What is DNA Structure and How Does Replication Work?
DNA is the molecule of heredity—a double helix wound from two complementary strands of bases paired by hydrogen bonds. Replication is semiconservative: each strand serves as a template, and DNA polymerase synthesizes a new partner, so every copy carries both an old and a new strand.
DNA structure: two antiparallel strands (5'→3' and 3'→5') wound in a double helix, with bases adenine–thymine (A–T, 2 H-bonds) and guanine–cytosine (G–C, 3 H-bonds) pairing across. Replication: strands unwind, DNA polymerase III builds new complementary strands on each template (5'→3' leading, 3'→5' lagging), producing two identical DNA molecules.
- 1↓Double Helix UnwindsHelicase breaks hydrogen bonds; two strands separate.
- 2↓Leading Strand SynthesisDNA Pol III synthesises 5'→3' continuously on the template 3'→5' strand.
- 3↓Lagging Strand SynthesisDNA Pol III synthesises short Okazaki fragments 5'→3' on the template 5'→3' strand.
- 4↓Fragment LigationDNA Ligase joins Okazaki fragments; new strands complete.
- 5Two Identical DNA MoleculesEach has one old strand + one new strand (semiconservative).
Step-by-step worked examples
A template DNA strand is 5'-ATCG-3'. What is the sequence of the newly synthesised complementary strand?
Template: 5'-ATCG-3' New strand synthesises 3'→5' antiparallel 5'-CGAT-3' (or read 3'-TACG-5' correctly)
In semiconservative replication, how many parental DNA strands are in each of the two daughter DNA molecules?
Each daughter molecule has: 1 original (parental) strand + 1 newly synthesised strand If you start with 2 parental strands, you end with 2 daughter DNA molecules, each hybrid.
Why is Okazaki fragment synthesis needed on the lagging strand?
DNA Pol III only synthesises 5'→3' On the lagging strand, template runs 5'→3' So short fragments (Okazaki) are made discontinuously, then ligated.
Flashcards
Quick quiz
Q1.If one strand of DNA is 5'-AATGC-3', the complementary strand is:
Q2.In semiconservative replication of a double helix, after one round, you have:
Q3.Which enzyme unwinds the DNA double helix?
Q4.Why is the leading strand synthesised continuously but the lagging strand discontinuously?
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Common mistakes
Both strands synthesise at the same speed. — Correct: Leading is continuous; lagging is in Okazaki fragments because of DNA Pol III directionality.
A–T and G–C have the same number of hydrogen bonds. — Correct: A–T has 2 H-bonds; G–C has 3 (stronger).
Replication produces two identical copies of parental DNA. — Correct: Semiconservative: each daughter DNA is a hybrid (1 old + 1 new strand).
Okazaki fragments are found on both strands equally. — Correct: They're found only on the lagging strand; the leading strand is continuous.
FAQ
What is the structure of DNA?
A double helix of two antiparallel strands held by hydrogen bonds between complementary bases (A–T, G–C).
Why is DNA replication called semiconservative?
Each daughter DNA retains one parental strand; 'semi' = half the parental DNA is conserved.
What is the role of DNA polymerase in replication?
DNA Pol synthesises new DNA strands 5'→3' using the old strand as a template.
What difference is there between the leading and lagging strands?
Leading is synthesised continuously 5'→3'; lagging is synthesised as short Okazaki fragments, then joined by ligase.