Start codon, AUG

The mRNA is read continuously from a start codon (AUG) to a termination codon (UAA, UAG, UGA). 

GG CAA Initial steps Messenger RNA is bound to ribosome with the start codon (AUG) at the P site. A transfer RNA molecule with the amino acid methionine (M) and the anticodon UAC has bound to the exposed start codon. The codon UCA is exposed at the A site. 

The genetic code is nonoverlapping (i.e., each nudeotide is used only once), beginning with a start codon (AUG) near the 5 end of the mRNA and ending with a termination (stop) codon (UGA, UAG, or UAA) near the 3 end. 

The start codon (AUG) sets the reading frame, the order in which the sequence of bases in the mRNA is sorted into codons (Fig. 15.3). The order of the codons in the mRNA determines the sequence in which amino acids are added to the growing polypeptide chain. Thus, the order of the codons in the mRNA determines the linear sequence of amino acids in the protein. 

Once associated with mRNA, the pre-initiation complex scans downstream to locate a start codon (AUG). This process is driven by ATP, and requires the helicase activity of an elF. The start codon is recognised by base paring between the anti-codon on tRNA and the AUG on the mRNA. Codon-anticodon interaction is facilitated by yet more elFs. Usually, the first AUG codon that is encountered is used, especially if it is surrounded by the so called Kozak consensus sequence (named after its discoverer Marilyn Kozak). Occasionally a later AUG is used if the first AUG not in the context of a consensus Kozak sequence, or is very close to the 5 cap. This is in contrast to prokaryotes, where the AUG that will act as a start codon is located at the future P site by the position of the ribosome after base-pairing between the 16S rRNA of the ribosome and the Shine Dalgamo sequence. 

The presence of stop signals in the code implies that start signals must also exist. There is only one initiation (start) codon, AUG, which also is the codon for the amino acid methionine. AUG functions as an initiation codon only when it occurs as the first codon of a sequence. When this happens, protein synthesis begins at that point (Section 11.8). 

During translation the protein is synthesized beginning at its N-terminus (Figure 26.12). The mRNA is read in its 5 3 direction beginning at the start codon AUG and ending... 

If the interest is in producing a 100 residue protein, then a 300 base DNA would be required, that is, n = 300 -t- 2 x 3, where one three is due to a triplet stop codon and the second three is due to the triplet start codon (AUG), which encodes for methionine but which is usually processed off. Therefore, 2n or 612 ATP equivalents are required to duplicate a 306 base strand of DNA to encode for a 100 residue protein. 

To begin protein synthesis on the ribosome, the mRNA start codon AUG associates with a tRNA-amino acid complex. Then, another tRNA delivers another amino acid, and a C—N bond forms between the two amino acids, making a small peptide. The peptide then grows further and becomes a protein. This process is called translation, because it translates a nucleotide sequence into an amino acid sequence. 

Initiation. Eukaryotic initiation factors (elFs) are proteins that cooperate with initiator methionyl tRNA ". This binds to the Psite of the 40S ribosomal subunit complex, which now recognises the 7-methylgnanosine cap (7-MG cap) at the 5 end of mRNA. The subunit scans along the mRNA until the START codon (AUG) is recognised. 

Formation of the SOS initiation complex. Recognition of the START codon (AUG) in mRNA is assisted by the preceding Kozak sequence (Table 71.3) before the pre-initiation complex is formed. Finally, the 60S ribosomal subunit combines with the 40S subunit to form the SOS initiation complex. 

Start codon (AUG) codes for Methionine N-formylmethionine (fMet)... 

During translation the protein is synthesized beginning at its N-terminus (Figure 26.12). The mRNA is read in its 5 3 direction beginning at the start codon AUG and ending at a stop codon (UAA, UAG, or UGA). Because the start codon is always AUG, the N-terminal amino acid is always methionine (as its A-formyl derivative). However, this... 

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