Deoxyribonucleic acid (DNA) is the hereditary material that carries the genetic blueprint for all living organisms. Its double‑helical strands are composed of nucleotides—adenine (A), thymine (T), cytosine (C), and guanine (G). Segments of specific nucleotide sequences form genes, the functional units that encode proteins and regulatory RNAs.
In multicellular eukaryotes, cells specialize, requiring only a subset of the genome to carry out their unique functions. Transcription is the first step in gene expression: a DNA segment is copied into a complementary RNA transcript. This RNA can then be translated into protein or serve regulatory roles.
RNA differs from DNA in that it uses ribose sugar, is typically single‑stranded, and substitutes uracil (U) for thymine (T). These minor differences enable RNA to function as a flexible messenger between the static genome and the dynamic cellular machinery.
Promoters are short DNA sequences located immediately upstream of a gene. Rather than coding for proteins, they act as regulatory “on‑switches.” When a cell needs to express a gene, RNA polymerase binds to the promoter, positioning itself to begin RNA synthesis. Additional transcription factors may also bind to the promoter, modulating the rate and timing of transcription.
