1. TRANSLATION AHL Topic 7.3 IB Biology Miss Werba
Information transferred from DNA to mRNA is translated into an amino acid sequence.
AHL Topic 7.3
IB Biology
Miss Werba

2. TOPIC 7 – HL MOLECULAR BIOLOGY
7.1
DNA STRUCTURE & REPLICATION
7.2
TRANSCRIPTION & GENE EXPRESSION
7.3
TRANSLATION
J WERBA – IB BIOLOGY 2

3. THINGS TO COVER U.1 U.2 U.3 U.4 U.5 U.6 U.7 U.8 U.9 Statement Guidance
Initiation of translation involves assembly of the components that carry out the process.
The names of the tRNA binding sites are expected as well as their roles.
U.2
Synthesis of the polypeptide involves a repeated cycle of events.
U.3
Disassembly of the components follows termination of translation.
Examples of start and stop codons are not required.
U.4
Free ribosomes synthesize proteins for use primarily within the cell.
U.5
Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
U.6
Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.
U.7
The sequence and number of amino acids in the polypeptide is the primary structure.
U.8
The secondary structure is the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding.
U.9
The tertiary structure is the further folding of the polypeptide stabilized by interactions between R groups.
Polar and non-polar amino acids are relevant to the bonds formed between R groups.
J WERBA – IB BIOLOGY 3

4. THINGS TO COVER U.10 A.1 S.1 S.2 NOS 3.7 Statement Guidance
The quaternary structure exists in proteins with more than one polypeptide chain.
May involve the binding of a prosthetic group to form a conjugated protein.
A.1
tRNA-activating enzymes illustrate enzyme–substrate specificity and the role of phosphorylation.
S.1
Identification of polysomes in electron micrographs of prokaryotes and eukaryotes.
S.2
The use of molecular visualization software to analyse the structure of eukaryotic ribosomes and a tRNA molecule.
NOS 3.7
Developments in scientific research follow improvements in computing
The use of computers has enabled scientists to make advances in bioinformatics applications such as locating genes within genomes and identifying conserved sequences.
J WERBA – IB BIOLOGY 4

5. RIBOSOMES Made of protein and ribosomal RNA (rRNA)
Have 2 subunits: large (50S) and small (30S)
Have 3 tRNA binding sites on the large subunit: A, P and E sites
Has a mRNA binding site
J WERBA – IB BIOLOGY 5

6. RIBOSOMES A site = ARRIVAL – tRNA binding site
P site = PROTEIN – holds the growing peptide
E site = EXIT – where tRNA leaves the ribosome
J WERBA – IB BIOLOGY 6

7. RIBOSOMES
Multiple ribsomes can work on the same mRNA at the same time to complete translation more efficiently
J WERBA – IB BIOLOGY 7

8. tRNA MOLECULES Clover-leaf structure made of a single chain of RNA
3 loops form in chain as complementary bases pair up forming double-stranded sections
The acceptor stem is where a tRNA-activating enzyme will attach an amino acid
The anticodon loop holds 3 bases complementary to mRNA codons
J WERBA – IB BIOLOGY 8

9. MOLECULAR VISUALISATION SOFTWARE
Skill: to be able to use molecular visualisation software to analyse the structure of eukaryotic ribosomes and a tRNA molecule.
Use the RCSB Protein Data Bank:
The large ribosomal subunit
The path of the mRNA through the ribosome
A tRNA molecule (article)
J WERBA – IB BIOLOGY 9

10. MOLECULAR VISUALISATION SOFTWARE
NOS.3.7
Developments in scientific research follow improvements in computing
The use of computers has enabled scientists to make advances in bioinformatics applications such as locating genes within genomes and identifying conserved sequences.
J WERBA – IB BIOLOGY 10

11. tRNA ACTIVATING ENZYMES
tRNA is activated by a specific tRNA activating enzyme
There are 20 tRNA activating enzymes – one for each of the 20 amino acids
The specific structure of each tRNA molecule means that each one only binds to the specific amino acid
ie. enzyme-substrate specificity
Addition of an amino acid to a tRNA molecule requires ATP
ie. phosphorylation
J WERBA – IB BIOLOGY 11

12. tRNA ACTIVATING ENZYMES
J WERBA – IB BIOLOGY 12

13. tRNA ACTIVATING ENZYMES
Ref: IB Biology HL, OSC
J WERBA – IB BIOLOGY 13

14. PROCESS OF TRANSLATION
U.1
U.2
U.3
Translation consists of initiation, elongation and termination.
J WERBA – IB BIOLOGY 14

