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Chapter 23: Q55P (page 1255)

Erwin Chargaff鈥檚 discovery that DNA contains equimolar amounts of guanine and cytosine and also equimolar amounts of adenine and thymine has come to be known as Chargaff鈥檚 rule:

G = C and A = T

(a) Does Chargaff鈥檚 rule imply that equal amounts of guanine and adenine are present in DNA? That is, does G = A?

(b) Does Chargaff鈥檚 rule imply that the sum of the purine residues equals the sum of the pyrimidine residues? That is, does A + G = C + T?

(c) Does Chargaff鈥檚 rule apply only to double-stranded DNA, or would it also apply to each individual strand if the double helical strand were separated into its two complementary strands?

Short Answer

Expert verified

(a) No, there is no relation between the amounts of guanine and adenine.

(b) Yes, Chargaff鈥檚 rule states that sum of purine residues equals the sum of the pyrimidine residues.

(c) Chargaff鈥檚 rule apply only to double-stranded DNA.

Step by step solution

01

Step-1.: Find Chargaff’s rule implies that equal amounts of guanine and adenine

Chargaff鈥檚 rule implies that total number of purine bases is equal to total number of pyrimidine bases, that is, number of guanine units is equal to cytosine units present and number of adenine units equal to thymine units. DNA composition is different in different species. Chargaff鈥檚 rule do not apply that number of adenine units are equal to number of guanine units.

02

Step-2.: Find Chargaff’s rule implies that the sum of the purine residues equals the sum of the pyrimidine residues

In double stranded structure of DNA,

A = T = 1

G = C = 1

Percentage of C + G does not necessarily equal to percentage of A + T.

Sum of purine residues is equal to sum of pyrimidine residues. In a double stranded DNA, number of purines and pyrimidines exist in 1:1 ratio as purines bond with pyrimidines in DNA.

03

Step-3: Find Chargaff’s rule applies only to double-stranded DNA

Chargaff鈥檚 rule apply only to double-stranded DNA. For each G in a strand, there exists a complementary C in the opposing strand but there is no relation between G and C in the same strand. Complementary base pairing is not applicable in case of RNA as it is single stranded, thus Chargaff鈥檚 rule is not applied to RNA.

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Most popular questions from this chapter

Draw the expected product of the reaction of the following sugars with excess methyl iodide and silver oxide.

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Question. (a) Figure 23-2 shows that the degradation of D-glucose gives D-arabinose, an aldopentose. Arabinose is most stable in its furanose form. Draw D-arabinofuranose.

(b) Ribose, the C2 epimer of arabinose, is most stable in its furanose form. Draw D-ribofuranose.

Question:

a) Draw D-allose, the C3 epimer of glucose.

b) Draw D-talose, the C2 epimer of D-galactose.

c) Draw D-idose, the C3 epimer of D-talose. Now compare your answers with Figure 23-3.

d) Draw the C4 鈥渆pimer鈥� of D-xylose. Notice that this 鈥渆pimer鈥� is actually an L-series sugar, and we have seen its enantiomer. Give the correct name for this L-series sugar.

Ruff degradation of D-arabinose gives D-erythrose. The Kiliani-Fischer synthesis converts D-erythrose to a mixture of D-arabinose and D-ribose. Draw out these reactions and give the structure of D-ribose.

Cytosine, uracil and guanine have tautomeric forms with aromatic hydroxyl groups. Draw these tautomeric forms.
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