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TranslationComments

Translation

Translation is a process by which the nucleotide sequence of mRNA is converted into the amino acid sequence of a peptide.

In translation, messenger RNA is decoded to produce a specific polypeptide according to the rules specified by the genetic code. It has initiation, elongation and termination. Inhibited by antibiotics anisomycin, cycloheximide, chloramphenicol and tetracycline. mRNA carries genetic information from chromosomes to ribosomes. Harboured on the ribosome (structures containing rRNA and a protein base), the mRNA information is matched (through hydrogen bonds) to the specific tRNA (tRNA is a small RNA chain (74-93 nucleotides) that transfers a specific amino acid to a growing polypeptide chain at the ribosomal site of protein synthesis). Aminoacyl tRNA synthetase is is enzyme catalyzing binding of amino acid to tRNA to form aminoacyl tRNA. It starts from the initiation codon, & then follows the mRNA sequence in a strictly "three nucleotides for one amino acid" manner.

This translation mechanism allows for gene overlapping. Translation is carried out by tRNA through the relationship between its anticodon & the associated amino acid. When a tRNA is brought to the ribosome by the pairing between its anticodon & the mRNA's codon, the amino acid attached at its 3' end will be added to the growing peptide. In bacteria, there are 30-40 tRNAs with different anticodons. In animal & plant cells, about 50 different tRNAs are found. However, there are 61 codons coded for amino acids. Suppose each codon can pair with only a unique anticodon, then 61 tRNAs would be needed.

Codon: 3-nucleotide sequence encoding an amino acid. More than one can encode for same amino acid (degeneracy). AUG is usually start codon. UAG, UGA and UAA are stop codons. Three complementary tRNA nucleotides are called anticodons.

  • RNAPI makes rRNA
  • RNAPII makes mRNA
  • RNAPIII makes tRNA & small RNAs
tRNA

tRNA has an anticodon complementing amino acid codon, binding one amino acid specifically to it's 3' end (forming anticodon). This is catalyzed by aminoacyl-tRNA synthetases, which recognize anticodon and compatible amino acid. There are 20 different aminoacyl-tRNA synthetases recognizing anticodon and corresponding amino acid and each add only one amino acid to compatible tRNA. For examble, Arg-tRNA specifies tRNA whose anticodon is for arginine codon. Arg-tRNAArg is with arginine attached.

Eukaryotic promoter has 4 parts
  • B Recognition Element
  • TATA box
  • Initiator
  • Downstream promoter element
Prokaryotes have no nucleus, so mRNA can be translated while it is still being transcribed. The translation is said to be polyribosomal when there is more than one active ribosome.

Initiation

The small ribosomal subunit binds to the start codon on the mRNA. The start codon (usu. AUG) indicates where mRNA starts coding for protein. Prokaryotic initiation small binding of small subunit facilitated by pairing to 8-13 upstream Shine-Delgarno sequence. Small subunit binds to upstreat site of _1. It proceed 5'-3' until AUG (region between cap and AUG is 5' UTR). In bacteria, protein starts instead with modified amino acid N-formyl methionine (f-Met). In f-Met, the amino group has been blocked by a formyl group to form an amide, so this amino group can not form a peptide bond. This is not a problem because the f-Met is at the amino terminus of the protein. The initiator tRNA binds to the P site on the ribosome. In eukaryotes, initiator tRNA carries methionine (Met). (Bacteria use a modified methionine designated fMet.)

Cap-Dependent Initiation
  • Proteins bind to special tag on 5' end of mRNA
  • These bind the small ribosomal subunit
  • Accompanied by some factors, subunit goes 5'-3' for AUG
  • Once found, the area between start and stop codons is translated
  • For euk. and arch., AUG encodes methionine
  • initiator tRNA charged with Met forms part of the ribosomal complex, thus all proteins start with this aminoacid (unless it is cleaved away by a protease in some subsequent steps)
Cap Independent Initiation

In eukaryotes it is the Internal Ribosome Entry Site IRES approach.

Elongation

Large 50S subunit forms a complex with the small 30S subunit, and elongation proceeds. A new activated tRNA enters the A site of the ribosome and base pairs with the mRNA. The enzyme peptidyl transferase forms a peptide bond between the adjacent amino acids. As this happens, the amino acid on the P site leaves its tRNA and joins the tRNA at the A site. The ribosome then moves in relation to the mRNA shifting the tRNA at the A site on to the P whilst releasing the empty tRNA, this process is known as translocation.An aminoacyl-tRNA (a tRNA covalently bound to its amino acid) able to base pair with the next codon on the mRNA arrives at the A site (green) associated with: an elongation factor (called EF-Tu in bacteria) GTP (the source of the needed NRG). The preceding amino acid (Met at the start of translation) is covalently linked to the incoming amino acid with a peptide bond (shown in red). The initiator tRNA is released from the P site. The ribosome moves one codon downstream. This shifts the more recently-arrived tRNA, with its attached peptide, to the P site & opens the A site for the arrival of a new aminoacyl-tRNA. This last step is promoted by another protein elongation factor (named EF-G) & the NRG of another molecule of GTP. Note: the initiator tRNA is the only member of the tRNA family that can bind directly to the P site. The P site is so-named because, with the exception of initiator tRNA, it binds only to a peptidyl-tRNA molecule; that is, a tRNA with the growing peptide attached. The A site is so-named because it binds only to the incoming aminoacyl-tRNA; that is the tRNA bringing the next amino acid. So, for example, the tRNA that brings Met into the interior of the polypeptide can bind only to the A site.

Termination

This step repeats until the ribosome encounters a stop codon (UGA, UAG, or UAA). At that point, translation is terminated, protein growth is stalled, factors are release & proteins mimicing tRNA enter A site & the ribosome is released. The nucleotides from the stop codon to the poly(A) tail make up the 3' UTR of the mRNA. No tRNAs have anticodons for stop codons. However, a protein release factor recognizes these codons when they arrive at the A site. Binding of this protein releases the polypeptide from the ribosome. The ribosome splits into its subunits, which can later be reassembled for another round of protein synthesis.