DNA Base Pairing and Hydrogen Bonds - Study Notes

DNA Orientation and Complementary Strands

  • DNA strands are anti-parallel: one strand runs 5'→3' while the complementary strand runs 3'→5'.

  • If a strand is described as running 5' to 3' and the sequence is ATG, the complementary strand runs 3' to 5' as TAC.

  • When writing the complementary strand in the standard 5'→3' orientation, the sequence is CAT.

  • This establishes the base-pairing rules that determine the other strand solely from the given sequence.

Base Pairing Rules

  • Adenine (A) pairs with Thymine (T): A ↔ T.

  • Guanine (G) pairs with Cytosine (C): G ↔ C.

  • Base pairing is complementary: A pairs with T, G pairs with C.

  • Hydrogen bonds in base pairs:

    • A–T pair forms 22 hydrogen bonds.

    • G–C pair forms 33 hydrogen bonds.

Complementary Strand Example

  • Given 5' ATG 3', the complementary strand is written as 3' TAC 5' (anti-parallel).

  • Writing the complementary strand in standard 5'→3' orientation gives CAT.

  • This illustrates how base-pairing rules determine the sequence of the opposite strand.

Hydrogen Bonds in DNA

  • A hydrogen bond is a non-covalent interaction where a hydrogen atom attached to an electronegative atom interacts with another electronegative atom.

  • In DNA, Hydrogen bonds hold the two strands together at the base pairs (A–T and G–C).

  • The concept of hydrogen bonds in DNA can be summarized as:

    • A–T base pair: 22 H-bonds.

    • G–C base pair: 33 H-bonds.

  • These bonds are weaker than covalent bonds, allowing strands to separate during replication and transcription, yet collectively provide considerable stability to the DNA duplex.

Common Misconceptions and Clarifications

  • Hydrogen bonds are a type of bond (non-covalent), not the same as covalent bonds, but they contribute to the stability of DNA.

  • The term "bond" here refers to non-covalent interactions that are reversible and crucial for DNA dynamics.

Quick Practice Questions

  • Practice A: If a strand runs 5' CGA 3', what is the complementary strand in the 3'→5' orientation? Answer: 3' GCT 5'. Writing in 5'→3' orientation gives 5' TCG 3'.

  • Practice B: Which base-pair has more hydrogen bonds, A–T or G–C? Answer: G–C, with 33 hydrogen bonds compared to 22 for A–T.

Connections to Core Principles

  • Complementary base pairing is a direct consequence of molecular structure and hydrogen bonding.

  • Anti-parallel orientation is essential for processes like replication and transcription.

  • The distribution of A–T and G–C pairs influences the DNA double helix stability and melting temperature, affecting how easily the strands separate during biological processes.