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Molecular and Cellular Biology
MCB 2050
Ray Lu

CH.19 Notes MCB  Eukaryotic gene expression can be regulated at the transcriptional, processing, or translational levels.  In eukaryotes, genes are regulated in a spatial dimension  Multicellular organisms contain many diff cell types organized into tissues and organs. e.g.- particular gene might be expressed in blood cells but never in nerve cells.  Or another gene could have an opposite expression profile  EUK gene expression is more complex, because EUK cells are compartmentalized by membranes  The membranes subdivide the cell into separate organelle: nucleus = stores genetic material (DNA), mitochondria & chloroplasts= recruit energy, reticulum= transports materials within the cell.  These subdivisions physically separates the events of gene expression  Transcription DNA->RNA = in nucleus & modification (capping, polyA tail, remove introns)  Resulting mRNAs then exported to the cytoplasm , associate w/ ribosomes (from endoplasmic reticulum) then translated into polypeptides.  Physical separation of events allows regulation to occur in DIFF places.  Regulation can occur in the nucleus at DNA or RNA level & in the cytoplasm at either RNA or POLYPEPTIDE level.  Controlled Transcription of DNA: before environmental signals can have any effect on the level of transcription, they must be transmitted from the cell surface, where they are usually received, through the cytoplasm & nuclear membrane and onto the chromosomes.  EUK need elaborate internal signaling systems to control the transcription of DNA  Environmental cues may have to pass through layers of cells in order to impact the transcription of genes in a particular tissue. Use INTRACELLULAR COMMUNICATION.  Transcriptional regulation is mediated by protein-DNA interactions.  + & - regulator proteins bind to specific regions of the DNA & stimulate or inhibit transcription = TRANSCRIPTION FACTORS  Alternate Splicing of RNA: after transcription in EUK introns are spliced out, turns into mRNA , formation of mRNA is mediated by spliceosomes.  Genes w/ multiple introns present a problem to the RNA splicing machinery. It has the opportunity to modify the coding sequence of an RNA by deleting some of its exons (apparent way of economizing on gentic info) – allows a single gene to encode diff polypeptides.  Cytoplasmic Control of Messenger RNA Stability: a particular mRNA can be translated by several ribosomes that move along it in sequential order, which continues until the mRNA is degraded (= another control pt. in process of gene expression).  mRNA that’s quickly degraded must be replenished by additional transcription or the polypep it encodes will cease to be synthesized (possible part of developmental program)  rapid degradation of mRNA would be a way of preventing undesired polypep synthesis.  UTR prior to poly A tail affects mRNA stability , as well as chemical factors.  Stability of mRNAs & translation of mRNAs into polypeps also regulated by small, noncoding RNA molecules= small interfering RNAs (siRNAs) or microRNAs (miRNAs)  They pair with sequences in specific mRNAs; once paired cause the mrNA to be cleaved & degraded, or prevent mRNA from being translated.  They Regulate the expression of genes involved in maturation & development  EUK gene expression can be induced by environmental factors such as heat and by signalling molecules such as hormones & growth factors  TEMPERATURE : The Heat Shock Genes – organisms subject to the stress of high temp, respond by synthesizing a group of proteins that help stabilize the internal cellular environment = heat shock proteins.  These proteins found in both PRO & EUK , 40-50% identical in diff organisms.  Expression of heat shock proteins is regulated at the transcriptional level; heat stress induces the transcription of the genes encoding the proteins.  HSP70 (heat shock protein , molecular weight = 70 kiladaltons), encoded by a family of genes located in 2 nearby clusters on one of the autosomes. 5-6 clusters of these genes in the 2 clusters.  When temp is above 33 degrees, genes transcribed into RNA, then translated to produce HSP70 popypeps. This is mediated by a polypep called the heat-shock transcription factor or HSTF.  When drosophila are heat stressed, the HSTF is chemically altered by phosphorylation when altered, It binds specifically to nucleotide sequences (heat shock response elements :HSEs) upstream of the hsp70 genes & makes genes more accessible to RNA polymerase 2 (transcribes protein-coding regions)  The transcription of the hsp70 gene is then vigorously stimulated.  Signal Molecules: Genes that respond to Hormones- in EUK one type of cell can signal another by secreting a HORMONE ( circulate through the body, make contact w/ their target cells, & then initiate a series of events that regulate the expression of particular genes)  Animal = 2 Classes of hormones. STEROID hormones = small, lipid-soluble molecules derived from cholesterol (no trouble passing through membrane) e.g. estrogen, testosterone . Once they have entered the cell, they interact w/ cytoplasmic or nuclear proteins called HORMONE RECEPTORS. Receptor/hormone complex is formed then interacts with the DNA where is acts as a transcription factor to regulate the expression of certain genes.  #2 PEPTIDE hormone: linear chains of amino acids (encoded by genes). E.g.= insulin, regulated blood sugar levels . Normally they are too large to pass freely through the membrane , therefore their signals must be transmitted to the interior of cells by membrane-bound receptor proteins. When a peptide hormone interacts with its receptor, it causes a conformation change in the receptor that eventually leads to changes in other proteins inside the cell. The hormone signal is then transmitted through the cytoplasm of the cell and into the nucleus, where it has the effect of regulating the expression of specific genes (called SIGNAL TRANSDUCTION)  Hormone-induced gene expression is mediated by specific sequences in the DNA (hormone response elements : HREs)  A gene with 2 response elements is transcribed more than twice as vigorously as a gene with only 1.  The transcription of EUK genes is regulated by interaction between proteins and DNA sequences within or near the genes.  Basal transcription factors needed for accurate initiation of transcription from EUK gene promoters. Each of these proteins bind to a sequence within the promoter to facilitate the proper alignment of the RNA polymerase on the template strand of DNA.  Transcription is EUK genes is also controlled by a variety of special transcription factors. These factors bind to response elements, or to sequences called ENHANCERS (located in the vicinity of the gene)  The special transcription factors that bind to these enhancers may interact with the basal transcription factors and the RNA polymerase, which bind to the promoter of a gene. The interactions that take place among the special transcription factors, and the RNA polymerase regulate the transcriptional activity of a gene.  ENHANCERS exhibit 3 fairly general properties: 1) They
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