DNA Structure

DNA is a polymer of nucleotides. Each nucleotide consists of three components:

  1. A phosphate group
  2. A deoxyribose sugar (a five-carbon sugar)
  3. A nitrogenous base: one of four types: Adenine (A), Thymine (T), Cytosine (C) or Guanine (G)

Two nucleotide strands wind around each other to form a double helix: like a twisted ladder. The sugar-phosphate groups form the "sides" of the ladder and the bases form the "rungs".

The bases pair by complementary base pairing:

  • A pairs with T (2 hydrogen bonds)
  • C pairs with G (3 hydrogen bonds)

This means if one strand reads A-T-C-G-A, the other strand reads T-A-G-C-T.

Genes, Chromosomes and DNA

  • A gene is a section of DNA that codes for a specific protein.
  • A chromosome is a long, coiled molecule of DNA. Humans have 46 chromosomes (23 pairs) in most body cells.
  • The sequence of bases in a gene determines the sequence of amino acids in the protein it codes for.
  • Every three bases (a triplet / codon) code for one amino acid.

Gene → mRNA → Protein: During protein synthesis, the DNA sequence is copied into messenger RNA (mRNA), which travels to ribosomes where amino acids are assembled in the correct order.

Why DNA Structure Matters

Complementary base pairing is essential for:

  • DNA replication: the two strands unzip and each strand acts as a template for a new complementary strand. This produces two identical DNA molecules before cell division.
  • Accuracy of copying: because A always pairs with T and C with G, the sequence is copied precisely. Errors (mutations) are rare.
  • Genetic inheritance: DNA is copied and passed to offspring, carrying instructions for their development.

The double helix structure is also stable: the hydrogen bonds between bases and the coiling protect the genetic information from damage.