3.1.5 Nucleic Acid (DNA)

Monomers of DNA and RNA:

Nucleotides

Components and Structure of DNA:

• Phosphate group

• Deoxyribose sugar

• Nitrogen containing base

Components and Structure of RNA:

• Phosphate group

• Ribose sugar

• Nitrogen-containing base

Bond formed between nucleotides during a condensation reaction:

Phosphodiester bonds

Bases of DNA:

• Adenine

• Guanine

• Cytosine

• Thymine

Bases of RNA:

• Adenine

• Guanine

• Cytosine

• Uracil

Structure of a DNA molecule:

A DNA molecule is a double helix with two polynucleotide chains held together by hydrogen bonds between specific complementary base pairs.

Structure of a RNA molecule:

RNA is a single polynucleotide chain (linear)

3 DNA replication theories:

• Conservative

• Semi-conservative

• Disperisive

DNA replication process (Semi-conservative):

• DNA Helicase breaks the hydrogen bonds between complementary bases

• The original strands act as a template to build a new strand

• Free-floating nucleotides in the nucleus attract to their complemenatry base pairs

• DNA polymerase catalyses a condensation reaction between adjacent nucleotides forming a phosphodiester bond and creating the phosphodiester bond

• Each DNA molecule contains a parent and a newly synthesised cell

Watson-Crick model evaluation:

Matthew Meselson & Franklin Stahl

• Carried out the Meselson and Stahl experiment using nitrogen isotopes (¹⁵N and ¹⁴N)

• Showed DNA replication is semi-conservative

✅ Strengths:

• Provided direct experimental evidence

• Effectively ruled out conservative replication

• Strong support for Watson–Crick prediction

❗ Limitations:

• Did not completely rule out dispersive replication in early stages

• Evidence was indirect (based on density, not visual structure)