BSCI330 Final Exam Study Guide Don t forget to review old exams in addition to this DNA information o First transcribed to RNA by RNA pol o The several processing steps to become mature messenger RNA mRNA o mRNA exported from nucleus to cytosol translated into protein by ribosomes Genetic info stored in chromosomes o Read and converted into proteins in CYTOSOL in order to be useful Some important RNA molecules do not code for proteins o Processed differently than mRNA Transcription and translation highly regulated o Many energy dependent steps fidelity Central Dogma DNA to RNA to protein Eukaryotic DNA packaged into chromatin Mechanisms similar between prok and euk Differences o Euk chromosomes highly packaged into chromatin o DNA in nucleus but protein synthesis machinery in cytosol o Euk genes are interrupted by large intervening sequences Introns must be removed Information content o DNA mRNA protein Aka DNA stores the most genetic information RNA transcription template strand Generates single stranded RNA molecule complementary to the DNA RNA synthesized 5 to 3 by RNA pol multi subunit enzyme o DNA read 3 to 5 Eukaryotes 3 different RNA polymerases o Synthesize different types of RNA Prokaryotes have only one RNA pol Transcription initiates at DNA sequence o Called promoters Binding sites for accessory proteins called general transcription factors Gtf help position RNA polymerase and start the process o Additional proteins required to modify chromatin structure and fully activate transcription Extension of RNA chain requires elongation factors o Use ATP hydrolysis to assist RNA polymerase to move along chromatin functional o RNA transcription stops when RNA polymerase encounters a special DNA sequence called a terminator o Most RNA requires post transcriptional processing before it can be o Unlike DNA single stranded nature of RNA allows it to fold into complex 3 D structures comparable to tertiary structure of proteins Pre mRNA processing Most protein coding genes contain intervening sequences introns that interrupt actual coding sequences expressed sequences exons Introns removed by RNA splicing o Carried out by machinery called spliceosome o Made up of small nuclear ribonucleoproteins snRNPs Small nuclear RNAs snRNAs multiple proteins o Directed by RNA sequences found at intron exon boundaries Spliceosome assembles on pre mRNA while it is still being transcribed o But splicing process may be delayed Splicing process extremely flexible o A given transcript may have many possible splicing patterns Once transcription complete RNA released from RNA pol o 3 end receives poly a tail First 3 end of original RNA cleaved off o Then series of about 200 A s are added by a poly A polymerase Poly a binding proteins bind to the tail o Important for export from the nucleus and later protein synthesis mRNA Processing For RNAs that are destined to encode proteins substantial processing required before they are considered mRNA First modification o Occurs immediately after 5 end of RNA exits polymerase of 7 methylguanosine cap to 5 end of RNA Marks RNA as an mRNA to be mRNA Export RNA synthesis and processing all occurs in the nucleus o But protein synthesis in the cytosol Only fully processed mature mRNA is exported from the nucleus o Depends on removal of some proteins snRNPs for example o and addition or retention of others exon junction complex at splice sites cap binding proteins poly A binding proteins etc o mature mRNA binds to nuclear export receptor guides it through the nuclear pore complex into the cytosol What about the other RNAs mRNA represents only 5 of cellular RNA up to 80 cellular RNA is ribosomal RNA rRNA o makes up the structural and catalytic core of ribosomes rRNA si synthesized by RNA pol III and pol I o rRNA is heavily processed and assembled with ribosomal proteins in the nucleolus a non membranous organelle within the nucleus Other non coding RNAs have functions o pre mRNA splicing snRNAs o ribosome assembly snoRNAs o protein synthesis tRNAs o regulation of gene expression siRNAs and miRNAs o telomere synthesis and more Protein Translation once mature mRNA has been exported to cytosol o translated into protein by the ribosome Transcription DNA RNA o One to one correspondence of subunits o Essentially the same language with minor changes U for ribose T for deoxyribose Translation RNA protein o No one to one correspondence o 20 amino acids but only 4 bases o totally different chemical language in order to accommodate 20 different amino acids genetic code must use combinations of at least 3 nucleotieds o double nucleotide code 4x4 16 different combinations o triple nucleotide code 4x4x4 64 different combinations Each set of 3 nucleotides codon o Since there are more codons than amino acids most amino acids have multiple corresponding codons Amino acids cant interact directly with mRNA o So intermediary is required Transfer RNA tRNA matches amino acids with codons o tRNAs are short RNAs with distinctive 3D structure o amino acid covalently attached to 3 end of tRNA o each tRNA contains a loop anticodon that is complementary to the appropriate amino acid s codon amino acid coupled to tRNA by aminoacyl tRNA synthetase each amino acid has a distinct synthetase amino acid is first activated by conjugation to AMP o energetically expensive since both high energy phosphates are used up in the process amino acid transferred from AMP to tRNA o resulting conjugate has high energy bond between amino acid and tRNA which allows later generation of peptide bond once complete synthetase proofreads for accuracy protein synthesis occurs in N term c term direction o new amino acid added to C term end of growing chain peptide chain is attached to last tRNA that was added new aminoacyltRNA replaces old tRNA extending the chain by one residue RNA message decoded by the ribosome o Eukaryotic ribosome has 4 rRNAs 80 proteins o rRNAs make up structural and catalytic core ribozyme two subunits one large L one small S ribosome reads mRNA from 5 to 3 o reading 3 bases at a time one codon at a time each mRNA has 3 potential reading frames ribosome must choose one and remain consistent or garbled translation occurs Translation begins with the codon AUG Met o This uses special initiator tRNA which is different nfrom the Met tRNA used for the rest of translation best results AUG should be in context of Kozak sequence gccRccAUGG step 2 peptidyl transfer step 3 4 translocation Translocation
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