DNA, RNA and DNA replication

Nucleotides and the double helix

A nucleotide is a pentose sugar, a phosphate group and a nitrogenous base. Nucleotides join by condensation reactions, forming phosphodiester bonds between the sugar of one and the phosphate of the next: the sugar-phosphate backbone.

• DNA: deoxyribose; bases adenine (A), thymine (T), cytosine (C), guanine (G); two antiparallel strands in a double helix, held by hydrogen bonds between complementary base pairs: A with T, C with G.
• RNA: ribose; uracil (U) replaces thymine; usually single-stranded and shorter.

Semi-conservative replication

1. DNA helicase unwinds the helix and breaks the hydrogen bonds between bases, separating the two strands.

2. Each strand acts as a template. Free DNA nucleotides line up by complementary base pairing.

3. DNA polymerase joins adjacent nucleotides, forming phosphodiester bonds.

4. Each new DNA molecule has one original strand and one new strand: semi-conservative replication.

Meselson and Stahl

E. coli were grown for many generations in a medium with heavy nitrogen (15N), so all their DNA was heavy. They were moved to light nitrogen (14N) and DNA was extracted after each generation and spun in a caesium chloride density gradient.

• After 1 generation: one band of intermediate density (rules out conservative replication).
• After 2 generations: one intermediate band and one light band.

This matched semi-conservative replication.

Worked example

A DNA sample has 30% adenine. What percentage is guanine?

1. A = T, so T = 30% and A + T = 60%.
2. C + G = 40%, and C = G.
3. G = 20%

Answer: 20% guanine.

Check yourself

1. A DNA molecule contains 18% thymine. What percentage of its bases are cytosine?
   Show answer

   32 %. A = T = 18%, so C + G = 64%, C = 32%.

2. Which base is found in RNA but not DNA?
   • Thymine
   • Adenine
   • Guanine
   • Uracil
   Show answer

   Uracil. Uracil replaces thymine in RNA.

3. Which bond links nucleotides in one strand of DNA?
   • Hydrogen bond
   • Phosphodiester bond
   • Peptide bond
   • Glycosidic bond
   Show answer

   Phosphodiester bond. Phosphodiester bonds form the sugar-phosphate backbone.

4. What is the role of DNA polymerase?
   • Breaking hydrogen bonds between strands
   • Unwinding chromosomes
   • Joining adjacent nucleotides to form the new strand
   • Making mRNA
   Show answer

   Joining adjacent nucleotides to form the new strand. DNA polymerase catalyses phosphodiester bond formation.

5. In Meselson and Stahl's experiment, what did DNA look like after one generation in 14N?
   • One band of intermediate density
   • One heavy band and one light band
   • One light band only
   • One heavy band only
   Show answer

   One band of intermediate density. Each molecule had one 15N and one 14N strand.

6. After two generations in 14N, the bands were
   • all intermediate
   • all light
   • half intermediate and half light
   • half heavy and half light
   Show answer

   half intermediate and half light. Intermediate molecules each give one intermediate and one light molecule.

7. Which pair of bases is joined by complementary base pairing in DNA?
   • Cytosine and guanine
   • Adenine and guanine
   • Thymine and cytosine
   • Adenine and uracil
   Show answer

   Cytosine and guanine. C pairs with G; A pairs with T.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)
• TutorChase: Meselson and Stahl's experiment (Edexcel A level Biology notes)

Last updated 7 November 2026. Spotted a mistake? Tell us.