Figure 1. The expression of DNA and consequences of mutation: (1) The DNA molecule consisting of two antiparallel deoxyribonucleic acids can harbour heritable information on either strand. Heritable information to be converted into protein structure and function resides in open reading frames (ORFs). The first step of DNA expression, called transcription, proceeds independent of whether or not the transcribed segment of DNA contains ORFs. It is initiated at a DNA element called promoter (P) and usually ends at a DNA element called terminator (T). This segment is referred to as a transcriptional unit (TU). Injury to the TU in form of micro- or macrolesions could abolish, modify or generate new DNA elements and hence change the function and size of the TU. (2) The product, the transcript, is a single-stranded ribonucleic acid (RNA) that extends from the transcriptional startpoint (tsp) to the terminator and represents a copy of the DNA sequence on the DNA strand with the coding information (coding strand). The transcript is always shorter than the TU as it lacks the promoter element. If the transcript contains ORFs, they are separated by intergenic sequence and flanked by untranslated leader and trailer sequences. Many bacterial transcripts contain more than one ORF. Dependent upon the kind of injury, a mutant transcript can have a smaller (red, caused by creating premature termination at site X), equal (due to point mutation at site O) or larger size (due to major insertion or abolishing the terminator (not shown) than the wild-type transcript. Transcripts that do not contain ORFs will join the cellular complement of macromolecules as structural and functional RNAs. (3) Transcripts that contain ORFs will be translated into proteins by the ribosome. Mutated ORF sequences can give rise to smaller (red, truncation due to shorter transcript; blue, nonsense mutation by substitution at site O), equal (silent or missense mutation by substitution at any location within the ORF) or larger (blue, missense mutation by deletion or insertion of one or two nucleotides at site O causing a frame shift; change of STOP codon by substitution causing read-through to the next STOP or to the end of the template, not shown) proteins than wild-type proteins.