Name: ______Period: _____

Point Mutations

Background Information: Mutations are changes in a DNA sequence. A point mutation is a change in a single base pair of a gene. Point mutations, or single nucleotide polymorphisms (SNPs), involve only one nitrogen base change of the three nitrogen bases in a codon. Perform this activity and witness the change a single point mutation in the DNA can have on a resulting protein. Dice will be used to determine the random change that will occur in the specified mutation location.

Substitution
A substitution is a mutation that exchanges one base for another (i.e., a change in a single “chemical letter” such as switching an A to a G). Such a substitution could:

  1. Change a codon to one that encodes a different amino acid and cause a small change in the protein produced. For example, sickle cell anemia is caused by a substitution in the beta-hemoglobin gene, which alters a single amino acid in the protein produced.
  2. Change a codon to one that encodes the same amino acid and causes no change in the protein produced. These are called silent mutations.
  3. Change an amino-acid-coding codon to a single “stop” codon and cause an incomplete protein. This can have serious effects since the incomplete protein probably will not function.

Insertion
Insertions are mutations in which extra base pairs are inserted into a new place in the DNA.

Deletion
Deletions are mutations in which a section of DNA is lost, or deleted.

Frameshift
Since protein-coding DNA is divided into codons three bases long, insertions and deletions can alter a gene so that its message is no longer correctly parsed. These changes are called frameshifts.
For example, consider the sentence, “The fat cat sat.” Each word represents a codon. If we delete the first letter and parse the sentence in the same way, it doesn’t make sense.
In frameshifts, a similar error occurs at the DNA level, causing the codons to be parsed incorrectly. This usually generates proteins that are as useless as “hef atc ats at” is uninformative.

Procedure:

1. Transcribe the DNA on the DNA Mutation Consequences Worksheet into mRNA.

2. Using the Universal Codon Chart translate the mRNA into its corresponding amino acid sequence.

3. Obtain a six-sided die. The first nucleotide that will be mutated is number 4. Roll the die and follow the instructions below.

4. Write the complete DNA sequence with one mutation in nucleotide 4.

5. Complete the mRNA sequence from the mutated DNA.

6. Translate the amino acid sequence from the mRNA.

7. Circle any differences from the original protein produced.

8. Perform this exercise again (Steps #1-7). Make sure that you roll a new number 

DNA Mutation Consequences Worksheet

Table A (1st Roll)

Analysis of Data: Answer the question below using complete sentences.

1. Did this mutation cause a change in the Amino Acid sequence produced? Explain why or why not.

2. Is it possible to have a mutation in nucleotide 4 that would produce the same amino acid?

3. What type of mutation occurred? ______

Table B (2nd Roll)

Analysis of Data: Answer the question below using complete sentences.

1. Did this mutation cause a change in the Amino Acid sequence produced? Explain why or why not.

2. Is it possible to have a mutation in nucleotide 4 that would produce the same amino acid?

3. What type of mutation occurred?

Mutations Analysis

Directions: Answer the questions below using the complete sentences.

1) Explain why deletion of a single nucleotide in a gene generally results in more severe defects in the polypeptide than a point mutation that changes a single nucleotide.

2) Muscular dystrophy includes several genetic diseases which cause increasing weakness of a person's muscles. Two different types of muscular dystrophy are caused by different types of mutations in the gene for a protein that helps to prevent muscle cells from dying prematurely.

Duchenne muscular dystrophy is more severe. A child with Duchenne muscular dystrophy begins to show symptoms of loss of muscle function by about age 3 and needs to use a wheelchair by about age 10. A person with Duchenne muscular dystrophy typically dies as a young adult, due to failure of the muscles in the internal organs.

Becker muscular dystrophy is milder. Symptoms do not begin until age 12 or later, and the person can live into their 40s or 50s.

The table shows the main kinds of mutations that are responsible for these two types of muscular dystrophy. Complete the second column to indicate which type of muscular dystrophy you think would be caused by each type of mutation.

Type of Mutation / Type of Muscular Dystrophy
Deletion Mutation 
# nucleotides deleted from mRNA is a multiple of 3
Deletion Mutation 
# nucleotides deleted from mRNA is not a multiple of 3
Point Mutation stop codon

Explain your reasoning.

3) What is the difference between a chromosomal mutation and a point mutation?

4) Most mutations are thought to be deleterious. Why, then, is it reasonable to state that mutations are essential to the evolutionary process?