The Genetic Code and Enzyme Formation
Introduction
DNA is composed of two long strands of repeating nucleotide bases (A, T, C, G). You have already learned how
DNA is able to replicate itself, but how is this simple molecule responsible for all of the traits that exist in every
living organism?
The flow of information in organisms, from DNA to traits, is called the Central Dogma. Today, you will use
paper models to show this important concept.
Procedure
As you work through the procedure, there will be questions asked at certain points. Record the answers to
these questions on your Genetic Code and Enzyme Formation Answer Sheet before moving on to the next
step.
Part 1: Transcription (DNA  mRNA)
Lay out your photocopy of “The Cell”. Notice that is shows a DNA molecule within the nucleus of the cell. The
line dividing the page is the nuclear membrane. The cytoplasm is on the right side of the page; imagine that
the cytoplasm extends off the page and on to your table.
Place the 3 Transfer RNA molecules (tRNA) and the 4 amino acid molecules in random arrangements on your
table (the cytoplasm of the cell).
The remaining molecule, mRNA, is called a messenger RNA. Place the mRNA molecule in the nucleus of the
cell next to the DNA molecule. Compare the DNA and mRNA molecules.
1. How many strands of DNA are there compared to mRNA?
2. What nucleotide base is found on mRNA, but not DNA? What nucleotide base of DNA is this replacing?
3. Is this nucleotide base of mRNA also found on the tRNA?
In an RNA molecule, the thymine (T) nucleotide in DNA is replaced by the uracil (U) nucleotide base. This
means that the U nucleotide will bond with the A nucleotide in mRNA and tRNA (as opposed to the A-T bond
found in DNA).
The DNA molecule in the nucleus of your paper cell has a “code” written down one of its strands. The letters
used in this code are the sequences of A, T, C, and G nucleotides. In this paper model, this code is the gene for
brown eyes (in real DNA, the gene for brown eyes would be thousands of base pairs long).
When this code reaches the cytoplasm, it will give instructions on how to make enzyme B, the enzyme needed
to make brown eyes. This code needs to somehow get from the DNA to the cytoplasm outside of the nucleus.
A helper molecule, mRNA (messenger RNA), accomplishes this task by carrying the message of the code from
the DNA to the cytoplasm.
Slide your mRNA molecule next to your DNA molecule. Every nucleotide base should fit next to each other.
4. What is the role of mRNA in the cell?
5. Which side of the DNA does the mRNA fit to?
In this model, the mRNA was fully formed. In a real cell, the mRNA molecule would be formed nucleotide by
nucleotide alongside the DNA until a long strand of mRNA is created. Each nucleotide of mRNA is paired with
the complementary DNA nucleotide.
6. Write down the sequence for DNA and the sequence for mRNA below the DNA.
7. What are the complementary bases (the bases that pair up) between DNA and mRNA?
Your mRNA is now completely formed. It carries the code from the DNA because it contains all of the
complementary nucleotide bases of the DNA strand. The mRNA is ready to carry its message to the cytoplasm.
Move your mRNA molecule through the nuclear pore (hole) in the nuclear membrane out into the cytoplasm.
You now have your code for instructions on how the make enzyme B (the enzyme needed for brown eyes) out
in the cytoplasm.
8. This process you just did is called transcription. What are the steps of transcription?
Part 2: Translation (mRNA  Protein)
Now try to fit each of the transfer RNA (tRNA) molecules to each of the four amino acid molecules found in the
cytoplasm. This fit actually represents a chemical bond formed between the tRNA and each amino acid. You
should have one amino acid, #11, that fits none of the three tRNAs. Leave this one alone for now.
Next, each tRNA carries (or transfers) its amino acid over to the mRNA molecule, which has attached itself
onto a ribosome in the cytoplasm.
The ribosome will help match the tRNA to the mRNA and bond the amino acids from each tRNA together to
form a protein. You will play the role of the ribosome. Move the tRNAs, with their bonded amino acids, over to
the ribosome. Slide the tRNAs along the mRNA until they fit together.
9. What is the role of tRNA in the cell?
10. What is the sequence, from top to bottom, of these three amino acids?
11. What is the role of the ribosome in the cell?
Note that the amino acids touch one another. Where this happens, a bond results. Using a small piece of
scotch tape, tape the 3 amino acids together at the tabs that show the bond between the carbon (C) and
nitrogen (N) atoms.
