In molecular biology, two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds are called a base pair (often abbreviated bp). In the canonical Watson-Crick base pairing, adenine (A) forms a base pair with thymine (T), and guanine (G) with cytosine (C) in DNA. In RNA, thymine (T) is replaced by uracil (U). Alternate hydrogen bonding patterns, such as the wobble base pair (e.g., G-U) and Hoogsteen base pair, also occur—particularly in RNA—giving rise to complex and functional three-dimensional structures. Importantly, pairing is the mechanism by which codons, tri-nucleotide sequences that map to amino acids, on messenger RNA molecules are recognized by anticodons, tri-nucleotide sequences that pair with a specific codon, on transfer RNA during protein translation. Some DNA- or RNA-binding enzymes can recognize specific base pairing patterns that identify particular regulatory regions of genes.