Protein TargettingProtein TargettingProkaryotes vs. EukaryotesProkaryotes vs. EukaryotesMutationsMutations

AP BiologyUnit 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 Targetting1. 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?– If primary sequence is wrong,

then sequence will also be wrong shape changes function lost

Leu Ala

Phe Gly