Chapter 11 1 11 11 1 Describe why where and when gene expression is regulated Gene Expression the overall process by which genetic information flows from genes to proteins genotype to phenotype as genes determine the nucleotide sequences or specific messenger RNA molecules and mRNA in turn determines the sequences of amino acids in protein molecules DNA RNA Protein attaches and initiates transcription Promoter site where the transcription enzyme RNA polymerase Operator a DNA control sequence located between the promoter and the enzyme genes that acts as a switch and determines whether RNA polymerase can attach to the promoter to start transcribing the genes Gene Regulation the turning on and off of genes Why is gene expression regulated Increase or decrease the production of certain gene products not every gene needs to be expressed in every cell then every cell would be alike Drives the process of cellular differentiation and creates different cell types which can produce different proteins and can fit them to their functions Where is gene expression regulated Nucleus DNA unpackaging transcription splicing Cytoplasm breakdown of mRNA translation cleavage modification activation breakdown of protein When is gene expression regulated In cell differentiation 2 Contrast the molecular interactions turning off the lac operon with those which turn it on Operon the cluster of genes and its functions all together in this case the entire stretch of DNA required for enzyme production They exist only in prokaryotes with some rare exceptions Turning off the operon Repressor this turns transcription off when there is no lactose in the cell s environment The repressor binds to the operator and physically blocks the attachment of RNA polymerase to the promoter Regulatory Gene located outside the operon codes for the repressor and is expressed continually so the cell always has a small supply of repressor molecules Turning the lac operon on Lactose interferes with the attachment of the lac repressor by binding to the repressor and changing its shape This shape cannot bind to the operator and the switch then remains on and able for RNA polymerase to bind to it The RNA can then transcribe the genes of the operon and the resulting mRNA carries coding sequences for those three enzymes needed for lactose metabolism Lac operon is an example of an inducible operon stimulated induced by a molecule to be turned on inducible operon an operon that is usually turned off but can be Activators proteins that turn operons on by binding to DNA and stimulating gene transcription They make it easier for RNA polymerase to bind to the promoter repressible operon operon that is normally turned on but can be inhibited e g Tryptophan a necessary protein our body needs but when we eat food already high in it E coli switches the operon off to save energy from making molecules that are already present in abundance 3 Compare DNA packaging modification which tends to enhance gene expression with that which tends to restrict it and explain the implications for X inactivation Chromosome Structure DNA packaging Histones proteins that help in the packaging of DNA make up half of the mass of DNA in eukaryotes Histones attach to the DNA double helix making a nucleosome Nucleosome the beads on a strong in DNA packaging consists of DNA wound around a protein core of eight histone molecules that are connected by short stretches of DNA linkers which is the string String is wrapped into a tight helical fiber which coils further into a thick supercoil Looping and folding can compact the DNA even more Modification Types Enhancing Gene Expression Restricting Gene Expression structure promoting transcription Addition of acetyl groups COCH 3 opens up the chromatin More loosely packed enhances expression Addition of methyl CH 3 group to some amino acids in histone proteins can cause chromosomes to become more compact reducing transcription certain enzymes can add the methyl group to the DNA bases without changing the actual sequence of the bases DNA methylation seems to play role in turning genes off not enough methylation can be bad for cell as well More tightly packed restricts expression X Chromosome Inactivation initiated in early embryonic development when one of the two X chromosomes in each cell is inactivated at random The one that is inactivated is a matter of chance but once an X is inactive all descendant cells have the same copy turned off This is an example of Epigenetic Inheritance Epigenetic Inheritance inheritance of traits transmitted by mechanisms Barr body the inactive X in each cell of a female condenses into this not directly involving the nucleotide sequence compact object One X cell is chemically modified and highly compacted rendering it almost entirely inactive this is why females don t make twice as many proteins encoded by genes on the X chromosome 4 Explain how transcription factors provide selective gene expression allowing differentiation of cell types Differentiation cell must become specialized in structure and function with each type of cell fulfilling a distinct role Almost all cells in an organism contain an identical genome but the subset of genes expressed in each cell is unique reflecting its specific function Transcription Factors proteins required by eukaryotic RNA polymerase to function 5 Describe the implication of alternate gene splicing and the functions of miRNA molecules alternative RNA Splicing process by which an organism can produce more than one type of polypeptide from a single gene Ex one mRNA molecule ends up with the green exon and the other with the brown exon Can lead to big differences in the organism in fruit flies gender depends on different patterns of RNA splicing determines which proteins and genes are expressed as a result Different RNA molecules can be created from the same RNA transcript microRNAs miRNAs small RNA molecules that can bind to complementary Each forms a complex with one or more proteins which can bind to any sequences on mRNA molecules have a role in gene regulation mRNA molecule with 7 8 nucleotides of complementary sequence Then the complex either degrades the target mRNA or blocks its translation This is how it regulates the expression by not allowing some parts to be transcribed 6 Describe multiple sites of gene expression control Breakdown of mRNA Enzymes in cytoplasm eventually breaks mRNA down the timing of this event is an important factor in regulating
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