Chapter 12, Problem 1MC

If a parental DNA strand has the base sequence A-T-T-G-C-A-C-T, DNA polymerase would synthesize a new strand with the sequence

a. A-T-T-G-C-A-C-T.

b. T-A-A-C-G-T-G-A.

c. C-G-G-T-A-C-A-G.

d. The sequence of the new strand cannot be determined from the information given.

Summary Introduction

Introduction:

Deoxyribonucleic acid (DNA) carrying the genetic information, is a double-helical structure. At the time of the replication, two strands of the DNA unwound from each other. The sole purpose of this mechanism is for the formation of the new strands which are complementary to each other. This is also known as Chargaff’s rule.

Answer to Problem 1MC

Correct answer:

According to the base pairing rule, if a parental DNA strand has A-T-T-G-C-A-C-T base sequence, its complementary strand will be formed having a T-A-A-C-G-T-G-A base sequence.

Explanation of Solution

Explanation for the correct statement:

Option (b) is given as, T-A-A-C-G-T-G-A. The formation of the new strands follows the complementarity rule. This states that A (adenine) will always pair with T (thymine) with a double bond whereas, G (guanine) will always pair with C (cytosine) with a triple bond. According to this rule, option (b) is correct.

Explanation for incorrect answers:

Option (a) is given as, A-T-T-G-C-A-C-T. Pairing either purine with purine or pyrimidine with a pyrimidine, imparts a strong repelling force thereby, hampering in the hydrogen bond formation between the bases. So, it is an incorrect answer.

Option (c) is given as, C-G-G-T-A-C-A-G. Formation of bonds depends upon the specific structure of the bases which allows them to form the hydrogen bond between them. Adenine shares only two atoms for the bond formation, which only welcomes thymine with its two atoms for the same. So, it is an incorrect answer.

Option (d) is given as the sequence of the new strand cannot be determined from the information given. Once the information about the bases is provided from one strand, it is possible to retrieve the other strand’s bases via Chargaff’s rule. As per the rule, A will always pair with T by two hydrogen bonds and G will always pair with C with three hydrogen bonds. So, it is an incorrect answer.

Hence, option (a), (c), and (d) are incorrect.

Conclusion

Therefore, it can be concluded that the retrieval of the other strand information at the time of replication is easy and useful. Once the information is provided about either of the strands, the further process can be studied with the help of complementary rule.