STPM Biology - chapter 3: Revision essay question 4

Explain the process of protein synthesis and where does it occurred. 


Answer:

-Protein synthesis starts with the process of transcription. Transcription is the copying of genetic information of a gene by an mRNA molecule. The RNA polymerase will binds to the promoter region at the starting point of gene. Promoter is a short region of DNA in front of a gene for the binding of RNA polymerase(RP). RP then moves along the gene from the direction of 3’ to 5’ and thus breaking the hydrogen bonds between the two strands of DNA molecule. This cause double helix strand to uncoil and separate. Only one of the two separated synthesis of mRNA. This strand is called a ‘sense strand’. The other strand not involved in this process is called ‘nonsense strand’. As RNA polymerase moves along the template strand, complementary ribonucleotide will pair with the bases of template strand. The pairing is specific. Adenine pair with Uracil or Thymine while Guanine pairs off with Cytosine. Transcription stops when the RP reaches the termination codon. The completed mRNA and RP detaches from the DNA template. The two DNA strand zipped up again to form double helix form. Then. The newly synthesis mRNA will move into the cytoplasm. After that, translation will occurred. Translation is the process of where the codons of the mRNA is converted into a sequence of amino acids in a polypeptides by ribosome and transfer RNA(tRNA). In the cytoplasm, ribosome binds to mRNA at the 5’ end. An amino acid-tRNA complex will pair with the initiation codon of mRNA. the initiation codon is AUG. A ribosome has two sites, named as P site and A site for tRNA  to bind and pair with mRNA codons. After the first amino acid-tRNA has paired to the initiation codon on the P site, another amino acis-tRNA complex will then bind to A site and pair the mRNA codon adjacent to the initiation codon. Both amino acid then linked by peptide bond. The first tRNA detaches from its amino acid and leaves the ribosome. This tRNA is then recycled into amino acid-tRNA complex. The ribosome then moves along one bu one codon along the mRNA towards the 3’ end and the correct anti-codon will pair with the mRNA codon. Polypeptide synthesis ends when the ribosome arrives at the termination codon of the mRNA. The termination codon of the mRNA is UAA, UAG or UGA. The completed polypeptide detaches from mRNA and ribosome. The first amino acid codede by the initiaton codon will be cut off. The large and small units of ribosomes detach from the mRNA and can return to 5’ of mRNA to be reused again.