DNA and Protein Synthesis Worksheet

See Genetic Code Chart at the end of the handout.

Part I – Transcription/Translation

1. The following sequence of DNA is found on one strand of a chromosome (DNA). What would the sequence of the opposite strand of the double stranded chromosome?

A T G G A G A T G C C G C T G C C C C C C G A T
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

2. What would be the sequence of the mRNA that is made from this sequence?

T A C C T C T A C G G C G A C G G G G G G C T A
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

3. Using the Amino Acid Codon Table on the last page, translate the following piece of mRNA into a peptide sequence:

A U G . G A G . A U G . C C G . C U G . C C C . C C C . G A U
Methonine . _____ . ______ . ______ . ______ . ______ . ______ . ______ . (example)

Part II. Mutations

A mutation is a change in the sequence of nucleotides in the DNA. Mutations are not necesarrily harmful, in fact they can even be beneficial. Mutations increase the genetic variability in populations. They’re essential for the survival of species through evolution.

Mutations can be caused by carcinogenic chemicals such as those found in pesticides, or radiation, such as UV exposure from the sun. If the mutation occurs within at coding region of a gene, it may lead to a change in the amino acid sequence of the protein.

Different types of mutations in a gene can lead to different consequences to the protein that is encoded by the gene. A point mutation occurs when there is a substitution of one nucleotide for another, and it will only impact one codon in a sequence. On the other hand, a frameshift mutation involves the insertion or deletion of one or more nucleotides from the sequence, which results in a shift of the nucleotides. This creates new codons and new amino acids sequences, and the results are always detrimental.

Point mutations:

A point mutation occurs when there is a substitution of one nucleotide for another, and it will only impact one codon in a sequence.

When the protein is translated, the amino acid sequence may or may not be different from the original sequence, because of “redundancy” of the codons. There are 64 potential 3-letter codons, but only 20 amino acids and one STOP codon are needed. Thus, the same amino acid can be added by more than one codon.

For example, leucine can be inserted into the growing protein by 6 different tRNAs that recognize the 3-letter codon sequences: UAA, UUG, CUU, CUC, CUA and CUG (See Figure 21.6).

Therefore some of the point mutations may encode the same amino acid. These are called “Silent Mutations”.

Questions 4-12. Compare different types of point mutations that have been created in the original DNA sequence.

4. What would be the amino acid sequence of the following point mutation, in the mRNA sequence in Question 3, above, when the third G is changed to an A?
The new sequence would look like this:

Point mutant:
A U G . G A A . A U G . C C G . C U G . C C C . C C C . G A U
Methonine . ______ . ______ . ______ . ______ . ______ . ______ . ______

5. Would this mutation cause a change to the original protein (compare to the sequence in Question 3, above)?

6. What would be the amino acid sequence of the following point mutation, in which the second G is changed to a U? The new sequence would look like this:

Point mutant:
A U G . U A G . A U G . C C G . C U G . C C C . C C C . G A U
Methonine . _____ . ______ . ______ . ______ . ______ . ______ . ______ .

7. In what way would this mutation cause a change to the protein sequence (compare to the sequence in Question 3, above)?

8. Which of these mutations (#4 or #6) is a “Silent mutation”?

Frameshift mutants. On the other hand, a frameshift mutation involves the insertion or deletion of one or more nucleotides from the sequence, which results in a shift of the reading frame in which the nucleotides are read. This creates new codons and new amino acids sequences, and the results are always detrimental.

An Insertion mutation is when one or more extra nucleotides is added to the DNA sequence.

9. Consider the following mutation of the original sequence in which an A is inserted before the third G. The new mRNA sequence would look like the sequence below. What would the amino acid sequence of this protein? What would be the sequence of the protein that is made from this mRNA sequence?

Insertion mutant:
A U G . G A A . G A U . G C C . G C U . G C C . C C C . C G A .
Methonine . _____ . ______ . ______ . ______ . ______ . ______ . ______ .

10. How would this insertion affect the protein sequence (compare to the sequence in Question 3, above), and the function of the new protein?

A Deletion mutationoccurs when one or more nucleotides is deleted from the DNA sequence. For example, the sequence below has had 2 nucleotides deleted at the asterisk *.

11. What would be the sequence of the protein that is made from this mRNA sequence in which 2 nucleotides were deleted at the asterisk? The new mRNA sequence would look like the sequence below.

Deletion mutant:
A U G . G A G . A U G . C C G . G*C C . C C C . C G A .
Methonine . _____ . ______ . ______ . ______ . ______ . ______ . ______ .

12. How would this affect the protein sequence (compare to the sequence in Question 3, above), and the function of the new protein?

Amino Acid Codon Table