One link in the chain
If DNA is a chain, the single link is a nucleotide. Every nucleotide has three parts snapped together: a sugar called deoxyribose, a phosphate group, and a nitrogenous base. The sugar and phosphate are the same in every nucleotide — they form the structural part — while the base is the part that varies and therefore the part that carries information.
Meet the four bases
DNA uses exactly four bases, and their first letters are the genetic alphabet: adenine (A), thymine (T), guanine (G), and cytosine (C). Every instruction in every genome on Earth is written with just these four letters — the staggering variety of life comes from their order, not from a large alphabet.
- The four bases fall into two chemical shapes: A and G are larger, double-ring molecules (purines); C and T are smaller, single-ring molecules (pyrimidines).
- This size difference is not trivia — in the next guide it explains exactly why A pairs with T and G pairs with C.
- A handy memory aid: the round-bodied letters C and G look like they belong together, and indeed they pair; A and T are the other couple.
Reading a short sequence
Biologists write DNA as a simple string of letters. Each letter stands for one nucleotide carrying that base. The string below is read left to right, and its meaning lives entirely in the order — swap two letters and you may have changed the instruction.
Sequence: A T G C G T A C Nucleotide: 1 2 3 4 5 6 7 8 Each position = one nucleotide = one sugar + one phosphate + one base. Reading the bases gives: A-T-G-C-G-T-A-C Count by type: A (adenine) : positions 1, 7 -> 2 T (thymine) : positions 2, 6 -> 2 G (guanine) : positions 3, 5 -> 2 C (cytosine) : positions 4, 8 -> 2 Total = 8 nucleotides in this strand.