T4 bacteriophage infecting an E. coli cell 0.5  m

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

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

Infection by Tobacco Mosaic Virus

Figure 18.4 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

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

Figure 18.02x2 Phages

Figure 18.9 Viral infection of plants

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

Table 18.1 Classes of Animal Viruses, Grouped by Type of Nucleic Acid

Figure 18.6 The reproductive cycle of an enveloped virus

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 18.4 The lytic cycle of phage T4

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.

Figure 18.x6 Herpes

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

Figure 18.7x1 HIV infection

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

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.

Figure 18.x3 Polio

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

Smallpox

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

Deer Mouse – Vector of Hantavirus

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

Model for how prions propagate Prion Normal protein Original prion New prion Many prions

Replication of the Bacterial Chromosome

E. coli

E. coli Dividing

Transformation of Bacteria - Griffith

Bacterium releasing DNA with plasmids

Plasmids

Transduction

Bacterial Mating “Male” with the F factor

Conjugation and recombination in E. coli

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

Insertion sequences, the simplest transposons

Insertion of a transposon and creation of direct repeats

Anatomy of a Composite Transposon