Cell Biology 3813 Exam II Gwendolyn Quintana 01187235 1 2 Which of the following nucleotide sequences would code for the polypeptide sequence arginine glycine aspartate d 5 CGG GGU GAC 3 3 4 5 6 7 A mutation in DNA generates a UGA stop codon in the middle of the RNA coding for a particular protein A second mutation in the cell leads to a single nucleotide change in a tRNA that allows the correct translation of the protein that is the second mutation suppresses the defect caused by the first The altered tRNA translates the UGA as tryptophan What nucleotide change has probably occurred in the mutant tRNA molecule Since tryptophan codon sequence is UGG and the previous stop codon was UGA the most likely nucleotide change is the single substitution between A and G Tryp has a codon that is 5 UGG 3 meaning the Trp tRNA antocodon of 5 CCA 3 If this tRNA contains a mutation in which the anti codon is changed to 5 UCA 3 it will recognize a 5 UGA 3 codon and lead to the incorporation of tryptophan instead of the intended stop of translation What consequences would the presence of such a mutant tRNA have for the translation of the normal genes in this cell Since the tRNA is binding to its initial intended target this means that the aminoacyl tRNA synthetase must also act in likewise manner to facilitate this Since the aminoacyl tRNA synthetase is acting in this manner it may have an effect on other amino acids G in the same of different DNA sequence 8 9 10 11 12 The enzymes for arginine biosynthesis are located at several positions around the genome of E coli and they are regulated coordinately by a transcription regulator encoded by the ArgR gene The activity of ArgR is modulated by arginine Upon binding arginine ArgR alters its conformation dramatically changing its affinity for the DNA sequences in the promoters of the genes for the arginine biosynthetic enzymes Given that ArgR is a repressor protein would you expect that ArgR would bind more tightly or less tightly to the DNA sequences when arginine is abundant The function of these Arg genes is to synthesize arginine When arginine is abundant expression of the biosynthetic genes should be turned off If ArgR acts as a gene repressor binding of arginine will increase its affinity for its regulatory sites allowing it to bind and shut off gene expression If ArgR acts as a gene activator instead then the binding of arginine will likely reduce its affinity for its regulatory DNA preventing its binding and thereby shutting off expression of the Arg genes 13 14 15 16 17 When enhancers were initially found to influence transcription many thousands of nucleotide pairs from the promoters they control two principal models were developed to explain this action at a distance In the DNA looping model direct interactions between proteins bound at enhancers and promoters were proposed to stimulate transcription initiation In the scanning or entry site model RNA polymerase or some other component of transcription was proposed to bind at the enhancer and then scan along the DNA until it reached the promoter There models were tested using an enhancer on one piece of DNA and a globin gene and promoter on a separate piece of DNA The globin gene was not expressed from the mixture of pieces However when the two segments of DNA were joined via a protein linker the globin gene was expressed Does this experiment distinguish between the two models The results of this experiment favor DNA looping which should not be affected by the protein bridge so long as it allowed the DNA to bend Yet the scanning model is predicted to be affected by the nature of the linkage between the enhancer and the promoter If the proteins enter at the enhancer and scan to the promoter they would have to traverse the protein linkage If such proteins are geared to scan on DNA they would likely have difficulty scanning across such a barrier 18 In bacteria but not in eucaryotes most mRNAs encode more than one protein b false 19 Which of the possible control points for gene expression is the most commonly used b transcription 20 Transcription is similar to DNA replication in that d nucleotide polymerization occurs only in the 5 to 3 direction 21 24 For each of the following sentences fill in the blanks with the best word or phrase selected from the list below Not all words or phrases will be used use each word or phrase only once In order for a cell s genetic material to be utilized the information is first copied from the DNA into the nucleotide sequence of RNA in a process called transcription Various kinds of RNAs are produced each with different functions mRNA molecules code for proteins tRNA molecules act as adaptors for protein synthesis rRNA molecules are integral components of the ribosome while snRNA molecules are important for splicing of RNA transcripts 25 26 You swtich a culture of bacteria from one culture medium to a second culture medium and you find that a previously inactive operon is activated Indicate whether each of the following explanations of the result is reasonable R or unlikely U 25 This operon produces anabolic synthetic enzymes and the second medium contains a much lower concentration of the end product synthesized by those enzymes a R second medium a R d TFIID 26 This operon produces catabolic enzymes and the initial substrates are much more concentrated in the 27 The first step in forming a transcription complex for mRNA transcription is binding of which of the following to the TATA box 28 Release of RNA polymerase II to initiate transcription appears to be the direct result of c phosphorylation of RNA polymerase by a protein kinase b can produce a product that is easily detected when expressed under the control of a regulatory 29 A reporter gene is one that sequence to which it has been ligated and cloned 30 The sigma subunit of bacterial RNA polymerase c recognizes promoter sites in the DNA 31 Mitochondrial genes are transcribed by d a separate mitochondrial RNA polymerase 32 The major product of Eukaryotic RNA polymerase II is precursors of mRNA pre mRNA 33 Located about 30 nucleotide pairs upstream from the transcription start site in many eukaryotic genes is an A and T rich region called the TATA box 34 35 What are two ways in which an enhancer differs from a promoter The promoter contains several short 10 bp sequence elements and dispersed over 200 bp An enhancer contains an array short 10 bp sequence elements and may be located several to many kb distant