Proteins are big molecules with various shapes. They conduct most of cell functions, including metabolism, digestion, delivery, signaling, structural support.
They are made of 20 types of amino acids as building blocks (comparing to 4 types of nucleotides for DNA and RNA), usually hundreds of amino acids long.
Protein structure hierarchy: primary structure (AA sequence) → secondary structure (alpha helix, beta sheet) → tertiary structure (3d structure) → quaternary structure (complex of protein molecules).
Codon: When a matured RNA is translated to proteins, it is read 3 letters (a codon) at a time. Each codon maps to an amino acid identity.
A codon is 3 nucleotides long, each nucleotide could any of the 4 choices (AGCU), resulting in 64 (4x4x4) combinations (codons). 61 codons correspond to amino acids, 3 are stop codons, indicating the end of a protein.