Using donor DNA from an E. coli strain of leu+ arg+ trp- you transform a resipient strain that is leu- arg- trp+ and select for leu+. The resulting 100 colonies are then screened for the arg and trp markers and the results are outlined in the table below.
genotype # of colonies
1. arg+ trp+ 8
2. arg- trp- 47
3. arg+ trp- 18
4. arg- trp+ 27
Determine the order of the genes and relative map distances.
To determine the order of the genes and their relative map distances, we can use the data from the table provided. This information is obtained through a process called "three-point cross analysis." Here's how to approach it:
1. Start by identifying the common genotype in all 100 colonies, which is leu+ arg+ trp-. This genotype was selected for, so all the colonies should have this trait.
2. Determine the number of colonies with each combination of the arg and trp markers:
- arg+ trp+: 8 colonies
- arg- trp-: 47 colonies
- arg+ trp-: 18 colonies
- arg- trp+: 27 colonies
3. Now, let's analyze the data to determine the order and relative map distances of the genes. Start with the gene in the middle, which is arg.
a) Calculate the recombination frequency (%) between arg and trp (arg and trp+ compared to arg+ and trp-):
Recombination frequency = [(arg+ trp-) + (arg- trp+)] / total colonies * 100
Recombination frequency = (18 + 27) / 100 * 100
Recombination frequency = 45%
b) Determine the order based on the recombination frequency:
If the recombination frequency was closer to 50%, it would suggest that arg and trp are located far apart on the chromosome.
If the recombination frequency was closer to 0%, it would imply arg and trp are very close or even adjacent to each other.
Since the recombination frequency is 45%, this indicates that arg and trp are moderately far apart on the chromosome.
4. Next, let's examine the genes leu and arg. Since we already know that arg is in the middle, we need to investigate the relationship between leu and arg.
a) Calculate the recombination frequency (%) between leu and arg (leu and arg+ compared to leu- and arg-):
Recombination frequency = [(arg- trp-) + (arg+ trp+)] / total colonies * 100
Recombination frequency = (47 + 8) / 100 * 100
Recombination frequency = 55%
b) Determine the order based on the recombination frequency:
A recombination frequency of 55% indicates that leu and arg are relatively far apart on the chromosome.
c) Combine the information:
From the above analyses, we know that arg is located between leu and trp. Therefore, the order of the genes is leu-arg-trp.
5. Calculate the relative map distances based on the recombination frequencies:
- The recombination frequency represents the percentage of time that recombination occurs between two genes during cell division.
- The higher the recombination frequency, the farther apart the genes are on the chromosome.
Assuming a linear chromosome and that the map distance between leu-arg and arg-trp is 1 unit, let's calculate the relative map distances:
- leu-arg: 55% recombination frequency = 1 unit (given assumption)
- arg-trp: 45% recombination frequency = 1 unit (given assumption)
- Therefore, relative map distance between:
leu-arg = 55/45 = 1.22 units
arg-trp = 1 unit (already mentioned)
- The overall order with the relative map distances is:
leu(1.22 units)-arg(1 unit)-trp
So, the order of the genes is leu-arg-trp, and the relative map distances are leu-arg (1.22 units) and arg-trp (1 unit).