Translation

True or False: the entire mRNA is not translated, so there are untranslated regions on the 5’ end and the 3’ end.

True

Triplet code

Groups of three nucleotides encode a single amino acid. Groups of three are called codons.

True or False: Genetic code is redundant, not ambiguous

True. Multiple codons can save for same amino acid, but each codon only codes for one amino acid.

Wobble

Identity of third nucleotide in codon is often not essential in determining the amino acid encoded.

tRNA

“adapter molecule” that enters the ribosomal complex to add amino acids to the elongating chain. They must be charged (appropriate amino acid is attached) by aminoacyl tRNA synthetase. They enter the ribosome and use their anticodons to temporary attach to the mRNA transcript.

Ribosomes

Consists of large and small subunits that come together during translation.

Binding sites:

A-site (amino-acyl site): Assembles charged tRNA with matching anticodon that binds to the mRNA

P-Site (peptidyl site): Holds tRNA with growing polypeptide strand

E-Site (exit site): where discharged tRNA exit ribosomes.

Initiation of Translation

Begins with the formation of the initiation complex, which involves small ribosomal subunit, mRNA, initiation factor proteins, and an initiator tRNA that interacts with the start AUG codon. The complex aligns with the 5’ cap and then scans the mRNA until start codon is found in the 5’→3’ direction. When initiator tRNA binds to start codon, initiation factor protein dissociates and large ribosomal subunit joins the complex. Initiator tRNA moves to the P site. A site is now ready to accept next aminoacyl tRNA, which possesses an anticodon complimentary to the mRNA codon.

Elongation of Translation

First tRNA with appropriate anticodon binds to mRNA at the A site. Peptide bonds forms between the amino acid on the initiator tRNA in the P site and the new amino acid in the A site. Translocation occurs, which is when the ribosome shifts over, placing the discharged initiator tRNA into the E site and the new tRNA into the P site. The uncharged tRNA exits and this process repeats.

Termination of Translation

Stop codon (UGA, UAG, UAA) enters the A site (no tRNA required). A release factor enters the A site, catalyzing an hydrolysis reaction that severs the bond between the polypeptide and the tRNA holding it in the P site. Large and small ribosomal subunits are dissociated, along with the mRNA, tRNA, and release factor. All components can be reused in additional rounds other transition until mRNA degrades.

What protein types do free ribosomes synthesize?

Free ribosomes synthesize proteins destined for the cytoplasm and nucleus of the cell.

What proteins do bound ribosomes synthesize?

Proteins that are incorporated into membranes, enclosed in organelles, and excreted outside the cell are produced by bound ribosomes.

What decides whether a protein will be synthesize on a free or bound ribosome?

mRNA encoding these proteins contains instructions for a signal sequence at the N terminus end of the polypeptide. Translation begins on a free ribosome, but the signal peptide is detected by a signal recognition particle that brings it to rough ER. Ribosomes exit tunnel is aligned with a pore on the surface of the rough ER so the polypeptide can enter the ER lumen.