Amino Acid Sequence in a Protein
Objective
To determine the amino acid sequence of a small protein.
Background Information
Enzymes are proteins. In order to carry on their very specific functions, the sequence of the amino
acids in their structure must be precise. The DNA in the chromosomes of cells, through its own
order of bases, is the determining factor in the amino acid sequence. Ribosomes, messenger RNA,
and transfer RNA are involved in the formation of the final protein molecule. In DNA, a specific
grouping of three bases (triplet code) represents a specific amino acid. Through mRNA (codons) and
tRNA (anticodons), we can determine the structure of a protein.
Materials
paper
pencil
table of codons (Nelson Biology 12, page 240)
Procedure
1. Examine the following segment from one side of a DNA molecule. Read the bases from left to
right.
T-A-C-A-C-A-C-G-G-A-A-T-G-G-G-T-A-A-A-A-A-A-C-T
2. Determine the sequence for the messenger RNA that corresponds to the given DNA structure.
(a) Draw the sequence. Are there any differences between the DNA and mRNA segments in
terms of chemical organization? Explain.
3. From left to right divide up the mRNA into consecutive groups of three bases. These are called
codons.
(b) List the codons in sequence in a column in your lab notebook.
4. Refer to the table of messenger RNA codons. Beside each of your listed codons, write the
appropriate codon information.
(c) Record the precise amino acid sequence of the protein molecule.
Application Questions
1. Do all of the codons on the messenger RNA represent specific amino acids? Explain.
2. If the original DNA strand in this investigation had been complementary to the one given, would
the same protein molecule have been produced? Explain.
3. Explain what would have occurred to your protein molecule if:
(a) the fourth base (adenine) in the original DNA strand had been thymine?
(b) the seventh base (cytosine) in the original DNA strand was deleted due to mutation?
4. Indicate the tRNA anticodon for the second DNA triplet code in the original DNA molecule.
5. Describe the role of the first and last triplet codes in the original DNA
molecule.
LSM 5.4-4
Student Worksheet Solutions
Additional Lab Exercise: Amino Acid Sequence in a Protein, Solution
•
Approximately 30 min to 40 min are required.
•
This exercise could be a class or homework assignment.
•
To accomplish everything in this investigation the students must exhibit full knowledge of the
DNA protein synthesis mechanism. This is a good activity for evaluation and/or reteaching.
•
Procedure 4 (c) is suggested in order to have the students physically link each amino acid to the
next. If the students have the background you could relate the bonding to dehydration
synthesis.
Answers to Observation Questions
(a) The triplet pairs in the mRNA codons are the reverse of the DNA triplets.
The base thymine (T) in DNA is replaced by uracil (U) in the mRNA.
A-U-G/U-G-U/G-C-C/U-U-A/C-C-C/A-U-U/U-U-U/U-G-A
The slash separates the bases into codons (procedure 3).
(b) AUG — start; UGU — cysteine; GCC — alanine; UUA — leucine;
CCC — proline; AUU — isoleucine; UUU — phenylalanine;
UGA — stop.
Answers to Application Questions
1. Some of the codons do not represent specific amino acids. They represent the start (start
codon) and end (stop codon) of the protein molecule.
2. The same protein molecule would not be produced. This is a different set of triplet codes that
result in a corresponding different set of codons and, therefore, different amino acids. Also,
no start or stop codons are present.
3. (a) When translated to an mRNA codon, it would change from UGU (cysteine) to AGU (serine).
(b) The next DNA triplet would now be GGA. Its codon would be CCU
(proline). The remaining triplets would also be altered by one base, producing a different
amino acid sequence. The terminator would be reduced to two bases, not enough for it to
function properly.
4. The DNA triplet is ACA; the codon is UGU; the anticodon is ACA.
5. These codes are responsible for determining the length of the protein by starting and stopping
synthesis of the molecule, preserving its precise number and sequence of amino acids.