According to Watson and Crick's model, DNA consists of two strands of nucleotides that are antiparallel. What does this mean? What evidence supports the antiparallel nature of the strands?
What will be an ideal response?
Answers may vary. DNA consists of two strands of nucleotides that are antiparallel. This means the sugar-phosphate groups of each strand are oriented in opposite directions. The 5' end of one strand is paired with the 3' end of the other strand and vice-versa. The antiparallel nature of the strands is necessary for Watson and Crick's model to work. This arrangement ensures that the bases are oriented in a way they can interact. Only in this arrangement will the DNA molecule have the width revealed by Franklin's X-ray diffraction pattern. This arrangement also agrees with Chargaff's rule of A = T and G = C, or complementary base pairing. If two pyrimidines were together, the DNA molecule would be too narrow and if two purines were together, the molecule would be wider than Franklin's work suggested. Therefore, the antiparallel model agrees with Chargaff' and Franklin's findings.
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