Bio 240 Chapter 13 notes!
Bio 240 Chapter 13 notes! Bio 240
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This 0 page Class Notes was uploaded by Izabella Nill Gomez on Thursday November 5, 2015. The Class Notes belongs to Bio 240 at University of Tennessee - Knoxville taught by Dr. Hughes in Summer 2015. Since its upload, it has received 23 views. For similar materials see General Genetics (Bio 240) in Biology at University of Tennessee - Knoxville.
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Date Created: 11/05/15
Biology 240 Chapter 13 Notes 1 Genetic code is linear using letters as the ribonucleotide base that composes mRNA derived from DNA 2 Each word of mRNA referred as a triplet code a codon 3 bases speci es one amino acid The code is unambiguous codes only one amino acid The code is degenerate a given amino can be speci ed by more than one codon 5 1 start codon 3 stop codons triplets initiate and terminate translation 6 Nonoverlapping after translation any one ribonucleotide of mRNA is part of only one triplet 7 Sequence is collinear sequence of amino acids making up the protein 8 Code is nearly universal almost the same for everyone PS An insertion of a single nucleotide causes the reading frame sequence of nucleus encoding polypeptide to shift changing all subsequent codonsframeshift mutations Sequence of protein altered radically If one or two insertionsdeletions protein cannot be produced original frame not established 3 insertionsdeletions reestablishes frame Nonsense codons blank codons If found during translation causes protein synthesis to stopterminate no protein made With Nirenberg and Matthei a cell free proteinsynthesizing system created and amino acids incorporated into polypeptides Begins with in vitro mixture with all the necessary items ribosomes tRNAs amino acids etc essential to translation To trace protein synthesis amino acids are made radioactive DNA is synthesized Probability of insertion of a speci c ribonucleotide is proportional to the availability of that molecule to other ribonucleotides RNA homopolymers are synthesized RNA molecule with only one type of ribonucleotide AAAUUUGGG Used to determine what amino acids resulted RNA heteropolymers were then used to predict following combinations that made amino acids based on proportions of ribonucleoside diphosphateused to predict triple codons Triplet binding assay was used later on for speci c assignments of triplet codons taking advantage of ribosome bonds that are made with RNA tRNA is attracted to a complimentary RNA sequence tRNA triplet sequence is the anticodon A radioactive amino acid was combined with tRNA making it 39charged39 NirenbergMatthei found out that way what sequences corresponded to what amino acids The genetic code is degenerate almost all amino acids are speci ed by 23 or 4 different codons A pattern of degeneracy exists sets of codons specifying the same amino acid are grouped so that the rst two letters are the same with only the third differing wobble hypothesis Initial two ribonucleotides are more critical than the third H bonding at the third position is less spatially constrained and provides more exibility for base pairing at the third base As a result only about 30 different tRNAs are needed to accommodate 61 codons for an amino acid Ordered genetic code chemically similar amino acids often share one or two middle bases in the different triples encoding them Buffers the potential effect of mutation on protein function While mutations in the second base of triplet codons result in a change of amino acids the chance is to one of similar chemical properties The protein function may not be noticeably altered n bacteria initiation of translation begins with fmet AUGsometimes known as the initiator codon Formyl is usually removed after protein synthesis or methionine as a whole is removed In eukarya methionine alone is the initial amino acid of polypeptide synthesis May be cleared afterward 3 other codons UAG UAA UGA serve as termination codonsnot recognized by tRNA which causes the end of translation Mutations that produce premature stop codons create partial poypeptidesnonsense mutation There are exceptions to the universal genetic code coding for other proteins within different organisms There is the possibility that a single mRNA may have multiple initiation points for translation overlapping genes Gene is considered an open reading frameDNA sequence that produces an RNA that has a start and stop codon and triplet codons specifying amino acids in between Multiple initiation points have been observed in 0X174 phage Optimizes the limited amount of DNA present the disadvantage is that it is possible to have mutations in more than one protein and can be lethal Applies also to mammals Transcription process from DNA template to RNA molecules Results in mRNA moecue complimentary to gene sequence of one of two strands of the double helix Each codon is then complimentary to the anticodon of tRNA which inserts the correct amino acid into the polypeptide chain during translation Transcription is the initial step in the process of information ow within the cell the amount of RNA is generally proportional to the amount of protein in a cell following synthesis RNA migrates to the cytoplasm from the nucleus where protein synthesis occurs RNA polymerase directs synthesis of RNA without a need for a primer to initiate NTPs serve as substrates for the enzyme which catalyzes the polymerization of nuceoside monophosphates or nucleotides into a poynuceotide chain Nucleotides are linked during synthesis by 539339 phosphate bonds Energy of the triphosphate cleavage supports the reaction The active form of RNA polymerase is a hooenzyme which contains its subunits beta and beta39 provides the catalytic mechanism and active site for transcription Sigma factor plays a regulatory function in the initiation of RNA transcription There is a single form of the enzyme but multiple sigma