Unit V: Activity 4

MUTATION WORKSHEET (teacher)

A mutation is a change in the nucleotide sequence of DNA. Mutations can involve large regions of the DNA or just a single nucleotide. How can small changes, such as altering a single nucleotide in the DNA sequence, cause such big changes in the phenotype, as the disease of Sickle Cell Anemia? In this activity we will be exploring several kinds of mutations.

A. Substitutions

1. Open and run the model: Mutations: Substitutions and Deletions. <http://xeon.concord.org/webstart/jnlpFiles/modelerPages/mutations.jnlp>

2. Note the model starts with a "flying bird" pattern (Arg-Ser-Gly-GLy-Gly-Ala-Gly-Gly-Gly-Arg-Gly-Gly-Gly-Ser-gly-Ala-Gly-Gly-Ala-Glu).

3. Change the C of the third codon (CGA) in DNA to A, making the codon AGA. Click "Run". Describe what happens, if anything, to the shape of the bird.

* Remember to make the mutation by changing the DNA, not the amino acids.

Fig 1. The first nucleotide "G" in the third codon GGA was substituted with an A (fig 1). The resulting codon is AGA (fig. 2). There is only a slight change in the shape of the protein. Most substitutions have little effect. Some substitutions have great effect, when they alter amino acids in critical places in the structure.

4. Now make three changes in the sequence of DNA by randomly substituting (replacing) any of the nucleotides with another. Record what nucleotide you changed (for example, "In the fifth codon, I changed the first nucleotide from A to T") and describe below how the replacement affected, if at all, the shape of the protein.

a. Answers will vary.

 

b. Answers will vary.

 

c. Answers will vary.

 

5. Explain why some substitutions of nucleotides in the DNA appear to have no effect on the protein.

Most substitutions have little effect, either because they don't change the properties of the amino acids for which they are substituting, or they are not in a critical location in the sequence.

 

6. Explain how one critical substitution of the nucleotides in the DNA can have a large effect on protein structure.

Some substitutions have a great effect when they alter the amino acids in critical places in the structure.

 

 

B. Deletions.

1. Scroll to the bottom of the page.

2. Click the link to the Deletion Challenge.

3. Change the same sequence of DNA by deleting the third nucleotide. Click "Run."

4. Describe what happens to the shape of the protein.

They should observe a radical change in shape. See below.

 

5. The following is a list of the original protein sequence.

Arg-Ser-Gly-Gly-Gly-Ala-Gly-Gly-Gly-Arg-Gly-Gly-Gly-ser-Gly-Arg-Gly-Gly-Arg-Glu

Record the new protein sequence below.

Arg-Ala-Glu-Glu-Gly-Gln-Glu-Gly-Gly-Glu-Glu-Gly-Gly-Ala-Glu-Gln-Glu-Gly-Gln-Asp

6. What happens to the DNA code after you deleted the third nucleotide? (You may want to reset the model by clicking the back button.)

All of the codons after the deleted nucleotide are changed.

7. Then how does this deletion in the genetic code affect the shape of the proteins produced? Briefly describe what happens, including the steps of making proteins.

As the codons are transcribed one after another by the ribosome, the wrong amino acids will be chained together in the protein. The protein will then fold in another way. This new structure may be lethal to the cell.

 

C. Comparison of Mutations.

Which type of mutation has the greater effect, substituting a nucleotide or deleting one? What might explain this difference?

Deletion has the greater effect. Once a nucleotide is deleted, the codons no longer read in the correct manner. Many codons "downstream" can become wrong. Some deletions can be lethal if they involve proteins essential for the survival.