Lecture 8 Flashcards
What are some common characteristics of a virus?
- Composed of nucleic acid (dsDNA, ssDNA, dsRNA, ssRNA), linear or circular, one piece or multiple, surrounded by a protein shell (capsid)
- Viruses alone are inert. Viral genetic material redicts the energies of a cell to making more virus.
- Each virus has narrow host range
- Classification based on nucleic acid sequence, host range, and mode of replication
- May have a membrane (rare for bacteriophage) derived from host cell. May ease entry with next cell.
What are viral enzymes?
Enzymes that a virus needs to do something unique which the host cell cannot provide. Carried inside nucleocapsid. Generally perform viral specific entry, replication, or release functions.
What are the 5 steps of lytic replication?
- Attachment: viral protein attaches to protein/lipid/carbohydrate on host cell surface. Determines host range specificity.
- Penetration by viral enzymes or conformation change in viral protein or host channel or endocytosis or membrane fusion
- Synthesis: nucleic acid replicated, viral mRNA synthesized, viral proteins produced. Viral enzymes may be needed.
- Assembly: Chaperones may help viral proteins fold, nucleocapsid self assembles.
- Release: Rupture cell or bud out. Will infect neighbors.
How can you map phage genomes?
Through double infection of one bacterial cells with different phage mutants. Phage lysate is then analyzed by spreading onto a bacterial lawn based on plaque morphology. RF = (# recomb plaques)/(total # plaques). Single cross overs produce viable recombinants if we assume viral genetic material is linear.
What does higher recombination frequency mean?
Genes are farther apart
What are intragenic recombinants?
Cross over within the same gene. Mutations sites within a gene differ for two phages. Recombination would result in a WT and double mutant. Very rare recombinants like this were discovered because of high yield of phage progeny.
What is transduction?
Mode of genetic transfer between bacteria by phages capable of picking up bacterial genes and carrying them from one cell to another.
What is the mechanism of generalized transduction?
Accidental incorporation of chromosomal DNA into phage head instead of phage DNA. The phage is then defective but it can still transfer DNA bc viral proteins are functional. Once inside the recipient, double cross over must occur to incorporate new DNA into the host DNA. Generalized because phage can carry any part of the bacterial chromosome.
What are virulent phage?
Phage that immediately lyse and kill the host cell
What are temperate phage?
Phage that remain within a host cell for a period of time without killing it. Phage DNA either incorporates into host chromosome and replicates with it or replicates independently in the cytoplasm. Also confers resistance to infection by other phages.
What is a prophage?
A phage with its DNA integrated into the bacterial chromosome.
What is a lysogen?
A bacteria harboring a phage
What is a cotransductant? How are they formed?
Example: Grow phage on A+B+ bacterial strain then grow phage lysate with A-B- bacterial strain. Select for A+ then select for A+ and B+. Strains transduced with A+ and B+ are cotransductants.
What does a large cotransduction frequency mean?
Genes are closer together
How do you calculate cotransduction frequencies?
(# progeny that have selected marker and genotype opposite the original recipient genotype)/(total # progeny)
