Question 19

The sequence of bases on the complementary strand reads 3' TCGATCGCA 5'

The complementary strand of DNA is formed by pairing adenine with thymine and cytosine with guanine. Given the original strand 5' AGCTAGGCT 3', the complementary bases are TCGATCGCA, which are arranged in the reverse order and antiparallel orientation.

A) 3' TCGATCGCA 5'

This option is correct as it accurately represents the complementary base pairing for the original DNA sequence. The bases match up correctly, with adenine pairing with thymine and cytosine pairing with guanine, arranged in a 3' to 5' direction.

B) 5' UCGAUCGCA 3'

This option is incorrect because it includes uracil (U), which is found in RNA, not DNA. The original sequence consists of DNA bases, and the complementary strand must contain thymine (T) instead of uracil.

C) 5' TCGTTCGCT 3'

This option is incorrect as it does not match the complementary bases of the original DNA sequence. The bases do not correspond correctly to the original sequence, resulting in an incorrect representation.

D) 3' AGCTAGGCT 5'

This option is incorrect because it replicates the original DNA sequence rather than providing the complementary strand. The complementary strand should have different bases that correspond to the original sequence.

Conclusion

Option A is definitively correct, as it follows the base pairing rules of DNA, producing the accurate complementary sequence. All other options fail either by using incorrect bases or by not providing the complementary sequence at all, highlighting the importance of understanding DNA structure and base pairing.