How to Read This Information Chapter

DNA stores information in the linear order of its bases, but a base sequence becomes biologically meaningful only through molecular interactions. Complementary pairing lets each DNA strand guide the construction of another. Promoters and regulatory sequences determine where transcription begins. Triplets in messenger RNA are interpreted as codons, and folded proteins act as enzymes, receptors, channels, pigments or structural components. The chapter therefore follows a causal chain from sequence to molecular product to phenotype.

Three distinctions prevent most errors in this topic. First, a template is read so that a complementary product can be built; the template and product do not normally have the same sequence. Second, synthesis has direction: polymerases add nucleotides to a 3′ end, so a new nucleic-acid strand grows 5′→3′. Third, expression is selective. A cell contains an entire genome but uses only part of it at a particular time, and the resulting proteome changes with cell type and conditions.

The labels 5′ and 3′ refer to carbon positions in the pentose sugar. They give a nucleic-acid strand polarity, rather like an arrow built into its chemical backbone. In double-stranded DNA, the two backbones run antiparallel. During replication, that geometry forces one daughter strand to be synthesized continuously and the other discontinuously. During transcription and translation, directionality determines which strand is read and which codon reaches the ribosome next.

When you meet a DNA sequence, identify whether it is the template strand or the coding strand before transcribing it. The mRNA is complementary and antiparallel to the template. Apart from uracil replacing thymine, it has the same base order as the coding strand when both are written 5′→3′. Genetic-code tables normally show mRNA codons, so translating a DNA triplet without deciding which strand is given can reverse the intended answer.

Replication copies almost all of a DNA molecule before cell division. Transcription copies a selected DNA region into RNA. Translation reads an mRNA to assemble a polypeptide. RNA processing and post-translational modification can create several final products from one initial transcript or polypeptide. Regulation can act at every stage: chromatin access, transcription-factor binding, mRNA lifetime, translation, protein modification and protein degradation all affect how much active product exists.

Mutation changes the nucleotide sequence; epigenetic regulation changes how a sequence is used without changing that sequence. Both can alter phenotype, but they have different mechanisms and patterns of persistence. Use the laboratory throughout the chapter as a single information system: change mode, predict which molecular relationship should change, and distinguish what the model preserves from what it simplifies.