 Solution

Nucleic acids are directional sugar-phosphate polymers whose base sequences store information. Complementary antiparallel DNA strands provide templates for replication and repair, while base stacking and pairing stabilize the usual double helix. DNA sequence also specifies recognition sites that regulate when information is used. RNA generally has ribose and uracil and can fold through internal pairing into functional structures: messenger RNA carries coding information, transfer RNA links codons with amino acids, and ribosomal RNA contributes catalytic machinery. Not all RNA is a passive messenger.

Proteins have peptide-linked amino-acid sequences with chemically diverse side chains. Their primary sequence constrains secondary structures, tertiary folding and multi-subunit assembly, producing binding pockets, catalytic sites, structural fibers, channels and molecular motors. Function depends on conformational dynamics, chemical context and interactions; a linear sequence is not itself a complete account of an active structure. Folding errors and modifications illustrate the need to connect multiple organizational levels.

The central dogma concerns directional transfer of sequence information. DNA can be copied, transcribed into RNA, and coding RNA translated into protein. Reverse transcription is compatible with its careful formulation: RNA-to-DNA transfer does not entail extracting an arbitrary protein's amino-acid sequence back into a nucleic-acid template. RNA replication also occurs. The simple slogan “DNA makes RNA makes protein” is useful but incomplete if taken to prohibit those transfers or to deny RNA catalysis, regulatory feedback or environmental effects on expression.

Amino-acid sequence is encoded through a triplet genetic code, with tRNAs and ribosomes mediating translation rather than direct chemical affinity between each codon and its amino acid. Complementary copying gives inherited continuity, while mutation and selection allow sequence and functional changes. Thus molecular structure supports both stable heredity and adaptable biochemical machinery without making every cellular process a one-way causal chain.^[ref]