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How does DNA replicate?

Class 12 BiologyNCERT chapter: Molecular Basis of Inheritanceइसे हिंदी में पढ़ें

Exam-ready answer

DNA replicates semiconservatively. At the origin of replication the two strands separate to form a replication fork, and each acts as a template. DNA-dependent DNA polymerase adds complementary nucleotides only in the 5'→3' direction, so one strand is made continuously and the other discontinuously as fragments joined by DNA ligase. Each daughter DNA keeps one parental strand.

Explanation

Watson and Crick proposed that the two strands of DNA separate and each acts as a template for a new complementary strand. Each daughter molecule therefore has one parental strand and one newly synthesised strand: the semiconservative scheme. Meselson and Stahl proved it in E. coli by growing cells in a medium with heavy nitrogen (15N), shifting them to normal 14N, and separating the DNA by density in a caesium chloride gradient.

Replication starts at a definite point called the origin of replication. The two strands cannot be separated along their entire length because that needs very high energy, so they open in a small region called the replication fork. The main enzyme, DNA-dependent DNA polymerase, uses deoxyribonucleoside triphosphates both as substrates and as the energy source. It works very fast and with high accuracy, since mistakes would cause mutations.

DNA polymerase can polymerise only in the 5'→3' direction. On the template strand with 3'→5' polarity, the new strand is made continuously. On the template with 5'→3' polarity, it is made discontinuously as short fragments, which the enzyme DNA ligase later joins. In eukaryotes, replication takes place in the S-phase of the cell cycle, and failure of cell division after replication leads to polyploidy.

Points to remember

  1. Semiconservative replication: each daughter DNA has one parental strand and one newly made strand.
  2. Proof: Meselson and Stahl used 15N and 14N in E. coli with caesium chloride density gradient centrifugation.
  3. Main enzyme: DNA-dependent DNA polymerase; works only in the 5'→3' direction; dNTPs act as substrate and energy source.
  4. Continuous synthesis on the 3'→5' template; discontinuous synthesis as short (Okazaki) fragments on the 5'→3' template.
  5. DNA ligase joins the discontinuous fragments; replication begins at the origin of replication and proceeds at the replication fork.

Exam tip

Draw the replication fork with arrows for the direction of synthesis, mark the 5' and 3' ends of both templates, show the continuous and discontinuous strands, and label DNA polymerase and DNA ligase.

Questions students also ask

Why is DNA replication called semiconservative?
Each of the two daughter DNA molecules keeps one strand of the parent DNA and gets one newly made strand. Half of the original molecule is conserved in each copy, so the process is called semiconservative, not conservative, where the parent would stay fully intact.
What did the Meselson and Stahl experiment show?
E. coli grown in 15N medium had heavy DNA. After one generation in 14N medium, all DNA had hybrid (intermediate) density. After two generations, equal amounts of hybrid and light DNA appeared. This pattern fits only semiconservative replication.
Why is one new strand made discontinuously?
DNA polymerase adds nucleotides only in the 5'→3' direction, and the two template strands are antiparallel. On one template the new strand can grow continuously towards the fork, but on the other it must be built in short pieces, which DNA ligase joins later.

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