Translation From RNA to Protein

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

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

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.

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

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.

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.

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.

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

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.

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!

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

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

(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!

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.

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.

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.