GENETICS QUESTION: For this experiment we focused on forward genetics: First, his- mutants were identified and then complementation analysis was used in order to determine which of the 7 his genes was mutated. ( The exact data can be seen at the bottom). In this experiment we screened for his- mutants that were generated via a random mutation process resulting from errors in DNA replication or induced by EMS aka Ethyl methanesulfonate. We compared a mutagenized culture that was treated with EMS and an un-mutagenized culture that was not treated with EMS. However, both cultures went trhough an enrichment process to increase the frequency of his- mutants.
Point mutations involve the substitution of one type of base pair for another. If the substitution replaces a purine with a purine (or a pyrimidine with a pyrimidine), it is called a transition. If the substitution replaces a purine with a pyrimidine (or a pyrimidine with a purine), it is called a transversion. Many mutagenic chemicals cause this type of mutation by alkylation of a nucleotide base. The chemically altered base can then be misread during replication or repair. For example the mutagen, ethyl methanesulfonate (EMS), adds an ethyl (-CH2-CH3) group to both guanine or thymine bases causing the modified guanine to pair with thymine and the modified thymine to pair with guanine.
1. Why might one gene be more susceptible to EMS mutagenesis than another gene?
2. Why might we have expected to find a higher frequency of polar mutants (his4A-B-C-) in the unmutagenized culture compared to the mutagenized culture?
Table 2. Experiment 1B class data summary (number of his-mutants by section)
Unmutagenized
Section
1
2
3
4ABC
4A
4B
4C
5
6
7
Total his- mutants
Tuesday
7
1
5
4
2
1
9
2
3
1
35
Wednesday
2
1
14
0
4
2
2
4
8
2
39
Total
9
2
19
4
6
3
11
6
11
3
74
% each mutant
12.1
2.7
25.7
5.4
8.1
4.1
14.9
8.1
14.9
4.1
~100%
Mutagenized
Section
1
2
3
4ABC
4A
4B
4C
5
6
7
Total his- mutants
Tuesday
2
5
4
2
6
5
5
0
4
1
34
Wednesday
9
1
15
0
9
1
2
2
9
4
52
Total
11
6
19
2
15
6
7
2
13
5
86
% each mutant
12.8
7.0
22.1
2.3
17.4
7.0
8.1
2.3
15.1
5.8
~100%
GENETICS QUESTION: For this experiment we focused on forward genetics: First, his- mutants were identified and then complementation analysis was used in order to determine which of the 7 his genes was mutated. ( The exact data can be seen at the bottom). In this experiment we screened for his- mutants that were generated via a random mutation process resulting from errors in DNA replication or induced by EMS aka Ethyl methanesulfonate. We compared a mutagenized culture that was treated with EMS and an un-mutagenized culture that was not treated with EMS. However, both cultures went trhough an enrichment process to increase the frequency of his- mutants.
Point mutations involve the substitution of one type of base pair for another. If the substitution replaces a purine with a purine (or a pyrimidine with a pyrimidine), it is called a transition. If the substitution replaces a purine with a pyrimidine (or a pyrimidine with a purine), it is called a transversion. Many mutagenic chemicals cause this type of mutation by alkylation of a nucleotide base. The chemically altered base can then be misread during replication or repair. For example the mutagen, ethyl methanesulfonate (EMS), adds an ethyl (-CH2-CH3) group to both guanine or thymine bases causing the modified guanine to pair with thymine and the modified thymine to pair with guanine.
1. Why might one gene be more susceptible to EMS mutagenesis than another gene?
2. Why might we have expected to find a higher frequency of polar mutants (his4A-B-C-) in the unmutagenized culture compared to the mutagenized culture?
| Table 2. Experiment 1B class data summary (number of his-mutants by section) Unmutagenized | |||||||||||
| Section | 1 | 2 | 3 | 4ABC | 4A | 4B | 4C | 5 | 6 | 7 | Total his- mutants |
| Tuesday | 7 | 1 | 5 | 4 | 2 | 1 | 9 | 2 | 3 | 1 | 35 |
| Wednesday | 2 | 1 | 14 | 0 | 4 | 2 | 2 | 4 | 8 | 2 | 39 |
| Total | 9 | 2 | 19 | 4 | 6 | 3 | 11 | 6 | 11 | 3 | 74 |
| % each mutant | 12.1 | 2.7 | 25.7 | 5.4 | 8.1 | 4.1 | 14.9 | 8.1 | 14.9 | 4.1 | ~100% |
| Mutagenized | |||||||||||
| Section | 1 | 2 | 3 | 4ABC | 4A | 4B | 4C | 5 | 6 | 7 | Total his- mutants |
| Tuesday | 2 | 5 | 4 | 2 | 6 | 5 | 5 | 0 | 4 | 1 | 34 |
| Wednesday | 9 | 1 | 15 | 0 | 9 | 1 | 2 | 2 | 9 | 4 | 52 |
| Total | 11 | 6 | 19 | 2 | 15 | 6 | 7 | 2 | 13 | 5 | 86 |
| % each mutant | 12.8 | 7.0 | 22.1 | 2.3 | 17.4 | 7.0 | 8.1 | 2.3 | 15.1 | 5.8 | ~100% |
