1. DNA and the Genome
Key Area 3c
Translation

2. Learning Intentions Translation of mRNA
State the location of translation
State 4 things that must be present for transcription to occur
Define ‘amino acid’, ‘polypeptide’ and ‘protein’
Describe the process of translation
Define ‘codon’
Define ‘anticodon’
Explain how the sequence of bases on mRNA acts as a code for protein synthesis
Describe the complementary pairing of bases between mRNA and tRNA
Explain how codons on mRNA recognise incoming tRNA
Explain the function of ‘start’ and ‘stop’ codons and identify them in a diagram
Name the bond formed between amino acids of a polypeptide
Describe the fate of tRNA as the polypeptide is formed

3. Translation
From mRNA to Protein

4. Translation
This is the process where ribosomes make proteins using the mature mRNA produced during transcription.
It occurs in the cytoplasm and requires:
ribosomal RNA (rRNA) to make the ribosome
transfer RNA (tRNA) to translate the mRNA into a polypeptide at a ribosome

5. Ribosomes
As we learned earlier, ribosomes are the site of protein production Ribosomes as composed of ribosomal RNA (rRNA) and proteins The ribosome has 2 subunits – the large subunit and the small subunit These subunits join together to make a ribosome when they attach to an mRNA molecule

6. Each ribosome has 3 binding sites for tRNA: P site: holds the tRNA carrying the growing polypeptide chain A site: holds the tRNA carrying the arriving amino-acid to be added to the chain E site: tRNA exits the ribosome from this site

7. Transfer RNA - tRNA This type of RNA is found in the cell’s cytoplasm.
Each molecule of tRNA only has one triplet of base pairs exposed. This triplet, called an anticodon, corresponds to a particular amino acid.
A U A
tRNA
anticodon
Site of attachment
Tyr
Amino acid
Each tRNA molecule picks up its corresponding amino acid from the cytoplasm.

8. Notice how the bases of the tRNA are bound to each other by weak hydrogen bonds so that only 2 sites are exposed:
Anticodon site (that will match with mRNA codons in ribosome)
site for amino acid attachment

10. 3 Stages of Translation – 1. Initiation
Translation of the mRNA into protein is split into 3 stages:
1. Initiation
The bringing together of the mRNA, the tRNA bearing the first amino-acid and the 2 ribosome subunits
The first codon of the mRNA is the start codon AUG which signals the beginning of translation
The first tRNA attaches to this codon by its anticodon at the p site and carries the first amino-acid methionine
The vacant A site is now ready to receive the next tRNA

12. 3 Stages of Translation – 2. Elongation
The second stage of translation is the elongation of the polypeptide chain:
2. Elongation
Amino acids are added one by one to the preceding amino acid:
The appropriate tRNA binds to the A site of the ribosome by formation of hydrogen bonds between complimentary base pairing of codon and anticodon
This brings the amino acids at the P and A site close enough for a strong peptide bond to form between them
The ribosome translocates (moves) the tRNA in the A site to the P site and the tRNA in the P site moves to the E site and exits the ribosome
This frees up the A site for the next tRNA to enter the ribosome
This process continues and causes the protein (polypeptide) to be formed

13. Elongation of polypeptide chain during translation.
E site
P site
A site
Elongation of polypeptide chain during translation.
Each tRNA leaves the ribosome as the polypeptide is formed so it can collect another amino acid

14. 3 Stages of Translation – 3. Termination
The third stage of translation is termination: 3. Termination Specific stop codons on mRNA do not code for amino acids, instead they act as signals to stop translation These triplets (UAA, UAG, UGA) cause the addition of a water molecule instead of an amino acid to the polypeptide chain (protein) This releases the tRNA in the P site, frees the polypeptide (protein) from the ribosome and causes the 2 subunits to break apart

15. Translation animation

16. Translation
The movement of the mature mRNA trough the ribosome translates the sequence of bases into a sequence of amino acids. Every 3 bases codes for one amino acid and the final sequence of amino acids forms the required polypeptide