Viruses and Bacteria

1. Viruses and Bacteria Ch. 18

2. Viruses • Parasite that requires a host cell in order to live • They take the host cell hostage and use the cell to create the proteins it needs to make new viruses • Typically creates thousands of new viruses and eventually kills the host cell • The range of organisms that a virus can attack is referred to as its host range • A new viral disease that harms humans (ex. Hanta virus) may result from a mutation in the virus that expanded its host range

3. Structure • Made of either DNA or RNA enclosed in a protein coat called a capsid • Many viruses also have a viral envelope which is a membrane that cloaks the capsids • A virus can only infect one specific type of cell • The reason for this is that the virus binds to specific receptors on the surface of the cell

5. Bacteriophages • Most complex virus; Infects bacteria • Also known as the shortened term “phages” • Can reproduce by either the lytic cycle or lysogenic cycle • Lytic Cycle: • Phage enters host cell • Takes control • Replicates itself • Causes cell to burst • This releases a new generation of phage viruses to go and do the same thing

7. Lysogenic Cycle • Replicate without killing the host cell • Incorporated into the host’s DNA • Stays dormant and is called a prophage • When host cell divides, prophage is replicated with it • Leads to a population of infected cells • Environmental trigger causes prophage to switch to the lytic phase • Viruses capable of both types of reproducing are called temperate viruses

9. Retroviruses • Viruses that contain RNA instead of DNA and replicate unusually • Virus infects host cell, uses its own RNA to create complementary DNA (cDNA) • Reverse the usual flow of information from DNA to RNA • This is why they are called retroviruses • Reverse transcription is made possible by an enzyme called reverse transcriptase • Newly made DNA integrates into a chromosome in the nucleus • The integrated viral DNA is called a provirus

11. Transduction • Phages carry bacterial genes from one host cell to another • There are two forms of transduction: • Generalized Transduction: moves random pieces of bacterial DNA as a cell is lysed by the phage and infects another (lytic cycle) • Restricted Transduction: moves specific pieces of DNA that were near the prophage site on the bacterial chromosome. Can only be done by a temperate phage.

13. Prions • Infectious proteins that can cause several brain diseases like “mad cow” disease in cattle • Prion is a misfolded version of a protein normally found in the brain • Causes normal proteins to misfold in same way • Prions are not considered cells or viruses

14. Bacteria • Chromosomes are circular, double-stranded DNA tightly packed into a nucleoid • A nucleoid has no nuclear membrane • Bacteria replicate DNA in both directions from a single origin of replication • This is called theta replication • Reproduction: • Conjugation: direct transfer of genetic material between two bacterial cells that are temporarily connected • Binary Fission: asexual reproduction

15. Plasmid: foreign, small, circular, self-replicating DNA molecule in a bacterium • A bacterium can have several plasmids • The first plasmid discovered is the F plasmid • F stands for fertility • Bacteria that contain the plasmid are F+ • Bacteria that do NOT contain the plasmid are F- • The F plasmid has the genes that produce pili • Pili are cytoplasmic bridges that connect cells and allow conjugation

16. Another plasmid worth knowing is the R plasmid • Resistance to antibiotics for the host bacteria • R plasmid can also be transferred from one bacteria to another by conjugation • Bacteria that have the R plasmid have a big advantage because they are resistant to antibiotics • Resistant bacteria are more likely to survive and reproduce quickly • This is bad news for doctors!

17. The Operon • Important to gene regulation • Operon is a set of genes and “switches” that control the expression of those genes • Discovered in 1940’s by Jacob and Monod • Two types of operons: • Inducible: switched off until it is “induced” to turn on • Repressible: always on until it is not needed and becomes “repressed” (turned off)

18. Tryptophan Operon • A repressible operon • Continuously turned on unless turned off by a corepressor • Repressor is encoded by the regulatory gene and is initially inactive • This means RNA polymerase is free to bind to the promoter and transcribe the structural genes, producing tryptophan • When the inactive repressor combines with the corepressor (tryptophan) it changes its shape and binds to the operator, stopping transcription • If tryptophan levels are high, no more is needed, so the production is turned off • Tryptophan is an allosteric effector

21. Lac Operon • Inducible Operon • Turned off until it is “induced” to turn on • Repressor is initially active; binds to the operator located on the promoter, which blocks RNA polymerase from binding there, stopping production • Allolactose (derivative of lactose) acts as the inducer • It binds to the repressor, changing its conformation (shape) • Now that the shape has changed the repressor cannot bind to the operator, which turns the operon on

24. Transposons • Transposable genetic elements • Also called “jumping genes” • Discovered by Barbara McClintock • Two types: • Insertion sequences: made of only one gene, which codes for transposase, the enzyme responsible for moving the sequence from one place to another • Complex transposons: longer and include extra genes. One or more genes located between two insertion sequences