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Control of Gene Expression The Central Dogma From DNA to Proteins Genotype DNA Transcription RNA Translation Protein Phenotype Review of Replication, Transcription and Translation DNA Replication Ch9 Q3 Note primase and Okazaki fragments Transcription + Translation Protein Synthesis Control of Gene Expression Every somatic cell has the same DNA Cells are very different because each cell makes certain proteins and not others How does a cell know which genes to transcribe and which not to? Transcription Factors Proteins which control the expression of other genes Link the genome with the environment Activated by signals from outside the cell (e.g. hormones, sugar, etc.) Allow RNA polymerase to bind to the promoter so that transcription can begin Gene must also be exposed –DNA must unwind in that area. RNA Processing The average total size of a gene is 27,000 bases but the average size of the coding portion is only 1,340 bases! mRNA transcripts are modified before use as a template for translation: Addition of capping nucleotide at the 5’ end Addition of polyA tail to 3’ end Important for moving transcript out of nucleus and for regulating when translation occurs RNA Processing -Splicing Splicing occurs, removing internal sequences Introns are sequences removed Exons are sequences remaining There is alternate splicing of mRNA in different tissues Introns =Intervening sequences Used to be called Junk DNA! The # of proteins (~200,000) far outnumbers the # of genes (~20,000) An intron in one context may be an exon in another context RNA Processing Figure 10.10 Translation: Multiple Copies of a Protein Are Made Simultaneously Figure 10.16 Protein Conformation Primary (1) structure Secondary (2) structure Folding of the protein into -helices and -pleated sheets Tertiary (3) structure Sequence of amino acids 3D shape Quaternary (4) structure Complex with other polypeptides (same or different proteins) The Proteasome Misfolded proteins have ubiquitin molecules attached to them Ubiquitinated proteins are sent to the proteasome to be degraded Prions Sole difference between normal and disease protein is conformation Normal PrPC conformation with many helices Infectious PrPSc conformation with -sheets All mammals have PrP but nearby proteins and polysaccharides keep it correctly folded Spread of Prion Disease PrPC PrPSc Spongiform Encephalopathies Gene Expression Can Change Over Time Example: Globin chain switching Hemoglobin molecules Transport oxygen molecules in the blood Composed of 4 globular proteins Globin Chain Switching Figure 11.2 Proteomics Looking at all of the proteins made in a particular cell (or tissue, organ, etc.) i.e. the proteome Chromatin Remodeling DNA is wrapped around histones to form nucleosomes Acetylation =acetyl groups are added to histones Chromosome packaging Exposes the primer so RNA polymerase can bind and transcription can begin Deacetylation =acetyl groups are removed from histones RNA Interference Occasionally, both DNA strands are transcribed Complementary strands bind to one another Gene sequence may allow formation of a “hairpin loop” RNA strand binds to itself Segments of dsRNA attract RNA-induced silencing complexes (RISCs) Can be used experimentally (clinically?) RNA Interference