DNA Structure and Replication: TEAS Practice Question with Answer & Explanation

Understanding DNA Complementary Base Pairing

To solve this problem on the TEAS, you must master two fundamental rules of DNA structure: Complementary Base Pairing and Antiparallel Orientation.

1. The Base Pairing Rules (Chargaff's Rules)

In a DNA molecule, nitrogenous bases do not pair randomly. They follow a specific "lock and key" pattern:

• Adenine (A) always pairs with Thymine (T).
• Cytosine (C) always pairs with Guanine (G).

Looking at the original sequence (5' GGTACGATT 3'):

• G pairs with C
• G pairs with C
• T pairs with A
• A pairs with T
• C pairs with G
• G pairs with C
• A pairs with T
• T pairs with A
• T pairs with A

This gives us the sequence: CCATGCTAA.

2. Antiparallel Directionality

DNA strands are "antiparallel," meaning they run in opposite directions. If the original strand is oriented 5' to 3', the complementary strand must be oriented 3' to 5'.

Therefore, the complementary sequence is 3' CCATGCTAA 5'.

Analysis of Incorrect Answers

• Choice A (5' GGTACGATT 3'): This is simply a copy of the original strand. DNA strands are complementary, not identical.
• Choice C (3' CCAUGCUAA 5'): This uses Uracil (U). Uracil is found in RNA, not DNA. The question specifically asks for the complementary DNA strand. Always check if the question asks for DNA or mRNA!
• Choice D (5' TTACGATGG 3'): This is the correct sequence of bases if read backward, but the orientation (5' to 3') is incorrect. In genetics, directionality is as important as the letters themselves.

TEAS Study Tip: The "GC-AT" Memory Hack

To remember which bases pair together, use one of these mnemonics:

1. Goal Crease / All Together
2. Garbage Can / Apple Tree
3. G-C looks like two curved letters; A-T are made of straight lines.

Crucial Test-Taking Advice: Always look at the 5' and 3' ends first. Many students lose points because they find the right letters but choose the wrong direction!