Week 4 Lecture 1, Viruses:

What are Viruses? -An infectious particle-Requires a host cell to reproduce-“Obligate Intracellular Parasite”

Viruses are simple and small-The smallest viruses are smaller than ribosomes. -Composed of:

-a genome-a protein shell (capsid)-sometimes a membrane-sometimes carries extra proteins (viral enzymes)

Viral Genomes are Unique-All prokaryotic and eukaryotic genomes are double-stranded DNA.

-Prokaryotic genomes-usually single, circular chromosome. -Eukaryotic genomes-usually multiple, linear chromosomes.

-Viral genomes may consist of single-or double-stranded DNA or RNA.

Viral Capsids-Protein shell encasing genome. -Subunits called capsomeres. -Capsid determines specificity of infection (host range).

-HIV infects humans-SIV infects apes (simians)-FIV infects cats (felines)-West Nile virus infects mosquitoes, birds, and humans

Generalized Viral Life Cycle-Infection-virus enters host cell -Virus commandeers host machinery and materials to replicate genome and make more viral protein. -New virus assembled and released.

Bacteriophages-Viruses that infect bacteria-Also known as phages-Best understood of all viruses because bacteria are easy to grow-dsDNA (double-stranded DNA) viruses can reproduce using the lytic or the lysogenic life cycle.

The Lysogenic Cycle-Doesn’t result in death of bacteria. -Virus reproduced with the bacteria because the viral genome inserts into the bacterial genome.

Temperate Phages-Temperate phages use both lytic and lysogenic life cycle to produce virus.

Animal Viruses with DNA Genome

Animal Viruses with RNA Genomes

RNA Genomes-Class IV viruses-RNA=mRNA-Class V viruses-RNA is template for mRNA-Class VI viruses (retroviruses)-RNA is template for DNA

Enveloped Viruses-Sometimes animal viruses have a membranous envelope surrounding the capsid. -Contains:

-host-derived phospholipids (host’s plasma membrane)-host-derived transmembrane proteins-viral proteins-viral glycoproteins

Week 4 Lecture 2, DNA as Genetic Material:

Which macromolecule is the genetic material? Protein with 20 amino acids or DNA with 4 nucleotides? This was the debate for scientists because they had DNA structured in some structure that lacked much diversity.

Transformation discovered by accident-Pneumonia caused by Streptococcus pneumoniaFrederick Griffith hoped to create a vaccine-Studied two strains of the bacteria

Rough bacteria transformed to smooth

DNA may be the transforming principle-Avery, McCarty, and McLeod found that cell extracts caused transformation-Removed protein, lipid, carbohydrates-Remaining sample caused transformation

Degraded RNA transformation (blue became red circles)Degraded protein transformation (blue became red circles)Degraded DNA no transformation (blue stayed blue circles)

Viruses transfer genetic material to host

Radioactivity tracks macromolecules

Radioactive viruses function normally

Blender separates viruses from cells

Viruses insert DNA into cells

DNA is the genetic material!-Griffith-1928

-Transforming principle converted non-infective rough bacteria into infective smooth bacteria (blue to red circles).

-Avery, MacLeod, McCarty-1944-DNA purified from cell extracts transforms cells. -Destroys DNA prevents transformation.

-Hershey and Chase-1952-Virus transfer DNA to host cells.

Experimental Basis for DNA Structure