Regulation of Gene Expression Prokaryotes Eukaryotes Virus

Gene Expression Functions of the three parts of an operon Role of repressor genes in operons Impact of DNA methylation and histone acetylation on gene expression The role of oncogenes, proto-oncogenes and tumor suppressor genes in cancer

Operons In bacteria, genes are often clustered into units called operons Three parts Promoter: RNA polymerase binding site; begins transcription Operator: controls access of RNA polymerase to genes Genes of the operon: entire stretch of DNA required

Operons Regulatory Gene Produce repressor proteins that may bind to the operator site Blocks the RNA polymerase and the operon is off Repressible Operon Normally on but can be inhibited Anabolic –builds molecules Inducible Operon Normally off but can be activated Catabolic – breaks down food molecules (energy)

Viral structure Virus: “poison” (Latin) Size infectious particles consisting of a nucleic acid in a protein coat Size Smaller than ribosomes Tiniest are about 20nm in diameter

Viral structure Capsid Viral envelope Protein shell that surrounds the genetic material Viral envelope Some viruses have these glycoprotein coats that surround the capsid and aid in infecting their hosts

Viral structure Genome DNA or RNA Double or single stranded Bacteriophages (phages) Viruses that infect bacterial cells It is believed there are approximately 1031phage particles on the planet with approximately 1025 infections per second. Bacteriophage are located anywhere you can isolate bacteria and are generally abundant in soil and water.

Common Structural Features Capsid (Head): Comprised of one or more proteins forming a protective covering around the nucleic acids. Can vary in shape and size. Genomic Material: Nucleic acids (DNA or RNA, but not both) contained with the capsid encoding essential viral proteins. May also encode additional genes obtained during viral passage. Tail (optional): Hollow protein tube used to transfer nucleic acids from phage capsid to bacterium during infection. Can be long or short. Base plate and fibers (optional): Involved in attachment of phage to bacteria prior to infection.

Viral reproduction: Viruses reproduce only in host cells Viruses have a very limited host range Human cold virus infects only the cells of the upper respiratory tract of Homo sapiens Has to do with receptor molecules on the cell membrane

Viral reproduction: Lytic Cycle Viral reproduction occurs only in host cells. Two variations Lytic Lysogenic The lytic cycle Attachment Injection Hydrolyzation Assembly Release (lysis) Results in death of host cell by rupturing the cell, releasing multiple copies of the virus Called a virulent virus

Viral reproduction: Lytic Cycle

Viral reproduction: Lysogenic Cycle Genetic material of virus becomes incorporated into the host cell DNA It is replicated along with the host cell’s DNA Called prophage DNA Called a Temperate virus phage capable of using the lytic and lysogenic cycles May give rise to lytic cycle under certain conditions Example is Herpes virus or cold sores

Viral reproduction: Lysogenic Cycle (Temperate)

RNA viruses: Retroviruses Use the enzyme reverse transcriptase to transcribe DNA from an RNA template (RNADNA) The new DNA then permanently integrates into the host DNA Host transcribes the DNA back to RNA and may be used to synthesize viral proteins or may be released in infect more cells Example: HIV

Viroids and prions Viroids Prions tiny, naked circular RNA that infect plants do not code for proteins, but use cause errors in regulatory systems that control plant growth Prions infectious proteins that cause misfolding of normal proteins in various animal species Example mad cow disease

Bacterial genetics Nucleoid: region in bacterium densely packed with DNA (no membrane) Plasmids: small circles of DNA Reproduction: binary fission (asexual)

Bacterial DNA-transfer processes Transformation: genotype alteration by the uptake of naked, foreign DNA from the environment (Griffith expt.) Transduction: phages that carry bacterial genes from 1 host cell to another •generalized~ random transfer of host cell chromosome •specialized~ incorporation of prophage DNA into host chromosome Conjugation: direct transfer of genetic material; cytoplasmic bridges; pili; sexual

Bacterial Plasmids Small, circular, self-replicating DNA separate from the bacterial chromosome F (fertility) Plasmid: codes for the production of sex pili (F+ or F-) R (resistance) Plasmid: codes for antibiotic drug resistance Transposons: transposable genetic element; piece of DNA that can move from location to another in a cell’s genome (chromosome to plasmid, plasmid to plasmid, etc.); “jumping genes”