15. PROCESS OF TRANSLATION
U.1
U.2
U.3
1) Initiation
[The tRNA-activating enzymes activate the tRNA molecules pre-initiation]
The mRNA binds to the small subunit of the ribosome
The small subunit moves in a 5’3’ direction along the mRNA molecule until it reaches the ‘Met’ (AUG) codon – ie. START codon
A tRNA molecule is brought in to the P site, carrying the complementary anticodon and the ‘Met’ amino acid
Then the large subunit of the ribosome binds to the tRNA and the small subunit.
J WERBA – IB BIOLOGY 15

16. PROCESS OF TRANSLATION
U.1
U.2
U.3
1) Initiation
J WERBA – IB BIOLOGY 16

17. PROCESS OF TRANSLATION
U.1
U.2
U.3
2) Elongation
A second tRNA with an anticodon complementary to the second codon binds to the A site of the ribosome
hydrogen bonds form between the base pairs
The amino acid carried by the tRNA in the P site is transferred to the amino acid in the A site.
the ribosome acts as a ribozyme and catalyses the formation of a peptide bond
= a condensation reaction
J WERBA – IB BIOLOGY 17

18. PROCESS OF TRANSLATION
U.1
U.2
U.3
2) Elongation
J WERBA – IB BIOLOGY 18

19. PROCESS OF TRANSLATION
U.1
U.2
U.3
2) Elongation
The ribosome moves one codon along the mRNA.
tRNA in the P site  moves to the E site and is released
tRNA in the A site  moves to the P site
Then the elongation process repeats the next tRNA and amino acid are brought in.
J WERBA – IB BIOLOGY 19

20. PROCESS OF TRANSLATION
U.1
U.2
U.3
3) Termination
When a STOP codon on the mRNA is reached, translation is terminated.
A release factor protein attaches to the STOP codon in the A site.
J WERBA – IB BIOLOGY 20

21. PROCESS OF TRANSLATION
U.1
U.2
U.3
3) Termination
The release factor allows the polypeptide to be released.
Once this occurs, the ribosomal subunits and other factors break apart.
The ribosome is free to pick up a new strand of mRNA.
J WERBA – IB BIOLOGY 21

22. PROCESS OF TRANSLATION
U.1
U.2
U.3
Initiation of translation involves assembly of the components that carry out the process.
Synthesis of the polypeptide involves a repeated cycle of events.
Disassembly of the components follows termination of translation.
J WERBA – IB BIOLOGY 22

23. PROKARYOTIC TRANSLATION
U.6
In prokaryotes, translation can occur immediately after transcription due to the absence of a nuclear membrane.
J WERBA – IB BIOLOGY 23

24. POLYSOMES
S.1
A polysome is a structure that consists of multiple ribosomes attached to a single mRNA strand.
J WERBA – IB BIOLOGY 24

25. POLYSOMES
S.1
Skill: Identification of polysomes in electron micrographs of prokaryotes and eukaryotes.
J WERBA – IB BIOLOGY 25

26. POLYSOMES
S.1
In prokaryotes, the chromosome may have numerous polysomes attached directly to it.
The polysomes can occur directly alongside the DNA due to the absence of the nuclear membrane.
J WERBA – IB BIOLOGY 26

27. POLYSOMES
S.1
In eukaryotes, the polysomes occur separately in the cytoplasm or on the Rough ER.
J WERBA – IB BIOLOGY 27

28. PROTEIN STRUCTURE
U.7
U.8
U.9
U.10
J WERBA – IB BIOLOGY 28

29. PROTEIN STRUCTURE
U.7
U.8
U.9
U.10
Primary structure: the sequence and number of amino acids in the polypeptide
Secondary structure: the formation of alpha helices and beta pleated sheets stabilized by hydrogen bonding
J WERBA – IB BIOLOGY 29

30. PROTEIN STRUCTURE
U.7
U.8
U.9
U.10
Tertiary structure: the 3D folding of the polypeptide resulting from interactions between R groups
Quaternary structure: exists when more than one polypeptide chain come together as a protein
J WERBA – IB BIOLOGY 30

31. TRANSLATION Q1. What is the function of the tRNA activating enzyme?
It links tRNA to ribosomes.
It links tRNA to mRNA.
It links tRNA to a specific amino acid.
It links an amino acid on one tRNA to an amino acid on another tRNA.
J WERBA – IB BIOLOGY 31

32. TRANSLATION
Q2. The antisense strand on the DNA molecule coding for three codons of a gene is: TATCGCACG
What are the anticodons of the three tRNA molecules that correspond to this sequence?
UAU, CGC and ACG
ATA, GCG and TGC
AUA, GCG and UGC
TAT, CGC and ACG
J WERBA – IB BIOLOGY 32

33. TRANSLATION
Q3. Outline the roles of the different binding sites for tRNA on ribosomes during translation. [4] Q4. Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards. [9]
J WERBA – IB BIOLOGY 33