Describe how a gene is used to synthesise a polypeptide

A gene is a section of DNA used to produce a single polypeptide, or protein molecule, which is made of amino acids. The process of protein synthesis can be divided into two parts.

The first part is transcription, which produces a molecule of messenger RNA (mRNA) from the gene. Transcription takes place in the nucleus of the cell. First, the gene to be transcribed dips into the nucleolus and the DNA is unwound, so the bases of the transcribing DNA strand are exposed. Complementary nucleotide base pairs which are in the nucleus then line up next to the exposed bases. C pairs with G, and A pairs with U, because U replaces T in RNA molecules. Phosphodiester bonds form between the RNA bases to form the mRNA molecule, catalysed by the enzyme RNA polymerase. This molecule of mRNA can then leave the nucleus through a nuclear pore, and the DNA rewinds.

The second part of protein synthesis is translation. It takes place outside the nucleus of the cell, on a ribosome. The mRNA produced in transcription attaches to the groove in the ribosome. Three bases on the mRNA make up a codon, and each group of three codes for a specific amino acid found in the cell. A tRNA molecule-another type of RNA-has the anticodon which is specific to each codon on the mRNA, and a specific amino acid is bound to the opposite end of the tRNA. tRNAs line up alongside the complementary codons on the mRNA, and then peptide bonds form between the adjacent amino acids. The mRNA moves through the ribosome, adding amino acids until the entire length of the mRNA has been translated. The completed polypeptide then leaves the ribosome and may go to another organelle for completion before it can perform its function.