Protein gel-running extension (optional activity) Proteins are essential tools for life. But how does a simple string of amino acids fold into an intricate three-dimensional structure capable of performing a specific task, such as emitting fluorescence? Using fluorescent proteins as a model, students will explore a protein's primary, secondary and tertiary structure and relate those structures to protein function in an integrative, hands-on lab. First, students will compare different fluorescent protein sequences at the primary, secondary, and tertiary levels and use that analysis to predict protein functions. Students will then express these fluorescent proteins using a simple cell-free protocol to test their functional predictions. This activity serves as an excellent interactive introduction to the basics of protein structure and function, as well as a springboard for discussing more advanced topics such as protein engineering.