Organisms within the same species and between different species often share common sequences of nucleotides within homologous genes. Homologous genes are genes shared by various species originating from a common ancestor. Common sequences of nucleotides from different organisms can be compared by physically aligning the sequences. These sequences are compared to an outgroup, a species with the same homologous gene but outside the group being studied. Evolutionary relationships may be indicated by the number of differences seen in the nucleotide alignments of the organisms being studied.
Objectives:
The student will be able to
1. Demonstrate the relationship between the nucleotide sequences within a gene common to several organisms and the evolutionary relationships between those organisms;
2. Align nucleotide sequences from several organisms to show the highest number of common bases at any location;
3. Determine amino acid sequences from nucleotide sequences using a Genetic Code Chart.
For diagrams and keys illustrating these activities, see Teacher Notes below.
Materials
Sequence for Outgroup (Serves as model for comparing sequences of other organisms)
Sequence for organism 1
Sequence for organism 2
Sequence for organism 3
Sequence for organism 4
Color coded cards representing each nucleotide (yellow for guanine, red for adenine, green for cytosine, and blue for thymine)
Chart of Genetic Code (DNA based code)
Instructions
Students will work in groups, preferably at lab tables.
1. Each group of students obtain and examine the sequence for the outgroup, placing the proper cards in order going across the top of the table or board.
2. Each group of students obtain and examine the sequences for Organism #1, Organism #2, Organism #3, and Organism #4.
3. Students will obtain the proper nucleotide cards for each of the four organisms.
4. Students will align the nucleotide cards for each of the 4 organisms so that each organism has the highest number of bases in common with the outgroup sequence.
5. Students will compare and contrast the four sequences, carefully determining at each nucleotide location if an organism matches the outgroup and the other three organisms.
6. Based on the comparison of nucleotide sequences, students will answer the following questions:
a. Which two organisms (1, 2, 3, 4) could be most closely related based on the sequence comparisons? Explain your answer.
b. Which two organisms (1, 2, 3, 4) could be least closely related based on the sequence comparison? Explain your answer.
7. Now use the chart of the Genetic Code to determine the sequence of amino acids coded for by each of the nucleotide sequences. List the sequences below. List first using the name of the amino acids. Then list the sequences using the letter designations for amino acids as found on the chart of the Genetic Code.
- Sequence for Outgroup:
- Sequence for Organism #1:
- Sequence for Organism #2:
- Sequence for Organism #3:
- Sequence for Organism #4:
Extension:
Most nucleotide sequences from homologous genes are much longer than the examples given here. Additionally, the nucleotide sequences from different organisms may differ in length. Using the sequences given below, align the sequences to provide the highest number of nucleotide matches. (Hint: You may need to leave spaces in one or more sequences to best match the other sequences.)
| 1. | Align the following
two sequences: TCTGAACGGGGACTGAGACAGGAA ACTGAACGGCGGCGGGGACAGACACAGCAA |
| 2. |
Now align the following sequences: ACTGCACGGGGGGGACTAGCAGCAGCAGCAGCAA |
Card Construction:
Using a computer, generate one page sheets for each of the four letters. Print on appropriately colored card stock (blue for adenine, red for thymine, yellow for guanine, green for cytosine). Be sure to print enough of each letter for the number of groups you have.
Genetic Code and Amino Acid Abbreviation Websites:
- http://psyche.uthct.edu/shaun/SBlack/geneticd.html
- http://www.nwfsc.noaa.gov/protocols/genecode.html
- http://archive.uwcm.ac.uk/uwcm/mg/docs/cd_amino.html
Sequence of Outgroup:
| ATG AGA ACA GGG GCC CAG TGT TAG |
Sequence of Organism #1:
| ATG TCT GAA CGG GGA CAG AGA TAG |
Sequence of Organism #2:
| ATG TTT CAA CGA GTA CAG AGT TAG |
Sequence of Organism #3:
| ATG ATT GTA GGG CAG CAG GGA TAA |
Sequence of Organism #4:
| ATG TTT CAA CGA GCA CAG AGT TAG |
Model of Sequence Comparison:
| SEQUENCE OF OUTGROUP: | |||||||||||||||||||||||
| A | T | G | A | G | A | A | C | A | G | G | G | G | C | C | C | A | G | T | G | T | T | A | G |
| SEQUENCE OF ORGANISM #1: | |||||||||||||||||||||||
| A | T | G | T | C | T | G | A | A | C | G | G | G | G | A | C | A | G | A | G | A | T | A | G |
| SEQUENCE OF ORGANISM #2: | |||||||||||||||||||||||
| A | T | G | T | T | T | C | A | A | C | G | A | G | T | A | C | A | G | A | G | T | T | A | G |
| SEQUENCE OF ORGANISM #3: | |||||||||||||||||||||||
| A | T | G | A | T | T | G | T | A | G | G | G | C | A | G | C | A | G | G | G | A | T | A | A |
| SEQUENCE OF ORGANISM #4 | |||||||||||||||||||||||
| A | T | G | T | T | T | C | A | A | C | G | A | G | C | A | C | A | G | A | G | T | T | A | G |
Key to Extension:
| 1. | TCTGAACGG--------------GGACTGAGACAGGAA |
| ACTGAACGGCGGCGGGGACAGACACAGCAA |
| 2. | TCTGAACGG-------GGACTAGCA---CAG-------------GAA |
| ACTGCACGGGGGGGACTAGCAGCAGCAGCAGCAA |
- See page one/Teacher References and Teacher Notes above.
- Science as Inquiry
- Molecular Basis of Heredity
- Biological Evolution