The primary structure of life is a linear string of letters
In short: Every living thing is governed by a succession of bases within a molecule. This nucleic acid sequence is more than just a code; it is the primary structure of the molecule itself. Since the covalent framework of each whole molecule is determined by its order, these polymers' linear nature makes sequence identification synonymous with structural definition.
A nucleic acid sequence is a succession of nucleotides forming alleles within DNA or RNA. In DNA, these are the bases adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, uracil (U) replaces thymine. These units are linked to a backbone of phosphate groups and sugars (deoxyribose for DNA, ribose for RNA).
While the primary structure is the linear order, nucleic acids also possess secondary and tertiary structures, such as the famous double helix. In the double helix, two strands run in opposite directions (5' to 3') and are held together by hydrogen bonds between complementary bases: A pairs with T via two hydrogen bonds, and G pairs with C via three. This uniform spacing allows the strands to coil into a right-handed B-helix, which makes one complete turn approximately every 10 base pairs. This geometry creates major and minor grooves, where proteins can interact with the sequence in a sequence-specific manner.
The sequence is the vehicle for genetic information, translated by cell machinery into amino acids to build proteins. Every three bases form a codon, which corresponds to a specific amino acid via the genetic code. Beyond the standard four bases, sequences can include ambiguity codes (like W for A or T) or modified bases like 5-methylcytidine (m5C) in DNA or pseudouridine (Ψ) in RNA. In bioinformatics, these sequences are aligned to identify mutations, determine ancestry, or calculate 'sequence entropy' to measure local complexity.
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Sources
- Summary by Intellect · source: Nucleic acid sequence
- Also: Nucleic acid | Definition, Function, Structure, & Types | Britannica
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