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23 March, 13:46

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)

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  1. 23 March, 16:19
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    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.
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