1. TRANSLATION
AHL Topic 7.4
IB Biology
Miss Werba

2. AHL TOPIC 7 – NUCLEIC ACIDS AND PROTEINS
7.1 DNA STRUCTURE
7.2
DNA REPLICATION
7.3
TRANSCRIPTION
7.4
TRANSLATION
7.5 PROTEINS
7.6 ENZYMES
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3. THINGS TO COVER Structure of tRNA tRNA activating enzymes
Structure of ribosomes
Process of translation
Initiation, elongation, translocation, termination
Polysomes, start codons, stop codons
Direction of translation
Structure of a peptide bond
Position of ribosomes v function
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4. tRNA MOLECULES Command term = EXPLAIN
7.4.1
tRNA MOLECULES Command term = EXPLAIN
The tRNA structure matches its function
Function?
To bring amino acids from the cytoplasm to the growing polypeptide and to attach them in the correct location.
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5. tRNA MOLECULES Command term = EXPLAIN
7.4.1
tRNA MOLECULES Command term = EXPLAIN
It has a clover-leaf structure made from a single chain of RNA
There are 3 loops and 2 ends
The 3’end has a CCA base sequence (known as the ACC terminal) and acts as the attachment site for the amino acid
The anticodon lies at the opposite end (on the centre loop). It is a sequence of 3 bases and will attach to the mRNA codons.
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6. tRNA MOLECULES Command term = EXPLAIN
7.4.1
tRNA MOLECULES Command term = EXPLAIN
tRNA is activated by a 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
Addition of an amino acid to a tRNA molecule requires ATP
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7. tRNA MOLECULES Command term = EXPLAIN
7.4.1
tRNA MOLECULES Command term = EXPLAIN
Ref: IB Biology HL, OSC
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8. STRUCTURE OF RIBOSOMES Command term = OUTLINE
7.4.2
STRUCTURE OF RIBOSOMES Command term = OUTLINE
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
Multiple ribsomes can work on the same mRNA at the same time to complete translation more efficiently
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9. DISTRIBUTION OF RIBOSOMES Command term = STATE
7.4.7
DISTRIBUTION OF RIBOSOMES Command term = STATE
Free ribosomes synthesize proteins for use primarily within the cell and bound ribosomes synthesize proteins primarily for secretion.
The distribution of ribosomes depends upon the function of the protein they make.
Free floating ribosomes synthesise proteins for use within the cell.
Ribosomes bound to the Rough ER produce proteins to be secreted out of the cell or for use in lysosomes.
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10. DIRECTION OF TRANSLATION Command term = STATE
7.4.4
DIRECTION OF TRANSLATION Command term = STATE
Translation is carried out in a 5’→ 3’ direction.
Like DNA replication & transcription, translation occurs in a 5’  3’ direction (of the mRNA).
Why?
b/c the start codon is at the 5’ end of the mRNA
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11. PROCESS OF TRANSLATION Command term = STATE
7.4.3
PROCESS OF TRANSLATION Command term = STATE
Translation consists of initiation, elongation, translocation and termination.
There are 4 main stages in translation:
Initiation
Elongation
Translocation
Termination
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12. PROCESS OF TRANSLATION Command term = EXPLAIN
7.4.6
PROCESS OF TRANSLATION Command term = EXPLAIN
1) Initiation
The tRNA-activating enzymes activate the tRNA molecules pre-initiation.
The mRNA binds to the small subunit of the ribosome at the ‘Met’ (AUG) codon – ie. START codon
The first tRNA molecule is brought in carrying the complementary anticodon and the ‘Met’ amino acid
This requires the help of the ATPase enzyme.
Then the large subunit of the ribosome arrives.
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13. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
1) Initiation
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14. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
The larger subunit of the ribosome has 3 tRNA binding sites – A, P and E.
The Arriving tRNA binds to the A site.
The P site holds the tRNA that carries the growing Polypeptide strand.
The tRNAs Exit the ribosome molecule via the E site.
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15. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
2) Elongation
Elongation is divided into 2 steps:
Codon recognition
Bond formation
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16. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
2) Elongation
Codon recognition
tRNA binds to a codon in the A site
The incoming tRNA contains the anticodon, which complements the codon
Hydrogen bonds form between the base pairs.
The tRNA carries the next amino acid in the chain.
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17. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
2) Elongation
Bond formation
The next amino acid is added
The large subunit acts as a ribozyme, using the enzymes within it to join the amino acids together.
The amino acids are joined by a peptide bond in a condensation reaction
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18. PEPTIDE BOND Command term = DRAW and LABEL
7.4.5
PEPTIDE BOND Command term = DRAW and LABEL
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19. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
3) Translocation
tRNAs and the mRNA strand “move down one site”.
The ribosome releases the used tRNA from the E site.
Then the elongation process repeats.... the next tRNA and amino acid are brought in.
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20. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
4) Termination
When a STOP codon on the mRNA is reached, translation is terminated.
A release factor protein recognises and binds to the STOP codon.
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21. PROCESS OF TRANSLATION Command term = EXPLAIN
7.3.3
PROCESS OF TRANSLATION Command term = EXPLAIN
4) 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.
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24. Sample questions
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.
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25. Sample questions
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
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26. Sample questions
Q3. Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards. [9]
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27. Sample questions
A1. and A2. C
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28. Sample questions
A3.
translation involves initiation, elongation/translocation and termination;
mRNA binds to the small sub-unit of the ribosome;
ribosome slides along mRNA to the start codon;
anticodon of tRNA pairs with codon on mRNA:
complementary base pairing (between codon and anticodon);
(anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon;
second tRNA pairs with next codon;
peptide bond forms between amino acids;
ribosome moves along the mRNA by one codon;
movement in 5′ to 3′ direction;
tRNA that has lost its amino acid detaches;
another tRNA pairs with the next codon/moves into A site;
tRNA activating enzymes;
link amino acids to specific tRNA;
stop codon (eventually) reached; 9 max
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