TRANSLATION Sections 5.2 & 5.4

Recall… Protein synthesis occurs in two distinct stages: Transcription, in the nucleus Product: mRNA Translation, in the cytoplasm Product: polypeptide

Translation The process of decoding the information carried in the mRNA. The base sequence is “translated” to generate a polypeptide chain.

Overview Three stages: Initiation Elongation Termination

Stage Events Initiation A ribosome binds to the mRNA Elongation tRNA delivers the appropriate amino acids to the ribosome for addition to the growing polypeptide chain Termination Transcription ceases, and the polypeptide is released

Initiation A ribosome recognizes a specific three-base sequence on the mRNA, and binds

Elongation The ribosome moves along the strand of mRNA, “reading” it three nucleotides at a time Each set of three nucleotides codes for one amino acid Each triplet of nucleotides is called a codon

The ribosome reads the codons, and a tRNA delivers the appropriate amino acid The ribosome catalyzes the formation of a peptide bond between each AA

Termination The ribosome reads a codon that tells it to stop elongation Ribosome falls off the mRNA, and releases the polypeptide chain

The genetic code How exactly does the base sequence of an mRNA dictate the order of amino acids?? The genetic code

A codon (triplet of nucleotides) encodes one amino acid Four bases (U, C, A, G) There are 20 amino acids

Codons consisting of three nucleotides is the only way to ensure that each amino acid can be encoded Number of nucleotides in a codon Possible combinations of the four bases 1 41 = 4 possible combinations 2 42 = 16 possible combinations 3 43= 64 possible combinations

e.g., alanine is encoded by four different codons (GCU, GCC, GCA, GCG) Only 20 amino acids. 64 possible codons. → the genetic code is redundant e.g., alanine is encoded by four different codons (GCU, GCC, GCA, GCG)

AUG (encodes methionine) is the universal start codon Several codons act as stop codons

THE DETAILS OF EUKARYOTIC TRANSLATION Section 5.4, Page 250 THE DETAILS OF EUKARYOTIC TRANSLATION

Important cellular machinery: Ribosomes tRNA (transfer RNA)

Ribosomes Free-floating in cytoplasm, or temporarily associated with the rough ER In eukaryotes, recognizes the 5’ cap of the mRNA transcript (recall post-transcriptional modifications)

Eukaryotic ribosome Two subunits: Large (50S) Small (30S)

Transfer RNA (tRNA) Small, single-stranded Cloverleaf structure Function: Delivers correct amino acids to the ribosomes

One arm: anticodon recognizes codon of the mRNA Opposite arm: carries corresponding amino acid at 3’ end

tRNA molecule + its corresponding AA = “aminoacyl-tRNA" Enzymes that adds AAs to each tRNA: aminoacyl-tRNA synthetases

Recognition between codon and anticodon is dependent on complementarity E.g., mRNA codon = UAU Complementary anticodon = ________ Opposite arm of tRNA carries _____________

“Wobble” base pairing Refers to the fact that the third base in the codon/ anticodon may not actually match

Wobble base pairing Allows some anticodons to bind to more than one codon Recall the genetic code: Multiple codons for one amino acid. Usually the third base in the codon is what differs. Helps overcome errors in transcription (no proofreading mechanism)

AUG = start = methionine Initiation First step: AUG codon is recognized AUG = start = methionine

Initiation Assembly of initiation complex: Small ribosomal subunit tRNA with methionine Large ribosomal subunit

Elongation Ribosome moves along the mRNA in the 5’  3’ direction (towards the 3’ end of the mRNA) Codons are read  New amino acids are added

Ribosome has three sites for tRNA: A (acceptor) site P (peptide) site E (exit) site – don’t worry about this one http://highered.mcgraw- hill.com/olc/dl/120077/micro06.swf

Steps: Assembly of initiation complex: tRNA carrying methionine at P site Next tRNA enters at A site. Peptide bond forms between methionine + 2nd AA Ribosome translocates over by one codon Next tRNA brings appropriate AA to the A site. Peptide bond forms. Ribosome shifts one codon down, process repeats over and over

Polypeptide trails into the cytoplasm tRNAs that exit from the P site are recycled by adding more AAs to them

Reading frame Positioning of ribosome on the mRNA is important: need to ensure correct reading frame The phase in which mRNA is read Recall a codon is a triplet of nucleotides Reading frame can differ, depending on the base pair from which the ribosome starts translating

Termination The ribosome reaches a stop codon Release factor protein dismantles the ribosome-mRNA complex, and the polypeptide chain is released

After termination… Polypeptides may be altered after translation

Homework Pg. 241 #6-13 Page 253 #1-9