5. Secret Codes

Example secret DNA codeExample secret DNA code

Summary
Kids love secret codes and secret messages. In this activity, kids first discover how codes work by reading and writing secret messages written in Morse code. Next, they make up their own secret codes and trade messages written in their self-created code. Finally, students learn how DNA codes for a “secret” protein message in a two step coding system – the genetic code. Since each of the 20 amino acids has a one letter abbreviation, student can discover the secret protein “messages” encoded in a DNA strand. Several secret DNA messages are provided for students to decode under the assessments section. For homework, students can be challenged to write a secret message to a friend using the genetic code.

Objectives
Can explain how DNA codes for a sequence of amino acids.
Can begin to explain some of the differences between DNA and RNA.
Can begin to describe the process of transcription and translation.

Vocabulary
DNA
Messenger RNA
Ribosome
Amino acids
Protein
Codon
Morse code
Genetic code
Transcription
Translation

AttachmentSize
5secret_codes.doc48.5 KB
codes_handout.doc43 KB

5. Secret Codes - Logistics

Time
45-55 minutes

Grouping
Individual

Materials

  • Copies of Secret Codes handout
  • Optional: internet access and a computer projection system to show students 2 web-based videos of transcription and translation from DNA Interactive.

Setting
classroom

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.

5. Secret Codes - Getting Ready

Getting Ready

  • Make copies of the Genetic Code handout.

5. Secret Codes - Lesson Plan

Lesson Plan

  1. Ask the students, “If you were a spy, how would you write a message to headquarters in a way that if the enemy intercepted it, they would not know what the message said?” Students will instantly respond with using a secret code. Ask for clarification of what a secret code is and how it is used.
  2. Give students the handout. Briefly discuss what Morse code is and how it is used.
  3. Have students try to decode the message at the bottom of the page. The secret message says, “I love learning about genetics!” Make up your own messages for students to decode as additional practice.
  4. Next, have students make up their own secret code by writing a different symbol (letter, number, picture) next to each letter of the alphabet on the right hand side of the page. Students can use their personal code to write a secret message to a friend on the bottom of the page.
  5. Collect the handouts when students are done, shuffle them, and redistribute them so students can practice decoding someone else’s message.
  6. Now that students are fluent with coding and decoding messages, introduce how DNA is a code for making proteins. Describe protein structure. Outline the 2 basic steps of the protein synthesis process: 1) DNA is transcribed into messenger RNA and 2) ribosome reads the RNA and assembles a chain of amino acids using the RNA codons.
  7. Have students flip their handouts over and look at the genetic code. Go over the examples, showing how the DNA codes for a protein message with a two step decoding process. First translate the top line of the DNA into RNA. Then use the table to identify the sequence of amino acids that matches each 3 nucleotide codon. The secret message at the bottom of the page says, “I like math”.
  8. Give students additional DNA codes to solve. See the Assessment section for ideas or create your own. Since there are only 20 amino acids, some letters cannot be used (B, J, O, U, X, Z).
  9. Optional: Show students the video of transcription and translation from the DNA Interactive website (click on the “Code” tab, then click the “Reading the Code” tab and finally click the “Putting it Together” tab.)

5. Secret Codes - Assessment

Assessment

  1. Have students decode one or more of the following DNA messages:
    • “Genetics”
      C C G C T T T T A C T C T G A T A G A C G A G T
      G G C G A A A A T G A G A C T A T C T G C T C A
    • “Science”
      T C G A C A T A T C T T T T A A C G C T T
      A G C T G T A T A G A A A A T T G C G A A
    • “DNA is neat”
      C T A T T G C G C T A G A G T T T G C T T C G T T G G
      G A T A A C G C G A T C T C A A A C G A A G C A A C C
    • “Why me?”
      A C C G T A A T A T A C C T C ?
      T G G C A T T A T A T G G A G
  2. Have students write their own secret word or short message using the genetic code. Warn them that some letters (B, J, O, U, X, Z) cannot be used. Trade these messages the following day.

Going Further

  1. Go into the details of the transcription and translation process. See the Protein Factory lesson and Comic Strip Assessment activities

5. Secret Codes - Sources and Standards

Standards
Grade 8
Chemistry of Living Systems (Life Sciences)
6. Principles of chemistry underlie the functioning of biological systems. As a basis for understanding this concept:
c. Students know that living organisms have many different kinds of molecules, including small ones, such as water and salt, and very large ones, such as carbohydrates, fats, proteins, and DNA.

Grade 9-12
Genetics
4. Genes are a set of instructions encoded in the DNA sequence of each organism that specify the sequence of amino acids in proteins characteristic of that organism. As a basis for understanding this concept:
a. Students know the general pathway by which ribosomes synthesize proteins, using tRNAs to translate genetic information in mRNA.
b. Students know how to apply the genetic coding rules to predict the sequence of amino acids from a sequence of codons in RNA.
e. Students know proteins can differ from one another in the number and sequence of amino acids.