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CHAPTER 11: Gene Expression Recall that gene information codes for the proteins a cell needs for structure & function. **But not all proteins are needed all the time. **How does a cell control when & which proteins are made??? http://upload.wikimedia.org I. Control of Gene Expression A. Gene Expression-Is the activation, or “turning on” of a gene that results in transcription & the production of mRNA. • Most of the mRNA is then translated into proteins. • Genome- is the complete genetic material contained in an individual. B. Prokaryotic Gene Expression • Since prokaryotes are simpler cells (single ring of DNA- no nucleus)- they are easier to study. • Scientists-Francois Jacob & Jacques Monod – in the 1960’s studied E. coli. – discovered how E. coli genes – control the metabolism of sugar (lactose). – Won noble prize 1965. http://zebu.uoregon.edu/~imamura/121/images/e_coli.gif • Operon- a series of genes that code for specific products plus the regulatory elements that control these genes. • Lac operon – is the operon in E. coli that codes for enzymes that regulate lactose metabolism. Parts of the lac operon & their purpose: 1. Structural genes: genes that code for polypeptides. – In E. coli, there are 3 structural genes right next to each other- these code for the 3 enzymes that are needed to break up lactose (a disaccharide) into 2 monsaccharides ( glucose & galactose) http://dspace.dial.pipex.com/town/park/gfm11/nomilkgif/LACTOSE.JPG 2. Promoter- a nucleotide sequence on a DNA molecule that is recognized by the enzyme RNA polymerase (& is where it binds), that initiates the transcription of a specific gene. 3. Operator – a short sequence of bacterial (or viral) DNA to which a repressor binds to prevent transcription (mRNA synthesis) of the adjacent gene in an operon. Lactose Metabolism http://www.phschool.com/science/biology_place/biocoach/images/lacoperon/pluslac2.gif Turing Operons on & off depends on presence of lactose. • Inducer- molecule that (if lactose is present) – turns on the gene with its 3 structural proteins – to start translating which makes 3 enzymes needed to break up lactose • Repressor protein turns gene off – If no lactose is present- inhibits genes from being expressed. – The repressor protein is coded for by a regulator gene. C. Eukaryotic Gene Expression • • • • • Very Different than prokaryotes. Eukaryotes MUCH more complex. Operons rarely used. Multicellular, Specialized cells – Some cells use different proteins – not all genes expressed at times 1. Eukaryotic Gene Structure • Chormosomes- tightly coiled DNA for cell division. – Some DNA remains tightly coiled & is not transcribed. – Some DNA uncoils- allows transcription • Euchromatin- uncoiled DNA molecule– Site of active transcription http://www.llnl.gov/str/June03/gifs/Stubbs1.gif Turning “on” Different than Prokaryotes • Beyond promotor = 2 sections of DNA • Introns- DNA section that is transcribed but not translated. • Exons –DNA section that is transcribed AND translated. Control After Transcription • In eukaryotic nucleus: -Pre-mRNA is large molecule of both introns & exons. -Splicing of introns results in mRNA • mRNA leaves nucleus & goes into cytoplasm where transcription takes place. • Transcription factors – regulatory proteins • Enhancers- located far from promoter, helps activate promoter. Section 1 Review Questions p. 222 http://gremlin1.zool.iastate.edu/~volker/img/ps-fig.jpg II. Gene Expression in Development & Cell Division • All multicellular, sexually reproducing organisms begin life as a zygote (fertilized egg). • Cell Differentiation- the development of cells that have specialized functions. • Morphogenesis- the development of form in an organism. Homeotic Genes • Regulatory genes that determine where certain anatomical structures, such as appendages, will develop in an organism during morphogenesis. • Considered “master genes” of development. • How they work: – Make regulatory proteins– switch on certain genes– controls growth rate in specific areas – Produces structural development. Homebox Sequences • A DNA sequence within a homeotic gene that regulates development in animals. • Example –Fruit Fly (Drosphilia sp.) – Specific homeoboxes control regional morphogenesis. – Can manipulate in lab – or if natural mutation- abnormality results. *Throughout an organism’s life: – Genes are switched on & off – Only a fraction of genes are expressed in any 1 cell. *DNA chip- tool used to track gene expression. -DNA fragments are “tagged” with a florescent compound & when mRNA is studied- look for “light-up” segments. -useful especially in medicine, cancer study. III. Cancers • Tumor- abnormal proliferation of cells. • Benign Tumor- growth of cells that remains within a mass. (doesn’t spread) • Cancer- uncontrolled growth of cells that can invade other parts of the body.( spreads- more dangerous, deadly) 2 ways to suppress uncontrolled cell growth. • Tumor suppressor genes – code for proteins that prevent uncontrolled cell division. • Proto-oncogenes- code for proteins that help regulate cell division. – are genes which regulate cell growth. – Ensure events occur properly. A mutation can causes changes in proto-oncogenes or tumor suppressor genes - turns them into Oncogenes. Onogene- gene can cause uncontrolled cell growth. • Metastasis- spread of cancer cells beyond original site. • Carcinogen- any substance that can induce or promote cancer. • Mutagen- agents that cause changes in the cell. Types of cancers • Carcinoma- growth in the skin, tissues that line the organs of the body. • Sarcomas- grow in bone, muscle tissue. • Lymphomas- solid tumors in lymphatic system tissue. • Leukemia- uncontrolled production of white blood cells. http://www.cancer-info.com/cancer.gif