factors such as in E coli creating variations of the holoenzyme Eukaryotes have distinct forms of RNA polymerase each consisting of a greater number of polypeptide subunits than in bacteria Transcription in bacteria Template binding from DNA occurs the site is established when RNA Pol or subunit recognizes the DNA promotors Located in the 539 region upstream from the initial point of transcription The helix is then denatured to make DNA accessible The point where transcription begins is the transcription start site Consensus sequences of DNA TATA box and TI39GACA is critical to transcription Sequences in regions adjacent to the gene are cisacting elementslocated on the same DNA molecule Transacting factors are molecules that bind to these DNA elements The degree of RNA Pol binding to different promoters varies greatly causing variable gene expression Mutations in promotors severely reduced initiation of gene expression Once RA Pol binds to the initiations site synthesizing begins in 539339 direction in terms of nascent RNA After ribonucleotides are added to the growing RNA chain the sigma subunit dissociates chain elongation proceeds under the core enzyme RNA Pol can proofread as it adds each nucleotide able to back up and correct mistakes Runs until termination signal important because of close proximity to an adjacent gene The unique sequence of nucleotides in the termination region causes newly the newly formed transcript to fold into a hairpin secondary structure held by H bonds Important to termination and also dependent on the termination factor rho piquot Physically interacts with the RNA transcript to ease termination of transcription When RNA transcript releases from the template the core enzyme dissociated ln bacteria groups of genes whose products are involved in the same pathway are clustered together on the same chromosomes genes are contiguous large mRNA is produced and encodes more than one protein Genes in phages are sometimes referred to as complementation groups where RNA is called polycistronic mRNA All proteins needed are produced at the same time Transcription in Eukarya 1 Transcription occurs within the nucleus under the direction of 3 separate forms of RNA Pol RNA transcript is not free to associate with other ribosomes prior to completion of transcription Must move into the cytoplasm rst 2 Initiation requires compact chromatin ber characterized by nucleosome coiling to be uncoiled and DNA to be made accessible to RNA Poland other regulatory proteins chromatin remodeling 3 Eukaryotic RNA Pol relies on transcription factors to scan and bind to DNA Enhancers and silencers may be located in the 539 regulatory region upstream of initiation within or downstream 4 mRNA processing must occurs before translation can occur involving the addition of a 539 cap and a 339 tail PremRNAs hnRNAs are usually larger than the mature mRNA Only 25 of hnRNA is produced to mature mRNA RNA Pol II is responsible for initial template binding depends on cis elements and transtransposon factors The rst of cis actingcore promoter determines where RNA ll binds to DNA and where copying begins The proximal promoter elements silencers and enhancers in uence the rate of transcription initiation TATA box is also cisacting located upstream of the start Enhancerssilences act in response to the requirement of gene product how much needed Transcription factors trans actingincude general transcription factors essentialalways requiredTFllBTFDTFllAbind to TATA and transcriptional activators and repressors that in uence the rate of RNA Pol II initiation With binding of GFFs TFllP RNA Pol llpreinitiation complex Promotor DNA positions itself over the 2 subunits of RNA Pol II securing the duplex called the clamp The small duplex region of DNA separates with the enzyme and is referred as the active center template scanned for the start site RNA synthesis is initiated and transcription is repeated a number of times before stable RNADNA hybrid transcript is formed and synthesis can continue Once the termination signal is received the complex becomes unstable and the clamp opens releasing DNA and RNA Process goes from unstable to stable to unstable again The 7mg cap is made during posttranslational modi cation and is placed at the 539 cap stabilizing mRNA and protecting form nuclease degradation also facilitates transport to the cytoplasm and translation Poly A sequence is added after the 339 end of initial transcript protects from degradation lntervening sequences are represented in the initial mRNA transcripts as expressed introns Expressed and retained mRNA consists of exons Splicing removes the unnecessary information from mRNA to produce functional proteins Heteroduplexes are the result of hybridization of nucleic acids that are not perfectly complimentary introns are present in DNA but absent in mRNA as they loop out and remain unpaved Few eukaryotic genes lack introns n splicing endonucleases cleave the ends of introns and allow RNA ligase to seal the gap between exons true in tRNAs of bacteria Ribozymes perform selfexcision as the intron serves as the source of enzymatic activity necessary for removal Group 1 introns In higher eukaryotes splicing occurs by the mediated spliceosome containing small nuclear RNAs that can be complexed with proteins to form small nuclear ribonucleoproteins Occurs within the nucleus Several cases are know where introns are present in mRNAs derives from the same gene spliced in a different way alternative splicing yields similar but variable mRNAs that result in protein isoforms Possible Exam Questions 1 Which of the following is a ribozyme catalyzed process Options exon splicing lariat formation during premRNA splicing transcription elongation or all of the above Exon splicing and lariat formation during premRNA splicing 2 After hnRNA is processed in eukaryotes it moves through the nuclear membrane to the cytoplasm True
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