1. Mr. Briner Unit 7.3 DNA Translation HL
American School of Milan
DP Biology
Unit 7.3
DNA Translation HL

2. 7.3.S2
The use of molecular visualization software to analyse the structure of eukaryotic ribosomes and a tRNA molecule.

3. Translation Structure of tRNA Many different types of tRNA in a cell
All tRNA molecules have:
A triplet of bases called the anticodon
A loop of 7 nucleotides
Two other loops
A CCA base sequence at the 3’ terminal
Forms a site for attaching an amino acid
Sections that become double-stranded by base pairing

4. Translation

5. (c) Complementary base pairing sequence
(a) Amino acid
Specific to each tRNA
(b) CCA base sequence
Where amino acid is attached by the ‘Activating Enzyme’
(c) Complementary base pairing sequence
Helical in shape

6. (f) Small open loop of RNA
(d) 8 free bases
Non-pairing giving one loop of RNA
(e) 7 free bases
Non-pairing giving second loop of RNA
(f) Small open loop of RNA
Variable in shape between different tRNA

7. (g) Anti-codon (3 bases) which binds to the mRNA codon
Specific to the amino acids being carried
Anti-codon is complementary to an mRNA codons

8. Translation Structure of tRNA
Structure allows tRNA to attach to binding sites on ribosomes and to mRNA
Variable features in each type of tRNA produce different properties
Allowing for the correct binding of amino acids to specific tRNAs

9. 7.3.a1
tRNA-activating enzymes illustrate enzyme–substrate specificity and the role of phosphorylation.

10. Translation Activation of tRNA Shape of each tRNA is different.
Defined by the loop and helical sections
Shape selects a specific enzyme
tRNA activating enzyme
Aminoacyl-tRNA synthetase
Enzyme adds a specific amino acid to the CCA sequence (at 3' end of the tRNA)

11. Translation Activation of tRNA
There are 20 different tRNA activating enzymes
One for each of the 20 amino acids
An example of a degenerate code
Each of these enzymes attaches one particular amino acid
To all of the tRNA molecules that have an anticodon corresponding to that amino acid

12. Translation Activation of tRNA tRNA Activating Enzyme uses energy
ATP hydrolysis provides the energy for amino acid attachment to tRNA
Stores energy that is also used later to link the amino acid to the growing polypeptide chain during translation

13. Translation

14. Translation Ribosome structure
Proteins and Ribosomal RNA combine in the structure
Protein (40% of mass)
rRNA (60% of mass)
Large sub-unit and a small sub-unit
Large sub-unit has three binding sites for tRNA molecules ( A, P and E site)
Small sub-unit has a binding site for mRNA

16. Translation Ribosome structure Ribosomes are enzymes
Catalyze the translation of mRNA into a polypeptide
Substrate is mRNA
Can catalyze transcription of any mRNA

17. 7.3.U2
Synthesis of the polypeptide involves a repeated cycle of events.

18. Translation Stages of translation
There are three stages in translation:
(Just like in transcription)
Initiation:
Ribosome, tRNA and mRNA come together to begin the translation

19. Translation Stages of translation
There are three stages in translation:
Elongation:
tRNA molecules attach to the mRNA
Based on the codon-anticodon recognition
Amino acids are brought together and polymerized into the primary structure of the polypeptide

20. Translation Stages of translation
There are three stages in translation:
Termination:
mRNA and the ribosomes detach
Polypeptide is released and the tRNA return to be charged with more amino acid

21. Translation Direction of translation
Translation happens in 5'  3' direction
Ribosomes move along mRNA in this direction
From the 5' free end to the 3' free end

22. Translation Peptide bonds between amino acids
During translation amino acids are joined to form a polypeptide
Sequence of amino acids is called the primary structure
A peptide bond joins each amino acid together
By condensation synthesis

24. Translation
Peptide bonds between amino acids

25. Translation Peptide bonds between amino acids N-C-C-N-C-C backbone
Polypeptides maintain this sequence no matter how long the chain
R-groups project from the backbone
As amino acids are added in translation, the polypeptide folds into specific shape
Secondary and tertiary structure

26. Transcription and Translation
Process of DNA Translation
Initiation:
5’ end of mRNA binds to the small subunit of the ribosome
Initial mRNA codon = AUG = start codon
tRNA anticodon binds to mRNA codon by complementary base pairing
tRNA with anticodon: UAC
Methionine attached to tRNA 3’ terminal

