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Lecture 19

BIOL 151 Lecture 19: Bio 151 - Chapter 19

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Virginia Commonwealth University
BIOL 151

BIO 151 – Chapter 19 19.1 Chromatin to Messenger RNA in Eukaryotes • Gene expression can be influenced by chemical modification of DNA or histones o In many eukaryotic organisms, gene expression is affected by chemical modification of certain bases in DNA, the most common of which is the addition of a methyl group to the base cytosine o Methylation occurs in cytosine bases adjacent to guanosine bases on DNA ▪ This is abbreviated as CpG (the p represents the phosphate) • The CpG sites are often clustered in small regions located in or near the promoter of the gene, the region where RNA polymerase and associated proteins bind to the DNA to initiate transcription o This cluster of sites is known as a CpG Island o Some CpG sites are methylated and some are not ▪ The methylation state can change over time or in response to environmental cues, providing a way to turn genes on or off o DNA in eukaryotes is packaged as Chromatin, a complex of DNA, RNA, and proteins that gives chromosomes their structure ▪ Chromatin must unravel to allow space for transcriptional enzymes and proteins to work • This happens through Chromatin Remodeling, in which the nucleosomes are repositioned to expose different stretches of DNA to the nuclear environment o One way which chromatin is remodeled is by chemical modification of the histones around which DNA is wound ▪ Modification usually occurs on Histone Tails, string of amino acids that protrude from the histone proteins in the nucleosome ▪ The pattern of modifications of the histone tails is thought to constitute a Histone Code that affects chromatin structure and gene transcription o Together, these modifications of bases, changes to histones, and alteration in chromatin structure are often termed Epigenetic • Gene expression can be regulated at the level of an entire chromosome o For most gens, there is a direct relation between the number of copies of the gene (the gene dosage) and the level of expression of the gene ▪ An increase in gene dosage increases the level of expression because each copy of the gene is regulated independently of other copies o XX females and XY males have different numbers of X chromosomes ▪ For genes contained in the X chromosome, the dosage of genes is twice as great in females than males • However, the level of expression of X-linked genes is about the same in both sexes o This implies that the regulation of X chromosomal genes is different in females and in males, and this difference of regulation is called Dosage Compensation BIO 151 – Chapter 19 o In mammals, including humans, dosage compensation occurs through the inactivation of one X chromosome in each cell in females ▪ This process is called X-inactivation • Transcription is a key control point in gene expression o Transcriptional Regulation: the mechanisms that regulate whether or not transcription occurs ▪ This regulation in eukaryotic cells requires the coordinated action of many proteins that interact with one another and with DNA sequences near the gene o An important group of proteins are the General Transcription Factors ▪ These proteins bind to the gene’s promoter, which is the region of a gene that recruits factors necessary to start transcription ▪ Once bound to the promoter, the transcription factors recruit the components of the RNA Polymerase Complex, which synthesizes the RNA transcript complementary to the template strand of DNA o The recruitment of the general transcription factors and components of the RNA polymerase complex is controlled by proteins called Regulatory Transcription Factors ▪ Transcription does not occur if the regulatory transcription factors do not recruit the components of the transcription complex to the gene ▪ Each factor has two bind sites • One binds with a particular sequence in the DNA in or near a gene known as an Enhancer • The second one recruits one or more general transcription factors to the promoter region • RNA processing is also important in gene regulation o The initial transcript, called Primary Transcript, undergoes several types of modification, collectively called RNA Processing ▪ The processing includes the addition of a nucleotide cap to the 5’ end and a string of tens to hundreds of adenosine nucleotides to the 3’ end to form the poly(A) tail o The primary transcript, which is usually longer than the mRNA used in protein synthesis, consists of regions that are retained in the mRNA (the exons) interspersed with regions that are excised and degraded (the introns) ▪ The introns are excised during RNA Splicing ▪ The exons are joined together in their original linear order to form the processed mRNA o RNA splicing can be done in different ways to yield different proteins in a process called Alternative Splicing o Some RNA molecules can become a substrate for enzymes that modify particular bases in the RNA, thereby changing its sequence and what it codes for ▪ This is known as RNA Editing BIO 151 – Chapter 19 19.2 Messenger RNA to Phenotype in Eukaryotes • Small regulatory RNAs inhibit translation or promote RNA degradation o Regulatory RNA molecules known as Small Regulatory RNAs work by binding to transcripts and blocking translation ▪ A small regulatory RNA called microRNA (miRNA) starts out just like the RNA transcribed from protein-coding genes, using the same RNA polymerase for transcription and gong though the same processes of capping, splicing, and polyadenylation • The processed RNA in miRNA folds back on itself to form one or more Hairpin structures, or step-and-loops, stabilized by base pairing in the stem o One strand from each RNA fragment is incorporated into a protein complex known as RISC (RNA-Induced Silencing Complex) ▪ The small, single-stranded RNA (the miRNA) targets the RISC complex to specific mRNA molecules by base pairing with short regions on the target mRNA o A second type of small regulatory RNA is known as Small Interfering RNA (siRNA) ▪ Transcription processing of siRNA and miRNA are virtually identical, including incorporation into a RISC complex • Translational regulation controls the rate, timing, and location of protein synthesis o Translation of mRNA into protein provides another level of control of gene expression o By either transport or repression, the proteins cause
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