DNA Structure and Complementary Base Pairing

Exam Tip: When describing the DNA structure, always mention that the strands are antiparallel. This means if one strand runs in a $5' \rightarrow 3'$ direction, its partner runs in a $3' \rightarrow 5'$ direction.

Nitrogenous Bases and Complementary Base Pairing

The true "code" of life resides in the nitrogenous bases. There are four types of bases found in DNA, categorized into two groups based on their chemical structure:

• Purines: These are larger, double-ringed structures. They include Adenine (A) and Guanine (G).
• Pyrimidines: These are smaller, single-ringed structures. They include Thymine (T) and Cytosine (C).

The most critical aspect of DNA function is how these bases interact. They do not pair randomly. Instead, they follow strict complementary base pairing rules. A purine must always pair with a pyrimidine to ensure that the distance between the two sugar-phosphate backbones remains constant throughout the entire molecule.

The specific pairings are:

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

These pairs are held together by weak hydrogen bonds. The A-T pair is held together by two hydrogen bonds, while the G-C pair is held together by three hydrogen bonds. This slight difference in bond strength is important for processes like DNA replication and stability.

Worked Example: Determining the Complementary Strand

Because of these strict pairing rules, if you know the sequence of one strand of DNA, you can mathematically determine the sequence of the opposite strand. This is the basis for DNA replication and many biotechnology techniques.

Problem: Given the following DNA sequence on the "sense" strand: $5' \text{— A T G C C G T A — } 3'$

Find the sequence of the complementary "antisense" strand.

Step-by-Step Solution:

1. Identify the pairs:
   • A pairs with T
   • T pairs with A
   • G pairs with C
   • C pairs with G
2. Apply the rules to the sequence:
   • A $\rightarrow$ T
   • T $\rightarrow$ A
   • G $\rightarrow$ C
   • C $\rightarrow$ G
   • C $\rightarrow$ G
   • G $\rightarrow$ C
   • T $\rightarrow$ A
   • A $\rightarrow$ T
3. Account for directionality: Remember that the strands are antiparallel. If the original strand is $5' \rightarrow 3'$, the complement will be $3' \rightarrow 5'$.

Final Answer: $3' \text{— T A C G G C A T — } 5'$

Summary Table of Base Pairing

Understanding this structure is the gateway to studying genetics, as the specific order of these bases constitutes the genetic code that dictates the traits of every living thing.