Insulin

Some cells, such as β-cells of the pancreas synthesise and secrete protein hormones such as insulin. In order for the genetic code on DNA to be expressed and the insulin formed, the DNA must be transcribed as a pre-mRNA molecule, spliced to form mRNA and transcribed as a protein. Enzymes are involved in each step. Helicase binds to the gene locus and cause the gene to unwind exposing the template strand. RNA polymerase joins adjacent nucleotides in a condensation reaction to form the pre-mRNA strand. Enzymes in the nucleus remove non-coding introns, and splice together the coding exons leading to the formation of an active mRNA which binds to a ribosome on the rough endoplasmic reticulum. Transfer RNA complexes line up with their anticodons on the codons on mRNA and bring two amino acids in contact with an enzyme in the ribosome that condenses them together by forming a peptide bond. The process is repeated to build up the primary structure of the insulin molecule. The action of the hormone insulin also involves phosphorylase enzymes which cause the condensation of glucose molecules into the storage polysaccharide glycogen in the liver by glycogenesis.