BSCI222 Exam 3 Study Guide Dr O Brochta Chapter 17 Control of Gene Expression in Eukaryotes I 17 1 Eukaryotic Cells and Bacteria Have Many Features of Gene Regulation in Common but They Differ in Several Important Ways Similarity DNA binding proteins influence the ability of RNA polymerase to initiate transcription Difference a Most eukaryotic genes are not organized into operons and are not transcribed together into a single mRNA molecule o Each structural gene of eukaryotes typically has its own promoter and is transcribed separately b Chromatin structure affects gene expression in eukaryotes DNA must unwind from histone proteins before transcription can take place c The presence of the nuclear membrane in eukaryotic cells separates transcription and translation in time and space d Larger genomes in eukaryotes greater sequence complexity II 17 2 Changes in Chromatin Structure Affect the Expression of Genes Gene control in eukaryotic cells is accomplished through modification of chromatin structure occur In nucleus histone proteins associate to form octamers around which helical DNA coils to create chromatin represses gene expression DNA needs to become more accessible in order for transcription and translation to a DNase I Hypersensitivity o As genes become transcriptionally active regions around genes become sensitive to action of DNase I DNase I Hypersensitive sites develop 100 nucleotides upstream of the start site suggesting chromatin has an open configuration during transcription o Correspond to binding sites for regulatory proteins 3 processes affect gene regulation by altering chromatin structure 1 Modification of histone proteins Histones in the octamer core of nucleosome have two domains 1 a globular domain that associates with other histones and DNA and 2 a positively charged tail domain that interacts with negatively charged DNA phosphate backbone Tails of histone proteins are often modified histone code encode information that affects how genes are expressed a Methylation of histones addition of methyl group to the tails of histone proteins bringing about either the activation or repression of transcription b Acetylation of histones addition of acetyl group CH3CO to histone proteins stimulating transcription by destabilizing chromatin structure acetyl groups added by acetyltransferase enzymes removed by deacetylase enzymes which represses transcription 2 Chromatin remodeling 3 DNA methylation chromatin remodeling complexes alter chromatin structure without altering the chemical structure directly they bind directly to DNA and reposition the nucleosomes allowing transcription to occur 1 cause the nucleosome to slide along DNA so it is in between nucleosomes and more easily accessible for gene expression 2 cause conformational changes in DNA nucleosomes or both so DNA is exposed C R complexes are targeted to specific DNA sequences by transcriptional activators or repressors addition of methyl group to cytosine bases yielding 5 methylcystosine heavy methylated DNA is associated with repression of transcription transcriptionally activate DNA unmethyated abnormal methylation associated with cancer DNA methylation is common on cytosines adjacent to guanine nucleotides CpG DNA regions with many CpG sequences are called CpG islands found near start sites CpG methyl group removed before initiation of transcription DNA methylation and deacetylation of histones repress transcription III 17 3 Epigenetic Effects Often Result from Alterations in Chromatin Structure Epigenetics alterations to DNA and chromatin structure that affect traits and are passed on to other cells or future generations but are not caused by changes in DNA base sequences everything talked about above o Effects a Epigenetics changes induced by maternal behavior more groming different methylation alters expression of certain genes b Epigenetic effects caused by prenatal exposure c Epigenetic effects in monozygotic identical twins Paramutation one allele of a genotype alters expression of another allele o Violates Mendel s law of segregation when gametes are formed each allele separates and is transmitted independently to the next generation Molecular mechanisms of epigenetic changes the fact that epigenetic marks are passed on to other cells and possibly future generations means that changes in chromatin structure associated with epigenetic phenotypes must be faithfully maintained when chromosomes replicate o After replication special methyltransferase enzymes recognize the hemimethylated state of CpG dinucleotides and add methyl groups to the unmethylated C bases creating two new DNA molecules that are fully methylated o Epigenome the overall pattern of chromatin modifications possessed by each individual organism helps explain stable patterns of gene expression i e distinguish between nerve cell and liver cell a Embryonic stem cells are undifferentiated cells that are capable of forming every type of cell a property referred to as pluripotency IV 17 4 The Initiation of Transcription Is Regulated by Transcription Factors and Transcriptional Regulator Proteins Control through the binding of proteins to DNA sequences that affect transcription o General transcription factors and RNA polymerase assemble into a basal transcription apparatus which binds to a core promoter located upstream of a gene o The basal transcription apparatus is capable of minimal levels of transcription transcriptional regulator proteins are required for normal transcription and they bind to a regulatory promoter upstream from core promoter as well as to enhancers Transcriptional activator proteins stimulate and stabilize the basal transcription apparatus at the core promoter directly or indirectly via protein coactivators Some activators and coactivators have acetyltransferase activity to stimulate transcription by altering chromatin structure Activators have two distinct functions o Binding DNA at a specific base sequence consensus sequence in regulatory promoter or enhancer transcriptional activator proteins contain one or more DNA binding motifs Affect assembly or stability of basal transcription apparatus at core promoter One of the components of the basal transcription apparatus is the mediator which makes contact with the activator proteins and affect rate of transcription initiation o Interact with other components of transcriptional apparatus and influence rate of transcription Transcriptional repressors inhibit transcription