5. Secret Codes - Background

Teacher Background
The genetic code is a set of rules that guide how the sequence of DNA nucleotides is read by a cells machinery and turned into a sequence of amino acids that make up a protein. Incredibly, nearly all living things use the same genetic code!

To understand how DNA provides the instructions for making proteins, one first needs to understand a little about what a protein is. A protein, like DNA, is a polymer (a long molecule that is a chain of smaller repeating subunits). The subunits in proteins are called amino acids. There are 20 different amino acids that can be linked together in an infinite array of sequences of different lengths. These chains of amino acids form the proteins that do all the work in our bodies – building cells, generating energy, transporting materials, and more. For this activity, it is important to note that each amino acids has been assigned a single letter abbreviation which allows students to create protein “words” and “messages” with different sequences of amino acids.

The DNA in a cell can be divided into functional units called genes. Each gene provides the instructions for making one protein. Thus, one can think of a gene as a long paragraph describing how to make a protein. The “letters” in the paragraph are the nucleotides (the A’s, T’s, C’s and G’s). The “words” within the gene are made of a sequence of 3 nucleotides, each of which specifies one amino acid in the protein. For instance, the DNA sequence, TAC, specifies the amino acid, methionine. Each nucleotide triplet that codes for an amino acid is called a “codon”.

The process of protein synthesis (reading the DNA codons and translating it into a sequence of amino acids) is a gorgeous, choreographed process involving many steps. For more detail on protein synthesis and the molecules involved, see the background section in the Protein Factory activity.

For the purposes of this activity, one needs to know that the DNA is trapped in the nucleus (at least in plant and animal cells) while the protein making apparatus, the ribosome, is located outside the nucleus in the cytosol. Therefore, a messenger molecule, messenger RNA, is used to copy the DNA message and bring it to the ribosome. RNA is closely related to DNA. They both are polymers of nucleotides with a sugar-phosphate backbone. The differences are that RNA is single stranded, while DNA is double stranded. Furthermore, RNA uses the nucleic acid, uracil, instead of thymine. Finally, instead of the sugar deoxyribose in the backbone, RNA uses the sugar ribose.

Protein synthesis can be summarized in 2 steps:

  1. One of the 2 strands of DNA is transcribed into a single stranded messenger RNA, which carries the DNA message to the ribosome.
  2. The ribosome reads the messenger RNA and assembles the appropriate sequence of amino acids.

In this activity, students become familiar with the idea of a coded message by practicing with Morse code. Morse code uses a sequence of dashes and dots to represent the letters of the alphabet, numbers and punctuation. It was developed in the 1830’s for early telegraph and radio communications. Once they get the idea, they have an opportunity to make up their own secret codes, with a different symbol (letter, number, picture) to represent each letter of the alphabet. They write a message to a classmate in their secret code. The keys and coded messages are traded and students can decode each others’ messages.

Then, they are told about how a strand of DNA can be turned into RNA and then into a string of amino acids. Using the one letter abbreviations of the amino acids, secret messages may be written and decoded using the genetic code. The idea of a genetic code can be compared to Morse code and their self-created codes. I have found that this activity makes the big picture of protein synthesis much easier for students to understand than jumping straight into the details of the molecules involved and how they all interact.

Student Prerequisites
A solid understanding of DNA structure is essential. A basic understanding of what a protein is and its structure is helpful but not required.