In samples of DNA isolated from two unidentified species of bacteria, adenine makes up 32 and 17%, respectively, of the total bases. 1. What relative proportions of adenine, guanine, thymine, and cytosine would you expect to find in the two DNA samples? (hint: bacterial DNA is circular plasmid) 2. What assumptions have you made? One of these bacteria was isolated from a hot spring 64 degree C. Which DNA came from this thermophilic bacterium? What is the basis for your answer? (hint: consider bonding strength A-T or G-C)

1. Sample 1 = A=32, T=32, G=18, C=18

Sample 2:

2. Chargaff rule (A=T and G=C)

3. Sample 2

Explanation:

Sample 1 = Adenine percentage = 32%

According to the Chargaff rule, thymine percentage in this sample DNA would be = 32%

Sum total of adenine and thymine in sample 1 = 32 + 32 = 64%

Sum total of GC content in the sample 1 = 100-64 = 36

Percentage of guanine bases in sample 1 = 36/2=18%

Percentage of cytosine bases in sample 1 = 18%

Sample 2

Adenine percentage = 17%

According to the Chargaff rule, thymine percentage in this sample DNA would be = 17%

Sum total of adenine and thymine in sample 2=17 + 17 = 34%

Sum total of GC content in the sample 2 = 100-34 = 66

Percentage of guanine bases in sample 2 = 66/2=33%

Percentage of cytosine bases in sample 2 = 33%

2. Percentage of adenine = percentage of thymine

Percentage of guanine = percentage of cytosine

3. DNA from sample 2 is from the bacterial living in hot water spring since it has a higher percentage of GC (66%) than sample 1 (36%). The GC bases have three hydrogen bonds between them and are relatively more stable base pairs than the AT pairs each with two hydrogen bonds.