# Nucleic Acids
DNA and RNA are nucleic acids that store and transmit genetic information. Understanding their structure, replication, and role in protein synthesis is fundamental to A-Level Biology.
1. Nucleotide Structure
A nucleotide consists of:
- Pentose sugar (deoxyribose in DNA; ribose in RNA)
- Phosphate group
- Nitrogenous base
Nucleotides joined by phosphodiester bonds (condensation) to form polynucleotides.
2. DNA vs RNA
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose | Ribose |
| Bases | A, T, C, G | A, U, C, G |
| Strands | Double (antiparallel) | Single |
| Structure | Double helix | Various (mRNA, tRNA, rRNA) |
| Location | Nucleus (mainly) | Nucleus and cytoplasm |
| Function | Long-term genetic store | Protein synthesis |
3. DNA Structure
- Two antiparallel polynucleotide strands
- Bases face inward; sugar-phosphate backbone outside
- Base pairing: A=T (2 H-bonds), G≡C (3 H-bonds)
- Twisted into double helix
- Hydrogen bonds between bases hold strands together
Why DNA Is Suited to Its Function
- Double strand: stable; one strand serves as template for replication
- Complementary base pairing: allows accurate replication
- H-bonds: weak individually (easy to separate) but many provide stability
- Very long: carries vast amount of genetic information
- Coiled and associated with histones: compact packaging
4. DNA Replication (Semi-Conservative)
- Helicase unzips DNA by breaking H-bonds between bases
- Each strand acts as a template
- DNA polymerase adds free nucleotides by complementary base pairing
- Ligase joins Okazaki fragments on lagging strand
- Two identical DNA molecules formed, each with one original and one new strand
Semi-conservative: each new molecule has one old strand and one new strand.
Evidence: Meselson and Stahl experiment with ¹⁵N and ¹⁴N.
5. RNA Types
| Type | Function |
|---|---|
| mRNA | Carries genetic code from DNA to ribosome |
| tRNA | Carries amino acids to ribosome; has anticodon |
| rRNA | Structural component of ribosomes |
6. Practice Questions
- Describe the structure of a nucleotide.
- Compare DNA and RNA.
- Explain the process of semi-conservative replication.
- Why are there two hydrogen bonds between A and T but three between C and G?
- Describe Meselson and Stahl's experiment and what it proved.
Want to check your answers and get step-by-step solutions?
Summary
- Nucleotides: sugar + phosphate + base; joined by phosphodiester bonds
- DNA: double helix; deoxyribose; A-T, C-G; stores genetic information
- RNA: single strand; ribose; A-U, C-G; mRNA, tRNA, rRNA
- Replication: helicase unzips → DNA polymerase adds bases → semi-conservative
- Meselson-Stahl: evidence for semi-conservative replication