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Transcription/ Translation From Gene to Protein (protein synthesis) • Transcription Protein is made outside of the nucleus at the ribosome, but DNA can’t leave the nucleus. • Transcription - is the making of mRNA from DNA. • messenger RNA (Ribo Nucleic Acid) is a copy of DNA that can leave the nucleus. • • • • • mRNA mRNA is single stranded mRNA contains no T’s (thymine) It replaces that T’s with U’s (uracil) It can leave the nucleus. It’s base pairs are read it sets of 3 called codons. • A-T-G • U-A-C C-G-G G-C-C T-T-A A-A-U DNA mRNA Transcription in Three stages • The three stages of transcription: – Initiation – starting transcription – Elongation – laying down the mRNA nucleotides – Termination – finalizing the mRNA Promoter • Promoter – the beginning of the DNA sequence to be copied. (RNA polymerase II attaches at the promoter) • TATA Box – the promoter begins with a sequence of DNA that includes the base pairs T and A. Transcription Factors • Transcription Factors – Proteins that allow the RNA Polymerase II to bind to the promoter. Transcription Initiation Complex • Transcription initiation complex – the promoter, RNA Polymerase II, and Transcription factors all combined and ready to start transcription. RNA Polymerase • RNA Polymerase – enzyme that unwinds DNA and lays down RNA nucleotides. Termination • A termination code caused the mRNA to be cut free from the DNA. (ex. AAUAAA)** • At this point the mRNA is pre-mRNA is needs a few modifications Alteration of mRNA • The 5 end receives a modified 5 cap – a modified guanine nucleotide with 3 phosphate groups • The 3 end gets a poly-A tail – several adenine nucleotides in a row (50+) • These modifications have several functions: – They allow mRNA to leave the nucleus – They protect mRNA from hydrolytic enzymes – They help ribosomes attach to the 5 end Eukaryotic RNA Splicing • Finally, the sections of the pre-mRNA that will not be used to code for the protein are cut out. • introns – section that are cut out of the pre-mRNA • exons –sections that are left in the finalized mRNA Fig. 17-10 5 Exon Intron Exon Exon Intron 3 Pre-mRNA 5 Cap Poly-A tail 1 30 31 Coding segment mRNA 5 Cap 1 5 UTR 104 105 146 Introns cut out and exons spliced together Poly-A tail 146 3 UTR Spliceosomes • Spliceosomes – carry out RNA splicing • Made of ribonucleoproteins (snRNPs) that recognize the splice sites Ribozymes • Ribozymes -are catalytic RNA molecules that function as enzymes and can splice RNA (present in snRNP’s) (ribozyme) Alternative RNA splicing • alternative RNA splicing – choosing different regions of introns or exons from the same premRNA sequence • So one gene can code for more than one protein. tRNA • • • • • • • • t-RNA – Transfer RNA (single stranded) Each tRNA has an anti-codon that matches up with a codon on the messenger RNA Each tRNA also has a particular amino acid attached to it. Amino Acids • Each tRNA also has a particular amino acid attached to it. • aminoacyl-tRNA synthetase – enzyme that attaches amino acids to tRNA Ribosomes • Ribosomes match the tRNA anticodons with mRNA codons in protein synthesis • The two ribosomal subunits are made of proteins and ribosomal RNA (rRNA) • small subunit holds mRNA • Large subunit holds the tRNA Building a Polypeptide • A ribosome has three binding sites for tRNA: – A site- holds the tRNA that carries the next amino acid to be added to the chain – P site- holds the tRNA that carries the growing polypeptide chain – E site- is the exit site, where discharged tRNAs leave the ribosome.** P A Fig. 17-17 3 U A C 5 5 A U G 3 Initiator tRNA Large ribosomal subunit P site GTP GDP E mRNA 5 Start codon mRNA binding site 3 Small ribosomal subunit 5 A 3 Translation initiation complex Fig. 17-18-4 Amino end of polypeptide E 3 mRNA Ribosome ready for next aminoacyl tRNA P A site site 5 GTP GDP E E P A P A GDP GTP E P A Polyribosomes • polyribosome (or polysome) -several ribosomes translate a single mRNA simultaneously, allowing a cell to make many copies of a protein very quickly Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Completing the Protein • signal peptide – a short peptide added to the finished protein.** • signal-recognition particle (SRP) – attaches to the signal peptide that that it may enter the ER. Predicting Proteins • • • • Predict the protein DNA = ATG CGG RNA = UAC GCC Prot. = Tyr Ala