(i) Termination codons (or stop codons) are ribonucleotide triplets with sequences UAA, UAG, and UGA that instruct the ribosome to cease polypeptide synthesis and release the resulting transcript. Thus, termination codons signal the termination of transcription process.
(ii) Lac operon is a cluster of three structural genes encoding proteins involved in lactose metabolism and the sites on the DNA involved in regulation of the operon. It's expressed only when lactose is present and glucose is absent.
(iii) Chemical carcinogens are factors including a great variety of man-made and naturally occurring substances, as well as radiation, both electromagnetic and particulate, with sufficient energy to break chemical bonds. These agents may damage DNA or interfere with its replication or repair that promotes carcinogenesis, the formation of cancer.
(iv) Immobilized enzyme is an enzyme attached by means of covalent or non-covalent bonds to an inert, insoluble material (silica, carbon-based, hydroxyapatite, synthetic and biopolymers etc.).