Central Dogma Information Flow from DNA to RNA to Protein Ch 6 The Big Picture o Genetic Information stored in chromosomes must be read and converted into proteins usually in the cytosol in order to be useful o DNA encoded information is first transcribed into an RNA intermediary by RNA polymerase o After transcription the RNA molecule must undergo several processing steps to become a mature messenger RNA mRNA o mRNA molecules are exported from the nucleus to the cytosol where they can be translated into proteins by ribosomes o Some important RNA molecules do not code for proteins and are processed differently than mRNA molecules o Both transcription and translation are highly regulated and use many energy dependent steps to ensure high fidelity The Central Dogma o Information flow in all prokaryotes and eukaryotes proceeds from DNA RNA protein o Although the mechanisms are very similar between prokaryotes and eukaryotes there are several differences that address specific difficulties in eukaryotes Eukaryotic chromosomes are highly packaged into chromatin DNA is in the nucleus but protein synthesis machinery is in the cytosol Eukaryotic genes are interrupted by large intervening sequences introns which must be removed o Information content DNA mRNA proteins DNA has most because it must have the information and the genes that regulate them A lot of information not present in protein itself mRNA has intermediate amount because it must have the information to regulate and transcribe the protein protein has least because it doesn t have regulatory information used to generate protein but does have some folding information RNA Transcription o Transcription generates a single stranded RNA molecules that is complementary o RNA is synthesized 5 3 and so DNA is read 3 5 by RNA polymerase a to the DNA template strand complex multi subunit enzyme o Eukaryotes have 3 different RNA polymerases which synthesize different types of RNA s prokaryotes have only one RNA pol RNA pol has a problem because Eukaryotic DNA is packaged into Chromatin needs promoters o Transcription initiates at special DNA sequences called promoters Binding sites for accessory proteins called general transcription factors which help position RNA polymerase and start the process o Additional proteins are required to modify chromatin structure and fully active transcription o Extension of RNA chain requires elongation factors which use ATP hydrolysis to assist RNA polymerase to move along chromatin Help to move it along Unwind DNA and needs energy o RNA transcription stops when RNA polymerase encounters a special DNA sequence called a terminator o Most RNA post transcriptional processing before it can be functional o Unlike DNA single stranded nature of RNA allows it to fold into complex 3 D structures comparable to tertiary structure of proteins o For RNA s that are destined to encode proteins substantial processing is required before they are considered mRNA o First modification occurs immediately after 5 end of RNA exits polymerase addition of 7 methylguanosine cap to 5 end of RNA marks RNA as an mRNA to be Protects 5 end of mRNA to indicate it will become messenger RNA mRNA Processing 5 end of mRNA to 5 of guanosine by 3 phosphate So it is different from other RNAs by 3 5 phosphates Pre mRNA Processing o Most protein coding genes contain intervening sequences introns that interrupt the actual coding sequences expressed sequences aka exons o Introns must be removed by the process of RNA splicing Carried out by complex machinery called spliceosome Made up of small nuclear ribonucleoproteins snRNPs small nuclear RNAs snRNAs multiple proteins Directed by RNA sequences found at intron exon boundaries o Spliceosome assembles on pre mRNA while it is still being transcribed but o Splicing process is extremely flexible a given transcript may have many splicing process may be delayed possible splicing patterns Different functionality comes from same gene by using alternative o Once transcription is complete and RNA is released from RNA polymerase the 3 splicing end received a poly A tail o First the 3 end of the original RNA is cleaved off and then a series of 200 A s are added by a poly A polymerase o Poly A binding proteins bind to the tail important for export from the nucleus and later protein synthesis After fully mature mRNA mRNA Export o RNA synthesis and processing all occurs in the nucleus but protein synthesis occurs in the cytosol o Only fully processed mature mRNA is exported from the nucleus depends on removal of some proteins e g snRNPs and addition or retention of others exon junction complex at splice sites to know it has been spliced cap binding proteins to know 5 cap has been added poly A binding proteins to know poly A tail has been added etc o Mature mRNA binds to nuclear export receptor which guides it through the nuclear pore complex into the cytosol Other RNAs o mRNA represents only 5 of cellular RNA o Up to 80 of cellular RNA is ribosomal RNA rRNA makes up the structural and catalytic core of ribosomes o rRNA is synthesized by RNA pol III 18S 5 8S and 26S and RNA pol I 5S 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 in pre mRNA splicing snRNAs ribosome assembly snoRNAs protein synthesis tRNAs regulation of gene expression siRNAs and miRNAs telomere synthesis and more Splicing mRNAs rRNAs tRNAs snRNAs Protein Translation o Once mature mRNA has been exported to the cytosol it can be translated into protein by the ribosome o Transcription DNA RNA One to one correspondence of subunits Essentially the same language with minor changes U for T ribose for deoxyribose o Translation RNA protein No one to one correspondence 20 amino acids but only 4 bases Totally different chemical language o in order to accommodate 20 different amino acids genetic code must use combination of at least 3 nucleotides double nucleotide code 4x4 16 combinations need 20 Triple nucleotide code 4x4x4 64 different combinations more than enough so minimum it could be o Each set of 3 nucleotides is called a codon Since there are more codons than amino acids most amino acids have multiple corresponding codons o Amino acids cannot interact directly with mRNA so an intermediary is required o Transfer RNA tRNA matches amino acids with codons o tRNAs are short RNAs with distinctive 3 D structures o Amino acid is covalently
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