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Protein Targetting Prokaryotes vs. Eukaryotes Mutations

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

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Protein Targetting Prokaryotes vs. Eukaryotes Mutations

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  1. Protein TargettingProkaryotes vs. EukaryotesMutations AP Biology Unit 2

  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)

  3. Protein Targetting • Ribosome begins translating protein and the amino acids of the signal peptide. • Signal peptide is recognized by SRP and bound by it. • The SRP and ribosome bind to the receptor on ER. • The protein is fed into the ER as it is made. • Signal sequence is removed by enzyme inside the ER.

  4. 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

  5. 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

  6. 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

  7. 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.

  8. Question… • What kind of molecules are doing all of these tasks (adding poly A tail, cutting out introns, etc.)? • Accomplished by many different enzymes

  9. 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”

  10. 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!

  11. Point Mutation • When a nucleotide is substituted for another one. • a variety of things can occur depending on the specific substitution

  12. 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

  13. 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

  14. Point Mutation: Nonsense • DNA sequence changes  RNA sequence changes  early stop codon introduced • Translation stops  Protein is incomplete

  15. 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? • If primary sequence is wrong, then sequence will also be wrong shape changes  function lost Leu Ala Phe Gly

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