16.1-3ITD - Campbell Biology

Question

: If Chargaff's rule—that the amount of A equals the amount of T and the amount of C equals the amount of G—is valid, then hypothetically, we could extrapolate this to the combined DNA of all species on Earth (like one huge Earth genome). To see whether the data in the table support this hypothesis, calculate the average percentage for each base in your completed table by averaging the values in each column. Does Chargaff's equivalence rule still hold true?

Source of DNA		Base Percentage
	Adenine	Guanine	Cytosine	Thymine
Sea urchin	32.8	17.7	17.3	32.1
Salmon	29.7	20.8	20.4	29.1
Wheat	28.1	21.8	22.7	27.4
E. coli	24.7	26.0	25.3	24.0
Human	30.4	19.8	19.8	30.1
Ox	29.0	21.0	21.0	29.0

Step-by-Step Solution

Answer

Source of DNA		Base Percentage
	Adenine	Guanine	Cytosine	Thymine
Sea urchin	32.8	17.7	17.3	32.1
Salmon	29.7	20.8	20.4	29.1
Wheat	28.1	21.8	22.7	27.4
E. coli	24.7	26.0	25.3	24.0
Human	30.4	19.8	19.8	30.1
Ox	29.0	21.0	21.0	29.0
Average %	29.11	21.18	21.08	28.61

The average percentage for the bases adenine is 29.11%, guanine is 21.18%, cytosine is 21.08%, and thymine is 28.61%.

Yes, Chargaff's equivalence rule still holds for all the purines and pyrimidines average percentages. It is because the average from the table follows Chargaff's A = T and G = C equivalence relationships.

Step 1: Chargaff's equivalence rule

Chargaff's equivalence rule states that the amount of complementary purine and pyrimidine must be equal. Consequently, DNA must have a 1:1 stoichiometric ratio for complementary purines and pyrimidines.

It implies that [A]=[T], [G]=[C], A+G=C+T, $A:T=1:1$ and $G:C=1:1$