1. Translation
From RNA to Protein

2. In prokaryotes
Transcription and translation occur together (can occur at the same time)
Prokaryotic cell. In a cell lacking a nucleus, mRNA produced by transcription is immediately translated without additional processing.
(a)
TRANSLATION
TRANSCRIPTION
DNA
mRNA
Ribosome
Polypeptide

3. In eukaryotes RNA transcripts are modified before becoming true mRNA
Eukaryotic cell. The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various
ways before leaving the nucleus as mRNA.
(b)
TRANSCRIPTION
RNA PROCESSING
TRANSLATION
mRNA
DNA
Pre-mRNA
Polypeptide
Ribosome
Nuclear
envelope

4. After processing, the now ‘mature’ mRNA exits the nucleus and enters the cytosol (aka Cytoplasm)
In the cytosol it is able to encounter ribosomes, which facilitate protein synthesis.

5. Cells are governed by a cellular chain of command:
DNA RNA protein

6. Codon-Anticodon
Each codon will only base pair with a specific anticodon. This anticodon can only be found on a specific tRNA-amino acid complex.
This ensures that the code on the mRNA is accurately made into the proper polypeptide.

7. The Machinery Ribosomes:
Remember – ribosomes are composed of protein and RNA (in this case rRNA)
A ‘ribosome’ is actually made up of a small ribosomal subunit and a large ribosomal subunit.
Q: Where are these subunits made?
A: In the nucleolus.

8. Transfer RNA (tRNA)
tRNA is the molecule that brings an amino acid to the ribosome-mRNA complex.
Each tRNA is bound to a specific amino acid based on its anticodon. This fact is the key aspect of protein synthesis specificity.
Q: What makes sure that each tRNA gets the right amino acid?
A: An enzyme! Aminoacyl-tRNA synthases (one for each amino acid) pair up the right tRNAs and amino acids.

12. The ribosome has three binding sites for tRNA
The A site
The P site
The E site
P site (Peptidyl-tRNA
binding site)
E site
(Exit site)
mRNA
binding site
A site (Aminoacyl-
tRNA binding site)
Large
subunit
Small
Schematic model showing binding sites. A ribosome has an mRNA binding site and three tRNA binding sites, known as the A, P, and E sites. This schematic ribosome will appear in later diagrams.
(b) E P A
Figure 17.16b

13. Synthesis of an RNA Transcript
Promoter
Transcription unit
RNA polymerase
Start point 5 3
Rewound
RNA
transcript
Completed RNA transcript
Unwound
DNA
Template strand of DNA
The stages of transcription are
Initiation
Elongation
Termination 1
Initiation. After RNA polymerase binds to
the promoter, the DNA strands unwind, and
the polymerase initiates RNA synthesis at the
start point on the template strand. 2
Elongation. The polymerase moves downstream, unwinding the
DNA and elongating the RNA transcript 5  3 . 3
Termination. Eventually, the RNA
transcript is released, and the
polymerase detaches from the DNA.

14. Initiation Events
The small ribosomal subunit recognizes the 5’ Cap of the mRNA and binds near the 5’ end.
The first tRNA (which carries the amino acid methionine “AUG”) binds to the start codon
The large ribosomal subunit binds, completing the complex.
Q: What is always the anticodon on the first tRNA?`
A: UAC – remember, the start codon is AUG!

16. Ribosomes: a closer look
There are three sites within the ribosome
A site: Aminoacyl site holds the tRNA with the next amino acid to be added
P site: Peptidyl site holds the tRNA that contains the growing polypeptide chain
E site: Exit site holds the tRNA that no longer has an amino acid and will soon leave

18. Elongation A single elongation cycle consists of:
(1) A new tRNA-amino acid entering the A site and binding to the appropriate codon.
(2) Formation of a peptide bond between the amino acid in the A site and the growing chain in the P site
(3) Translocation of the A site tRNA to the P site and the P site tRNA to the E site

19. (Translocation)
During translocation the tRNAs remain bound to the mRNA, dragging it with them as they move.
This ensures that the next (new) codon will be exposed in the A site.
Q: What reaction occurs during the formation of a peptide bond?
A: dehydration synthesis!

21. Termination
Termination occurs when a stop codon is present at the A site.
A releasing factor binds to the codon. This induces the ribosome to transfer water to the peptide instead of another amino acid.
The polypeptide is released, tRNAs dissociate, and the ribosomes detach from the mRNA.

23. Some notes
It is quite common for many ribosomes to translate the same mRNA at the same time.
This speeds protein synthesis and can amplify the effects of turning on a gene.

24. Free vs. Bound
When a ribosome begins to synthesize its protein, there can be a ‘signal sequence’ on the end of the polypeptide that binds to a specific signal sequence protein in the cytosol. (the signal recognition particle (SRP))
This protein directs the ribosome-mRNA-polypeptide complex to the membrane of the rough ER where it becomes a ‘bound ribosome.’
Q: What is the difference between bound ribosome proteins and free ribosome proteins?
A: bound ribosome proteins are targeted for export from the cell or locations within plasma membrane while free ribosome proteins float free within the cytosol.