Protein Targetting Prokaryotes vs. Eukaryotes Mutations AP Biology Unit 2 Protein Targetting • Secretory proteins have molecular tags that help direct them to the ER = signal peptide • Signal peptide = sequence of amino acids at beginning of protein that binds to the SRP (signal recognition particle) Protein Targetting 1. Ribosome begins translating protein and the amino acids of the signal peptide. 2. Signal peptide is recognized by SRP and bound by it. 3. The SRP and ribosome bind to the receptor on ER. 4. The protein is fed into the ER as it is made. 5. Signal sequence is removed by enzyme inside the ER. Protein Synthesis in Eukaryotes • Between transcription and translation there is an extra step = RNA Processing – RNA is modified before it is translated – Pre mRNA = the RNA before RNA processing RNA Processing • 3 main things happen in RNA processing: – 5’ (G) cap is added – Poly A tail added – Introns removed • Step 1: – Modified G nucleotide is added to the 5’ end of the pre mRNA – Helps mRNA bind to ribosome – Prevents RNA from being broken down by enzymes RNA Processing • Step 2: – Poly A tail is added to the 3’ end of the pre mRNA (100-300 A nucleotides) – NOT the same thing as the termination sequence – Helps direct the mRNA out of the nucleus – Makes the RNA more stable RNA Processing • Step 3: RNA splicing – Introns are noncoding regions (“junk” DNA) – Exons are sections that code for part of protein – Introns are cut out of the pre mRNA and exons are joined together. Question… • What kind of molecules are doing all of these tasks (adding poly A tail, cutting out introns, etc.)? – Accomplished by many different enzymes Protein Synthesis in Prokaryotes • In prokaryotes, translation can start even as transcription is still occurring. • What characteristic of prokaryotic cells allows this to happen? – Absence of a nucleus– it can all happen in “one room” Mutations • A change in the DNA sequence • Could be due to a substitution, insertion or deletion of nucleotide(s) • Wildtype = “normal” sequence – NOT a mutation! Point Mutation • When a nucleotide is substituted for another one. • a variety of things can occur depending on the specific substitution Point Mutation: Silent Mutation • DNA sequence changes  RNA sequence changes  still codes for the same amino acid • No effect on the amino acid sequence Gly Point Mutation: Missense Mutation • DNA sequence changes  RNA sequence changes  codes for a different amino acid • Could affect the functioning of the protein • Under what conditions might the protein not be affected much by a missense mutation? – If the amino acid has similar properties as the wild type amino acid (+ charged  + charged) Ser Point Mutation: Nonsense • DNA sequence changes  RNA sequence changes  early stop codon introduced • Translation stops  Protein is incomplete Frameshift Mutations • Caused by the insertion or deletion of nucleotide(s) • Can cause nonsense, extensive missense, or the insertion/deletion of single amino acids • Why would a frameshift mutation cause a protein to lose its function? Leu – If primary sequence is wrong, then sequence will also be wrong shape changes  function lost Phe Gly Ala