Protein Synthesis

Definition of Transcription: A segment of DNA acts as a template to direct the synthesis of a complementary sequence of RNA, with the enzyme RNA polymerase. It occurs in the nucleus.

Sequence of events for transcription:

• The enzyme DNA helicase breaks the hydrogen bonds between the bases in a region of the DNA molecule, causing the two strands to separate and unwind, exposing the nucleotide bases.
• The enzyme RNA polymerase binds to the template strand of DNA at the beginning of the sequence to be copied.
• Free RNA nucleotides align opposite the template strand, based on the complementary relationship between bases in DNA and free nucleotides.
• RNA polymerase moves along the DNA, forming bonds that add RNA nucleotides to the growing strand. This results in the synthesis of a molecule of mRNA alongside the unwound portion of DNA. Behind the RNA polymerase, the strands rewind, reforming the double helix.
• The RNA polymerase separates from the template strand when it reaches stop signal.
• The production of the transcript is complete, the newly formed RNA detaches from the DNA.

Definition of Translation: The sequence of codons on the mRNA is used to assemble a specific sequence of amino acids into a polypeptide chain, at the ribosomes in the cytoplasm.

Sequence of events for Translation:

• Initiation: a ribosome attaches to a start codon at one end of the mRNA molecule. The first tRNA, with an anticodon complementary to the first codon on mRNA, attaches to the ribosome. 3 bases on codon of mRNA bond to 3 bases on the anticodon on the tRNA, with hydrogen bonds. A second tRNA, with an anticodon complementary to the second codon on mRNA attaches to the other attachment site, codon and anticodon bond with hydrogen bonds.
• Elongation: the two amino acids are close enough for a ribosomal enzyme to catalyse the formation of a peptide bond between them. The first tRNA leaves the ribosome, leaving its attachment site vacant, returning to the cytoplasm to bind to another copy of its specific amino acid. Ribosome moves one codon along the mRNA strand, the next tRNA binds.
• Termination: the sequence repeats until a stop codon is reached. The ribosome - mRNA - polypeptide complex separates.