If a portion of a strand of DNA bases reads 5' AGCTAGGCT 3', what would the sequence of bases on the complementary strand read?

Rationale

3' TCGATCCGA 5'

The complementary strand of DNA is formed by pairing each base with its corresponding mate: adenine (A) pairs with thymine (T), and cytosine (C) pairs with guanine (G). The sequence of the complementary strand is thus derived from the original strand read in the opposite direction, resulting in a specific base pairing.

A (3' TCGATCGCA 5') CORRECT

This choice correctly pairs the bases with the original sequence. The adenine (A) from the original strand pairs with thymine (T), cytosine (C) pairs with guanine (G), and guanine (G) pairs with cytosine (C). The directionality is also reversed, resulting in the correct complementary sequence.

B (5' UCGAUCGCA 3') YOUR ANSWER

This sequence incorrectly replaces thymine (T) with uracil (U), which is found in RNA, not DNA. In DNA, thymine should always pair with adenine, making this choice invalid for a DNA complementary strand.

C (5' TCGTTCGCT 3') YOUR ANSWER

This option does not accurately reflect the original DNA sequence, and the base pairing is incorrect. The second base in the original strand is a guanine (G), which would pair with a cytosine (C) on the complementary strand, but this choice shows a thymine (T) instead.

D (3' AGCTAGGCT 5') YOUR ANSWER

This choice is simply a repetition of the original strand in reverse order. While the direction is correct for a complementary strand, the base pairing is not, as it does not reflect the necessary A-T and C-G pairings.

Conclusion

The complementary DNA strand must adhere to specific base pairing rules where adenine pairs with thymine and cytosine pairs with guanine. Analyzing the original sequence, 5' AGCTAGGCT 3', the correct complementary sequence is 3' TCGATCCGA 5', as reflected in choice A. This demonstrates the essential principles of DNA structure and base pairing.

If a portion of a strand of DNA bases reads 5' AGCTAGGCT 3', what would the sequence of bases on the complementary strand read?

3' TCGATCCGA 5'

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