Now, move the chain of amino acids away from the tRNAs, and move the tRNAs away from the mRNA. These
tRNA molecules are now free to bond with new amino acids and bring them back to the mRNA to produce a
new chain of amino acids.
This chain of three amino acids now represents enzyme B. All enzymes are proteins. You have now created a
protein. This is the way the cell builds all proteins needed for cell functions. It is important to note that the
amino acid sequence (#7-#3-#19) of this protein, enzyme B for brown eyes, could have had no other sequence.
12. Explain why this enzyme, which you just made, could have no other sequence of amino acids as long as
the DNA sequence inside the cell remains the same.
Now that enzyme B is formed, it can convert a colorless substance into brown pigment for the eyes.
13. The process of creating a protein from mRNA is called translation. What are the steps in translation?
Let’s review the procedure starting the with gene code for brown eyes on the DNA molecule in the nucleus of
the cell. The gene code for brown eyes is CCG-ATT-CAT. This sequence in DNA directs the formation of the
mRNA sequence: GGC-UAA-GUA. The process of DNA to mRNA is called transcription.
This message is taken out into the cytoplasm of the cell by the mRNA where a tRNA molecules will bond with
specific amino acids and bring them to the mRNA. A ribosome will match the tRNA to the mRNA. The
sequence of mRNA nucleotides will determine how the amino acid sequence will appear. The resulting amino
acid chain is a protein. This protein is enzyme B, which produces the brown pigment needed to make brown
eyes. This process of mRNA to protein is called translation.
14. Where does transcription occur in the cell?
15. Where does translation occur in the cell?
Part 3: Mutations
Our DNA does not stay the same. It will have slight changes in the sequence of nucleotide bases. This could be
due to a copying error when replicating our DNA, or environmental factors such as sunlight and certain
chemicals that can change our DNA. Every single change in our DNA sequence is called a mutation, and
mutations can lead to variations in organisms.
16. What are some things that can cause mutations?
Now, suppose that we could use an instrument to reach into the nucleus of a fertilized egg cell and change the
DNA sequence from CCG-ATT-CAT to CCG-ATT-CAC. This is a change in only one nucleotide base.
17. What would the new mRNA sequence be? Use your complementary base pairs to figure this out.
Remember to replace T with U.
18. Instead of having the amino acid sequence #7-#3-#19, what would the new amino acid sequence be?
This change produces a completely new sequence of amino acids. This means a new enzyme is created, an
enzyme that CANNOT produce brown eye pigment. Instead, this enzyme will have a different name and a
different function.
You should have ended up with a new amino acid sequence of #7-#3-#11. If you did not, you may want to
recheck your work. This enzyme sequence is a mutation of the eye color gene. It produces enzyme b, which
creates pigment for blue eyes. As you can see in this model, the only difference between two traits was a
change in a single nucleotide base of DNA. In fact, any variation in a human, animal, plant, or other living
organism, is due to a change in the DNA sequence.
Name: _______________________________
Date: ____________________
Period: _______
_______________________________
Genetic Code and Enzyme Formation Answer Sheet
Part 1: Transcription (DNA  mRNA)
1. How many strands of DNA are there compared to mRNA?
2. What nucleotide base is found on mRNA, but not DNA? What nucleotide base of DNA is this replacing?
3. Is this nucleotide base of mRNA also found on the tRNA?
4. What is the role of mRNA in the cell?
5. Which side of the DNA does the mRNA fit to?
6. Write down the sequence for DNA and the sequence for mRNA below the DNA. Match up the
nucleotides between the two strands.
DNA:
mRNA:
7. What are the complementary bases (the bases that pair up) between DNA and mRNA?
8. This process you just did is called transcription. What are the steps of transcription?
Part 2: Translation (mRNA  Protein)
9. What is the role of tRNA in the cell?
10. What is the sequence, from top to bottom, of these three amino acids?
11. What is the role of the ribosome in the cell?
12. Explain why this enzyme, which you just made, could have no other sequence of amino acids as long as
the DNA sequence inside the cell remains the same.
13. The process of creating a protein from mRNA is called translation. What are the steps in translation?
14. Where does transcription occur in the cell?
15. Where does translation occur in the cell?
Part 3: Mutations
16. What are some things that can cause mutations?
17. What would the new mRNA sequence be? Use your complementary base pairs to figure this out.
Remember to replace T with U.
18. Instead of having the amino acid sequence #7-#3-#19, what would the new amino acid sequence be?