1. TRANSLATION
Sections 5.2 & 5.4

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

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

4. Overview
Three stages:
Initiation
Elongation
Termination

5. 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

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

7. 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

8. 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

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

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

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

12. 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

14. 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)

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

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

17. Important cellular machinery:
Ribosomes
tRNA (transfer RNA)

18. 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)

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

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

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

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

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

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

26. 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)

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

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

29. 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

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

31. 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

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

33. 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

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

36. After termination…
Polypeptides may be altered after translation

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