Molecular Genetics of Viruses Viruses are parasites of cells. Typical virus –Penetrates a cell –Takes over the metabolic machinery –Assembles hundreds of new viruses –Host cell is destroyed as the virus leaves the cell to infect other cells

Molecular Genetics of Viruses Specie specific and cell specific. Bacteriophage (phage) –Viruses that attack only bacteria. Structure –Capsid (Protein coat) –Envelope (phospholipids and proteins) Assist in penetrating host cell. –Genetic material (DNA or RNA)

Molecular Genetics of Viruses Reproductive Cycles (2) Lytic Cycle –Penetrates the membrane –Uses host enzymes to replicate viral DNA –Transcribes viral DNA into RNA –Translates RNA to protein –Assembly of DNA and protein into virus particle –Erupts from cell

Molecular Genetics of Viruses Reproductive Cycles (2) Lysogenic cycle –Viral DNA is temporarily incorporated into the DNA of the host –Dormant state: remains inactive until some trigger, usually an external environmental stimulus (radiation or certain chemicals) Provirus Prophage (bacteriophage) –Activated and starts the lytic cycle

Molecular Genetics of Viruses RNA virus –Viral RNA is used directly as RNA –Retroviruses Use Reverse transcriptase to make a DNA complement –RNA  DNA  Transcription  mRNA  Protein –OR –Lysogenic cycle

Molecular Genetics of Bacteria Reproduction- Binary fission –Chromosome replicates and the cell divides into two cells, each cell bearing one chromosome. –No spindle, microtubules, or centrioles. No nucleus to divide. –Plasmids Short, circular DNA molecules outside the chromosome. Replicate independently of the chromosome

Molecular Genetics of Bacteria Causes of Genetic Variation in the Genome of Bacteria Conjugation- DNA exchange between bacteria –Donor bacterium produces a tube or pilus that connects to a recipient bacterium. –Exchange of chromosomal or plasmid DNA. –F plasmids- genes for production of pili. –R plasmid- provides bacteria with resistance against antibiotics

Molecular Genetics of Bacteria Causes of Genetic Variation in the Genome of Bacteria Transduction- introduction of new DNA into a bacteria by a virus –When a virus is assembled during a lytic cycle, it is sometimes assembled with some bacterial DNA in place fo some the viral DNA. –When this aberrant virus infects another cell, the bacterial DNA that it delivers can recombine with the resident DNA.

Molecular Genetics of Bacteria Causes of Genetic Variation in the Genome of Bacteria Transformation- absorption of DNA from their surroundings. –Specialized proteins on the cell membranes of some bacteria facilitate this kind of DNA uptake.

Regulation of Gene Expression Gene regulation in prokaryotes, E. coli, is controlled by sequences of DNA called operons. Four major components of an operon. 1. Regulatory gene- produces a repressor protein that prevents gene expression by blocking the action of RNA polymerase.

Regulation of Gene Expression Four major components of an operon cont’ 2. Promoter region- RNA polymerase attaches to this region to begin transcription. 3. Operator region- can block the action of the RNA polymerase if the region is occupied by a repressor protein. 4. Structural genes- DNA sequences that code for several related enzymes that direct the production of some particular end product.

Regulation of Gene Expression Example: lac operon- controls the breakdown of lactose –Regulatory gene in the lac operon produces an active repressor that binds to the operator region. –When the operator region is occupied by the repressor, RNA polymerase is unable to transcribe several structural genes that code for enzymes that control the uptake and subsequent breakdown of lactose.

Regulation of Gene Expression Example: lac operon- cont’ –When lactose is available, some of the lactose combines with the repressor to make it inactive. –When the repressor is inactivated, RNA polymerase is able to transcribe the genes that code for the enzymes that breakdown lactose.

Regulation of Gene Expression Example: trp operon- produces enzymes for the synthesis of tryptophan –Regulatory gene produces an inactive repressor that does not bind to the operator. –RNA polymerase proceeds to transcribe the structural genes necessary to produce enzymes that synthesize tryptophan. –When tryptophan is available from the surroundings, the bacteria does not need to manufacture it. –Available tryptophan reacts with the inactive repressor and makes it active (tryptophan = corepressor) –Active repressor now binds to the operator region, prevents RNA polymerase from working