Rebecca Davenport BIOL 1406 BWI 11 6 16 Ch 17 blueprint 1 Gene expression information stored in DNA is expressed as protein a Central Dogma DNA to through transcription RNA to through translation Protein Polypeptide b Transcription and Translation i ii Transcription rewrites DNA into RNA form 1 DNA is the template for RNA synthesis 2 DNA nucleotide Deoxyribose A T G C Double stranded RNA Nucleotide Ribose A U G C Single stranded 3 Where does transcription occur a Prokaryotes cytosol b Eukaryotes nucleus 4 The promoter is the start site of Transcription 5 Three stages of transcription a Initiation b Elongation c Termination 6 RNA synthesis is catalyzed by RNA polymerase which pries the DNA strands apart and joins together the RNA nucleotides 7 RNA polymerase does not need any primer 8 The stretch of DNA that is transcribed is called a transcription unit 9 a Initiation Stage 1 i The DNA sequence where RNA polymerase attaches is called the promoter 1 In eukaryotes Transcription factors bind the promoter first then recruit Rpol and the initiation of transcription forming the transcription initiation complex 2 In prokaryotes Rpol will bind to Promoter Rpol will separate DNA starnds and use 1 strand as a template Rpol will lay down a few nucleotides 3 A promoter called a TATA box is crucial in forming the initiation complex Rpol unwinds DNA and ii Rpol binds the promoter unwinds DNA and starts RNA synthesis iii b Elongation stage 2 i As RNA polymerase moves along the DNA it untwists the double helix ii Nucleotides are added to the 3 end of the growing RNA molecule iii As Rpol moves along the DNA the helix reforms and then RNA molecule hang off Rpol like a tail iv RNA peels away from DNA strand and the DNA helix reforms v A gene can be transcribed simultaneously by several RNA polymerases vi c Termination stage 3 i Rpol has copied entire gene and can down dissipate ii The mechanisms of termination are different in bacteria and eukaryotes iii In bacteria the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification iv In prokaryotes Rpol hits terminator sequence causing Rpol to fall off DNA v In eukaryotes polyadenylation signals sequence and it falls off 1 AAUAAA RNA 2 RNA polymerase II transcribes the polyadenylation signal sequence the RNA transcript is released 10 35 nucleotides past this polyadenylation sequence vi RNA processing its goal is to modify the RNA that was made after transcription 1 Only happens in eukaryotes a Happens in nucleus 2 Edit the pre mRNA that was made during transcription a Enzymes in the nucleus modify premRNA before the genetic messages are dispatched to the cytoplasm 3 Modify ends 5 cap and 3 poly A tail a During RNA processing both ends of the primary transcript are usually altered b Why modify ends i Export from the nucleus ii Protection from degradation iii Attachment to the ribosome 4 Modify the middle Splicing a EX Netflix cuts out the annoying commercials Introns so you can just watch Exons greys anatomy b Exon Expressed as protein we NEED this c Intron intervening sequence noncoding we NEED to get rid of these d Also usually certain interior sections of the molecule are cut out and the remaining parts spliced together remove introns and join together the exons vii Alteration of mRNA ends1 Each end of a pre mRNA molecule is modified in a particular way a The 5 end receives a modified nucleotide 5 cap b The 3 end gets a poly A tail 2 These modifications share several functions a They seem to facilitate the export of mRNA to the cytoplasm b They protect mRNA from hydrolytic enzymes c They help ribosomes attach to the 5 end 3 viii Split genes and RNA Splicing1 Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions 2 These noncoding regions are called intervening sequences or introns 3 The other regions are called exons because they are eventually expressed usually translated into amino acid sequences 4 RNA splicing removes introns and joins exons creating an mRNA molecule with a continuous coding sequence 5 Spliceosomesa In some cases RNA splicing is carried out by spliceosomes b Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins snRNPs that recognize the splice sites c The RNAs of the spliceosome also catalyze the splicing reaction d d Example coding i Template 3 TACGTGTCACTAATT 5 coding 5 TGCACAGTGATTAA 3 Transcription mRNA 5 AUGCACAGUGAUUAA 3 4x4x4 64 codons o 61 of the codons amino acid o 3 of codons stop codons o Codons are on mRNA coded message analogy Redundancy multiple codons give the same amino acids o Wobble spot third spot varies but gives same amino acid o CCC Pro CCG Pro CCA Pro CCU Pro Polypeptide Sequence Met His Ser Asp iii Translation mRNA nucleotide to Protein amino acid 1 Changing the language from nucleotides to amino acids 2 Translator tRNA transfer RNA 3 Occurs on a ribosome in the cytosol 4 Genetic information flows from mRNA to protein through the process of translation 5 Accurate Translation Requires Two Steps a First a correct match between a tRNA and an amino acid done by the enzyme aminoacyl tRNA synthetase b Second a correct match between the tRNA anticodon and an mRNA codon c Flexible pairing at the third base of a codon is called wobble and allows some tRNAs to bind to more than one codon i Key player 1 tRNA 1 A cell translates on mRNA message into protein with the help of transfer RNA tRNA 2 tRNAs transfer amino acids to the growing polypeptide in a ribosome 3 Molecules of tRNA are not identical a Each carries a specific amino acid on one end b Each has an anticodon on the other end the anticodon base pairs with a complementary codon on mRNA 4 a Top end of tRNA is Amino acid attachment site b Bottom is anticodon ii Key player 2 Ribosomes 1 Facilitate specific coupling of tRNA anticodons with mRNA codons in protein synthesis 2 The two ribosomal subunits large and small are made of proteins and ribosomal RNA rRNA 3 A ribosome has three binding sites for tRNA a The P site peptidyl holds the tRNA that carries the growing polypeptide chain b The A site Aminoacyl holds the tRNA that carries the next amino acid to be added to the chain c The E site is the exit site where discharged tRNAs leave the ribosome d d Aminoacyl tRNA synthetase i Enzymes that match a tRNA with the correct amino acid ii Uses ATP hydrolysis iii Results in an aminoacyl tRNA aka a
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