The Genetics of Viruses & Bacteria Chapter 18

Overview Viruses and bacteria –are the simplest biological systems –provided evidence that genes are made of DNA –Working out most of the major steps in DNA: Replication Transcriptio translation

Viruses Viruses are –small and simple –Not cells because they lack: Cellular structure, & metabolic machinery of cells –Aggregate of nucleic acids and protein i.e. Genes in a protein coat

Viruses vs Bacteria TMV experiments (sap): –Could only reproduce within host –Could not be cultivated on nutrient media –Was not inactivated by alcohol –Generally lethal to bacteria

The Virus Genome Depending of the kind of the virus, the virus genome may consists of: –Double stranded DNA –Single stranded DNA –Double stranded RNA –Single stranded RNA

According to the nucleic acid making the viral genome, a virus is either called: –A DNA virus, or, –An RNA virus

Whether DNA or RNA, the virus genome is usually organized as a: – single linear nucleic acid or – circular molecule of nucleic acid The smallest viruses have only four genes The largest viruses have several hundreds

The Capsid The capsid is: –The protein shell enclosing the viral genome Capsids are built of: –Large number of protein subunits –These subunits are called capsomeres –The number of different kinds of protein is small –Capsid of TMV has more than 1000 copies of the same protein

Some viruses have accessory structures: –A membranous envelope surrounding the capsid –Enzyme molecules within caspid Vruses that infect bacteria are called: –Bacteriophages or phages

Reproduction of Viruses Viruses are obligate parasites They can reproduce only within a host cell Isolated viruses can’t reproduce They lack enzymes for metabolism They lack ribosomes for protein synthesis

Host Range Each type of virus can infect & parasitize: –Only a limited range of host cells (host range) Some infect several species Others infect only a single species Cell recognition is thru surface receptors

Host Specificity West Nile Virus can infect: –Mosquitoes –Birds –Horses –Humans Measles virus can infect: –Only humans

Tissue Specificity Most viruses of eukarotes attack specific tissues Human cold virus infect: –Only cells lining upper respritory tract The AIDS virus binds: –Only to certain white blood cells

Viral Infection Begins when viral genome enters a host cell. Once inside the cell, the viral genome reprogramming the host cell to: –copy the viral nucleic acid –manufacture proteins from the viral genome. The host provides: –Nucleotides –Ribosomes –tRNAs –Amino acids –ATP –Other components for making the viral components dictated by viral genes.

Most DNA viruses use: –the DNA polymerases of the host cell to: synthesize new genomes along the viral DNA templates RNA viruses use: –special virus-encoded polymerases that can use: RNA as a template. Nucleic acid molecules and capsomeres self- assemble Assembly produces Viral particles Viral particles then exit the cell.

The simplest type of viral reproductive cycle ends with the : –exit of many viruses from the infected host cell –a process usually damages or destroys the host cell.

Biol Break Slide Monday, April 23

Reproductive Cycles of Phages Some double-stranded DNA viruses can reproduce by two alternative mechanisms: –the lytic cycle –the lysogenic cycle.

The Lytic Cycle the lytic cycle : –culminates in the death of the host. –the bacterium lyses (breaks open) –phages produced within are released –Released phages infect other healthy cells. Virulent phages reproduce only: – by a lytic cycle.

The Lytic Cycle the lysogenic cycle : –The phage genome replicates without destroying the host cell. –the viral DNA molecule is incorporated into: A specific site on the host cell’s chromosome A process called genetic recombination –Phage is in a prophage stage –Its genome is largely silent (self repressed) –phage DNA is copied every time the host divides –phage DNA copies passes to host daughter cells. –Thus, virus propagates without killing its host cells.

Temperate phages Temperate phages: –phages using both lytic and lysogenic cycles. Ex. phage lambda, It infects E. coli