Unique Structures Determines How Bases Pair
Adenine (A) and Thymine (T)
- Adenine and thymine form two hydrogen bonds between them:
- A partially positive hydrogen atom on adenine forms a bond with a partially negative oxygen atom on thymine.
- A hydrogen atom on thymine interacts with a nitrogen atom on adenine.
These two hydrogen bonds make the A-T pairing stable but not overly rigid, allowing the DNA strands to separate during processes like replication and transcription.
Guanine (G) and Cytosine (C)
- Guanine and cytosine form three hydrogen bonds:
- A hydrogen atom on guanine bonds with an oxygen atom on cytosine.
- Another hydrogen atom on cytosine bonds with a nitrogen atom on guanine.
- A third bond forms between a hydrogen atom on guanine and an oxygen atom on cytosine.
- Because of the extra hydrogen bond, G-C pairs are slightly stronger than A-T pairs.
- This difference in bonding strength has biological implications, as regions of DNA with high G-C content are more stable and harder to separate.
- For instance, organisms living in extreme environments, such as high temperatures, often have DNA with higher G-C content.
- The increased number of hydrogen bonds helps stabilize their genetic material under harsh conditions.
- Many Paper 2 questions ask students to explain how complementary base pairing ensures accurate DNA replication or protein synthesis.
- Answers should always include both the specific base-pairing rules and the role of hydrogen bonds.
Why Complementarity Matters in DNA
1. Accurate Replication
- During DNA replication, the two strands of the double helix separate, and each serves as a template for a new complementary strand, resulting in semiconservative DNA replication.
