DNA Structure and Replication
The Blueprint of Life
DNA Structure
DNA (deoxyribonucleic acid) is a double helix discovered by Watson and Crick in 1953 (building on X-ray diffraction work by Rosalind Franklin and data from Chargaff). Each strand is a polymer of nucleotides, each consisting of:
- A deoxyribose sugar
- A phosphate group
- A nitrogenous base: Adenine (A), Thymine (T), Guanine (G), or Cytosine (C)
The two strands are held together by hydrogen bonds between complementary base pairs: A–T (2 H-bonds) and G–C (3 H-bonds). The strands are antiparallel — one runs 5'→3', the other 3'→5'.
DNA Replication
Replication is semi-conservative — each new double helix contains one original and one new strand (proved by Meselson-Stahl experiment, 1958). Key steps:
- Helicase unwinds and separates the strands at the replication fork
- Primase adds short RNA primers to provide a 3'-OH start
- DNA polymerase III adds nucleotides 5'→3', reading the template 3'→5'
- Leading strand is synthesised continuously; lagging strand in Okazaki fragments
- DNA polymerase I removes RNA primers; DNA ligase seals nicks
- Topoisomerase relieves torsional stress ahead of the fork
Chargaff's Rules
In any DNA molecule: [A]=[T] and [G]=[C]. The ratio (A+T)/(G+C) varies between species — this tells us the base composition is species-specific. These rules were critical clues for Watson and Crick's model.
Which enzyme joins Okazaki fragments on the lagging strand during DNA replication?
DNA ligase forms phosphodiester bonds to seal the nicks between Okazaki fragments after RNA primers are replaced by DNA polymerase I.
Correct answer: DNA ligase
The Meselson-Stahl experiment (1958) proved that DNA replication is:
Bacteria grown in ¹⁵N then shifted to ¹⁴N produced hybrid DNA after one generation and a mix of hybrid and light DNA after two — proving semi-conservative replication.
Correct answer: Semi-conservative
Adenine always pairs with which base in DNA?
A pairs with T via 2 hydrogen bonds; G pairs with C via 3 hydrogen bonds in DNA (Chargaff's rules).
Correct answer: Thymine
Explain what is meant by 'antiparallel' strands in DNA, and why this matters for replication.
Antiparallel structure dictates the asymmetry of DNA replication.
Correct answer: The two strands of DNA run in opposite directions: one 5'→3' and the other 3'→5'. This matters for replication because DNA polymerase can only add nucleotides in the 5'→3' direction, requiring the leading strand to be synthesised continuously and the lagging strand discontinuously (Okazaki fragments).