1. Protein Targetting Prokaryotes vs. Eukaryotes Mutations
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.
Binding the SRP temporarily stops protein synthesis until the ribosome attaches to the ER.
Binding the SRP to the complex converts the receptor into a channel protein that feeds into 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: Step 1:
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 ( 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”
Prokaryotes and eukaryotes also have slightly different ribosomes

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
Note: book doesn’t call all of these frameshifts, but just make sure the students know the concept of inserting/deleting bases and the effects. Not crucial for them to know the name as much as what is happening.
Leu Ala
Phe Gly