Question

Be sure to include processes such as formation of mRNA (use correct codons, etc.), attachment of mRNA to a ribosome, attachment of tRNA to mRNA-ribosome complex, and so on.

Answer 1

The first step is transcription in coding and synthesis of tetra peptide. RNA polymerase catalyzes the synthesis of mRNA. The DNA double helix unwinds at a point near the gene for Gly-Leu-Pro-Cys. Ribonucleotides line up along the unwound DNA strand in a sequence determined by complementary base pairing of the DNA strand bases and ribonucleotide bases. The enzyme catalyzes the synthesis of mRNA in the 5’ to 3’ direction, moving along the DNA template in the 3’ to 5’ direction.

Since we need a mRNA strand that codes for Gly-Leu-Pro-Cys, we need to use the genetic code to pick mRNA codons that represent those amino acids, as follows:

(Note: glycine has 4 codons GGA, GGC, GGG, and GGU. Leucine has 6 codons CUA, CUC, CUG, CUU, UUA, and UUG. Proline has 4 codons CCA, CCC, CCG, and CCU. Cysteine has 2 codons UGC and UGU.)

To get initiation codons, for making mRNA and protein synthesis, insert the initiation codons (AUG) before the oxytocin sequence. Then it appears as AUG-GGA-CUA-CCA-UGC. This sequence is corresponding to the DNA template 3’ TAC-CCT-GAT-GGT-ACG 5’ for the section of the DNA strand used to make the 5’-AUG-GGA-CUA-CCA-UGC-UAG 3’ mRNA strand.

The mRNA moves into the cytoplasm, where the ribosome organelles are found. The initiation process begins when the mRNA strand aligns itself on the surface of a small ribosomal subunit. This is the way that the initiating codon (AUG) occupies the peptidyl site (P site) on the ribosome.

Next, a tRNA molecule carrying a N-formylmethionine (fMet) residue binds to the codon through hydrogen bonds. The resulting complex binds to the large ribosomal subunit. This forms a unit called an initiation complex. There is another site called the aminoacyl site (A site) located on the mRNA-ribosome complex next to the P site. The A site is where the next incoming tRNA carrying the next amino acid will bond. Each of the tRNA molecules representing the 20 amino acids can try to fit the A site, but only the one with the correct anticodon (complementary to the next codon on the mRNA) will fit properly.

Once at the A site, this second amino acid (in this case, glycine) is linked to N-formylmethionine by a peptide bond using the enzyme peptidyl transferase. After the peptide bond forms, the tRNA bound to the P site is “empty”. The beginnings of the polypeptide chain is now attached to the tRNA that is bound to the A site.

In the next phase (elongation), the whole ribosome moves one codon along the mRNA strand, towards the 3’ end. As the assembly moves, the empty tRNA is released from the P site, and tRNA attached to the peptide chain moves from the A site to the P site. This movement of the ribosome as it shuffles along the mRNA is a process called translocation. It makes the A site available to receive the next tRNA (that possesses the proper anticodon). This tRNA is carrying the next amino acid (leucine) and it bonds to the peptide chain, continuing the elongation process. This occurs repeatedly until the entire oligopeptide has been synthesized.

Once the ribosome meets the UAG or UAA or UGA stop codon, a protein called a termination factor binds to the stop codon and catalyzes the hydrolysis of the completed polypeptide chain from the final tRNA. The tetrapeptide has been synthesized and the mRNA strand can bind to fresh ribosomes to begin the process all over again.