1. Nucleic Acids
7.3 Translation

2. Recap video

3. The Structure of the Ribosome
Proteins and RNA molecules (rRNA)
2 sub-units (large and small)
3 binding sites for tRNA on the surface:
E = exit site
P = peptidyl site
A = aminoacyl site
2 tRNA molecules can bind at the same time to the ribosome
Binding site for the mRNA on the surface

4. The Structure of the Ribosome
Protein database

5. Structure of tRNA
Loops: sections that become double stranded by base pairing
Base sequence CCA forms a site for attaching amino acid
Anti-codon: triplet of bases
Part of a loop of 7 unpaired bases

6. tRNA activating enzymes

7. Initiation of Translation
To begin translation:
An mRNA molecule binds to the small ribosomal subunit at the mRNA binding site
An initiator tRNA carrying Methionine (MET) binds to the start codon “AUG” in the “P-site”
The large ribosomal subunit binds to the small one

8. Initiation of Translation
The next codon signals another tRNA to bind in the “A-site”
A peptide bond is formed between the amino acids in the P and A sites

9. Elongation Following initiation
The ribosome translocates 3 bases along the mRNA
Moving the tRNA in the “P-site” to the “E-site”
This tRNA is set free
A new tRNA with the correct anticodon binds to the next codon in the “A-site”
This series of steps repeats

11. Termination of Translation
The process continues until a stop codon is reached
The polypeptide is released
The components disassemble
Note: direction of movement from 5’ to 3’

12. What does 5’ to 3’mean?

13. Free Ribosomes
Free ribosomes synthesize proteins for use primarily within the cell

14. Bound Ribosomes
Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes

15. Free vs Bound Ribosomes
Proteins perform specific functions. They must be sorted to go to the correct location.
Whether a ribosome is free in the cytosol or bound to the ER depends on a signal sequence in the polypeptide
The signal sequence is at the beginning of the polypeptide

16. Free vs Bound Ribosomes
As the signal sequence is made it becomes bound to a signal recognition protein
Translation stops until the ribosome attaches to a receptor on the ER
Translation begins again, with the polypeptide moving into the lumen of the ER.

17. Translation and Transcription in Eukaryotes vs Prokaryotes
Cellular function is compartmentalized No
compartmentalization
Modifications of mRNA after transcription before exiting nucleus
No modifications
Delay between transcription and translation
No delay

18. Polysomes
Polysomes are multiple ribosomes attached to a single mRNA molecule.
Strings of polysomes attached to DNA molecule in a prokaryote

19. Polysomes
In prokaryotes, multiple polysomes are visible associated with one gene.
In eukaryotes, polysomes occur on the cytoplasm and next to the ER.
Multiple ribosomes translating a single mRNA molecule with the cytoplasm. The polypeptides are longer the closer to the end of the mRNA.