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Lecture 15

Lecture 15-Gene Expression

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Western University
Biology 1202B
Brenda Murphy

Lecture 15: Control of Gene Expression Human Development o Human egg when released from the ovary is almost completely metabolically inactive o Within seconds of the egg & the sperm meeting, rapid cell division (mitosis) o Mitosis (growth) produce cells of the body o Cells differentiate into specialized cells with different functions from the same template o Every nucleated cell of the body contains the same DNA template and genes i.e. liver cells do not contain different DNA than skin cells Then how do cells of a developing embryo, with the same set of DNA/genes create vastly different specialized? o Structural and functional differences in cell types result from the presence or absence of the products resulting from expressed (transcribed or transcribed and translated) genes rather than the actual presence of genes themselves on DNA. o In order for cells to become specialized it is not just enough to have a gene present in DNA. Gene must be expressed or not expressed in the correct tissue (special) and at the correct time (temporal). This is a very complex system Gene Expression is like music played by an orchestra o Gene is present in DNA but is it “On” or “Off” o On – gene is expressed and actively transcribed o Off – gene is not expressed and not actively transcribed  In different tissues and times this may change o Gene is individually fine tuned o Gene’s tuning is dynamic and aware of its surrounding Regulation of Gene Expression in Prokaryotes o Simple, single celled organisms with generation times in minutes o Rapid and reversible alterations so they can adapt quickly to changes in their environment o Genes are organized into a functional unit called an operan i.e. coordinated synthesis of proteins with related functions. o Each operon can contain several to many genes and each is transcribed as a unit from a promoter into a single messenger RNA  The lac operan is an example of transcriptional regulation demonstrating how genes are turned on and off  Operator – a short segment to which a regulatory protein binds Regulation of Gene Expression in Eukaryotes  Multicellular cells  Produce a large number of cells  Produce a large number of different types of cells  More complicated since nuclear DNA is bound to histones, thus need chromatin remodeling to loosen histone DNA interaction (acetylases add acetyle groups to histones) or slide nucleosomes away from gene’s promoter region Chromatin Remodeling  Remodeling is a crucial initial event because it opens the way for transcription to occur  Promoter DNA not accessible to proteins (RNA polymerase, TF’s) that bind to DNA to initiate transcription.  DNA is wrapped around a core of two molecules each made of histones H2A, H2B, H3 and H4. Forming the nucleosome. Higher level of compaction when histone H1 is linked to adjacent nucleosomes. o Gene is inactive  Using ATP energy, chromatin remodeling complex binds and slides nucleosomes along DNA Steps in Gene Expression in Eukaryotes - Which genes are transcribed - Types & availability of mRNAs to ribosomes - Rate at which proteins are made - Availability of finished protein DNA that does not encode mRNA  Introns  Promoters, enhancers  Intergenic sequence  Repeats – telomeres and centromeres DNA that does encode for mRNA, & does get translated into a protein in eukaryote  Genes are scattered around the genome  Genes consist of protein-coding sequences and adjacent regulatory sequences Organization of a Eukaryotic Gene Emphasizing the Regulatory site involved in Gene Expression  The transcription unit is the segment transcribed into pre-mRNA, it contains the 5’UTR, exons, introns and 3’ UTR  Further upstream is the promoter region, which often contains the TATA box. Transcription factors bind to TATA and then recruit RNA polymerase II.  Promoter proximal elements are part of a regulatory system for increasing the rate of transcriptio
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