruses • Which scientists used bacteriophages to prove that DNA was the hereditary material? • Hershey and Chase • What are the two mechanisms of phage infection? • Lytic and Lysogenic cycles
Figure 18.5 The lysogenic and lytic reproductive cycles of phage , a temperate phage
Bacterial Defense • What defense do bacteria have against phage infection? • Restriction enzymes • What do restriction enzymes do? • They cut up DNA The bacterial DNA is modified to protect it from the restriction endonucleases.
Animal Viruses • What is the viral envelope? • An outer membrane that helps the virus to invade the animal cell. • The invasion of the virus has the following stages ...
1. Attachment 2. Entry 3. Uncoating 4. RNA and protein synthesis 5. Assembly and release
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.
Glycoprotein Viral envelope Capsid Reversetranscriptase RNA(two identicalstrands) The structure of HIV, the retrovirus that causes AIDS
The reproductive cycle of HIV, a retrovirus The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 1 HIV Membrane of white blood cell 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis ofa second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNAhybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. mRNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 7 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 9 New viruses bud off from the host cell. New HIV leaving a cell
Viral Evolution What is the current hypothesis concerning how viruses evolved?
Viral Disease • Some viruses have toxic components and cause infected cells to release enzymes from lysosomes • Recovery involves ability to repair damaged region of the body. Ex: polio may permanently damage nerve cells.
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
Emerging Viruses • HIV • Ebola • Influenza • From where do these viruses emerge? • From mutated versions of current viruses • Jump from current host to new host • Move from a previously isolated region of the world
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.
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
Originalprion Prion Many prions Normalprotein Newprion Model for how prions propagate
Bacterial Mating “Male” with the F factor
Viruses and Cancer • Hepatitus B virus can cause liver cancer • Some viral genes can trigger cancerous genetic conditions • Oncogenes = viral genes that trigger cancerous characteristics • proto-oncogenes = genes already found in normal cells, usually regulate growth factors