Beyond the central dogma

Picks up where Reading the code left off: a deeper look at RNA, especially the non-coding RNAs (miRNA, siRNA, lncRNA) that regulate genes, after a quick recap of mRNA, tRNA, and rRNA (Module 9).

You already met the RNAs of the central dogma in Module 2 (Reading the code): mRNA carries the message, tRNA delivers amino acids, and rRNA forms most of the ribosome and helps catalyse the bonds between amino acids. This module picks up where that left off. The cell makes dozens of RNA types, and most of them never become protein at all. Instead they regulate which genes are active, guide the processing of other RNAs, catalyse reactions, and shape how chromosomes are packaged. After a brief recap of the coding RNAs, this module focuses on that wider, non-coding world.

What is RNA?

RNA (ribonucleic acid) is synthesised from a DNA template during transcription. An enzyme called RNA polymerase reads one strand of a gene and builds a complementary RNA strand, following base-pairing rules: A pairs with U, G pairs with C.

RNA is chemically similar to DNA but differs in two key ways. First, it uses ribose as its sugar rather than deoxyribose. Second, it replaces the base thymine (T) with uracil (U), which still pairs with adenine. Unlike DNA, RNA is normally single-stranded, though it can fold back on itself to form complex 3D shapes that are essential to its function.

The many roles of RNA

RNA is not just a passive messenger. Different classes of RNA perform distinct, active roles in the cell. Ribosomal RNA (rRNA) forms most of the ribosome's structure and catalyses the bond-forming reaction that builds proteins. Transfer RNA (tRNA) physically delivers amino acids to the ribosome, matching each three-letter mRNA codon to the correct amino acid.

Beyond these, small non-coding RNAs regulate gene expression without being translated into protein. They silence target genes, guide RNA processing, and control developmental timing. This diversity shows that RNA is one of the most versatile molecules in biology, and that the genome's instructions are expressed in RNA long before a protein ever forms.