27. Transcription and Translation
Process of DNA Translation
Initiation:
Large ribosomal subunit binds
Completes ribosomal structure
Producing three ribosomal binds sites: A, P, & E
A- (Amino acid) is where the new tRNA binds
P- (Polypeptide) is where the amino acid on the tRNA adds to the polypeptide
E- (Exit) is where the tRNA (without amino acid) is the released from the ribosome

28. Transcription and Translation
Process of DNA Translation
Initiation:

29. Translation Process of DNA Translation Start codon
mRNA triplet codon AUG is universally the start codon
Marks the beginning of the coding sequence of a gene
Thus, tRNA with the anticodon UAC and carrying the amino acid methionine is always the first tRNA to enter the P-site during translation

30. Transcription and Translation

31. Transcription and Translation

32. 7.3.U1
Initiation of translation involves assembly of the components that carry out the process.

33. Transcription and Translation
Process of DNA Translation
Elongation:
tRNA with anticodon complementary to 2nd mRNA codon binds to ribosomal A site
Correct amino acid attached to tRNA 3’ end
Enzymes in ribosome catalyze formation of peptide bond between methionine and 2nd amino acid

34. Transcription and Translation
Process of DNA Translation
Elongation:
P site tRNA, now separated from methionine, move to E site
Exits ribosome
Ribosome moves one codon toward the 3’ end of mRNA
Shifting previous A-site tRNA to P-site
Opening A-site

35. Transcription and Translation
Process of DNA Translation
Elongation:
tRNA with anticodon complementary to A- site mRNA codon binds to ribosomal A-site
Correct amino acid attached to tRNA 3’ end
Enzymes in ribosome catalyze peptide bond between 2nd and 3rd amino acids

36. Transcription and Translation
Process of DNA Translation
Elongation:
P site tRNA, now separated from its amino acid, move to E site
Exits ribosome
Ribosome moves one codon toward the 3’ end of mRNA
Shifting previous A-site tRNA to P-site
Opening A-site
Repeats process until stop codon reached

37. Transcription and Translation

38. Transcription and Translation

39. Transcription and Translation

40. Transcription and Translation

41. 7.3.U3
Disassembly of the components follows termination of translation.

42. Translation Process of DNA Translation Stop codon
There are three stop codons
None of these have a corresponding tRNA
When a ribosome encounters a stop codon, a release factor binds to the stop codon
Which terminates translation and allows the separation of all of its components

43. Transcription and Translation
Process of DNA Translation
Termination:
When ribosomal A-site reaches stop codon
No tRNA has a complementary anticodon
Release factor protein binds to ribosomal A-site stop codon
Polypeptide and mRNA are released
Protein further modified in endoplasmic reticulum or golgi, or secreted in a vesicle
Large and small ribosomal subunits separate

44. Transcription and Translation

45. Transcription and Translation

46. Transcription and Translation
ANIMATION

47. Transcription and Translation

48. 7.3.U6
Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.

49. Transcription and Translation
Process of DNA Translation
In eukaryotes:
mRNA needs to leave nucleus to be translated by 80S ribosomes in the cytoplasm or on the RER
In prokaryotes:
mRNA can immediately be translated by free 70S ribosomes

50. 7.3.S1
Identification of polysomes in electron micrographs of prokaryotes and eukaryotes.

51. Translation Process of DNA Translation Polysomes
Several ribosomes translating the same mRNA into protein
Each moving in the 5’ to 3’ direction
Allows many proteins to be made from the same mRNA
Speeds up protein production

52. Translation

53. Transcription and Translation
ANIMATION

54. 7.3.U4
Free ribosomes synthesize proteins for use primarily within the cell.
7.3.U5
Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.

55. Translation Free vs. bound ribosomes Free ribosomes Bound ribosomes
Synthesize proteins for use primarily within the cell itself
Bound ribosomes
Synthesize proteins primarily for secretion or for lysosomes

56. 7.3.U7
The sequence and number of amino acids in the polypeptide is the primary structure.

57. 7.3.U8
The secondary structure is the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding.

58. 7.3.U9
The tertiary structure is the further folding of the polypeptide stabilized by interactions between R groups.

59. 7.3.U10
The quaternary structure exists in proteins with more than one polypeptide chain.

60. MAJOR SOURCES
Thank you to my favorite sources of information when making these lectures!
John Burrell (Bangkok, TH)
Dave Ferguson (Kobe, JA)
Stephen Taylor (Bandung, IN)
Andrew Allott – Biology for the IB Diploma
C. J.Clegg – Biology for the IB Diploma
Weem, Talbot, Mayrhofer – Biology for the International Baccalaureate
Howard Hugh’s Medical Institute –
Mr. Hoye’s TOK Website –
And all the contributors at