DNA (deoxyribonucleic acid) is the macromolecule that encodes genetic information in all living organisms. Structurally, it is a polymer composed of nucleotide monomer building blocks arranged in a double helix.
🔑 Key Principle
Each DNA nucleotide is composed of three distinct components: a phosphate group, a 2-deoxyribose sugar, and a nitrogen-containing organic base. These form the monomer via two condensation reactions.
The repeating monomer unit of nucleic acids, formed by the condensation of a pentose sugar, a nitrogenous organic base, and a phosphate group.
Structure of a DNA Nucleotide
During the formation of a single nucleotide from its three constituent components, two molecules of water are lost:
- One water molecule is lost when the phosphate group links to Carbon 5 (\\( \text{C}^5 \\)) of the deoxyribose pentose sugar.
- A second water molecule is lost when the organic base links to Carbon 1 (\\( \text{C}^1 \\)) of the deoxyribose sugar.
There are four nitrogenous bases in DNA, classified into two groups:
- Purines (double-ring structure): Adenine (A) and Guanine (G)
- Pyrimidines (single-ring structure): Thymine (T) and Cytosine (C)
Polymerisation and the Phosphodiester Backbone
Nucleotides link together via further condensation reactions to form a single DNA strand. The phosphate group on one nucleotide reacts with the hydroxyl group on Carbon 3 (\\( \text{C}^3 \\)) of the deoxyribose sugar of the adjacent nucleotide. This covalent linkage is called a phosphodiester bond.
This repeating sugar-phosphate sequence forms the structural phosphodiester backbone of DNA, with the nitrogenous bases projecting inwards from the sugar rings.
The strong, covalently linked chain of alternating sugar and phosphate groups that forms the structural support of a DNA strand.
Complementary Base Pairing and the Double Helix
DNA exists as a double-stranded helix. The two strands run antiparallel to each other and are held together by hydrogen bonds that form between complementary base pairs on opposite strands:
Specific electrostatic attractions between partial positive hydrogen atoms and partial negative oxygen or nitrogen atoms on complementary bases, stabilizing the double helix.
- Adenine (A) pairs with Thymine (T): stabilized by 2 hydrogen bonds.
- Guanine (G) pairs with Cytosine (C): stabilized by 3 hydrogen bonds.
Replication Role
The specificity of hydrogen bonding ensures that bases pair in a highly complementary manner. During cell division, the double helix unwinds, and the hydrogen bonds break. Each separate single strand then acts as a template, binding free nucleotides with high fidelity to replicate the original double helix perfectly.
Examiners routinely test the number of hydrogen bonds between bases. Always write out: Adenine pairs with Thymine using 2 hydrogen bonds, and Guanine pairs with Cytosine using 3 hydrogen bonds. Getting these numbers transposed is a common source of lost marks.
Step 1: Identify base pairing partners.
- Adenine (A) pairs with Thymine (T)
- Cytosine (C) pairs with Guanine (G)
- Guanine (G) pairs with Cytosine (C)
- Thymine (T) pairs with Adenine (A)
Step 2: Replace each base in sequence.
- A → T
- C → G
- G → C
- T → A
- A → T
- T → A
- G → C
- C → G
Answer: The complementary strand base sequence is T G C A T A C G.
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