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Question:

Draw a neat labelled sketch of a replicating fork of DNA.

Solution:

A replicating fork of DNA is a Y-shaped structure formed during DNA replication. It's where the double helix unwinds and separates into two single strands, creating a replication fork. The sketch should show:

  • The parental DNA double helix: Depict the two antiparallel strands (5' to 3' and 3' to 5') with base pairs (A-T, G-C) connected by hydrogen bonds. Show the double helix unwinding.
  • The replication fork itself: Clearly indicate the Y-shaped structure where the strands are separating.
  • Leading strand: Show the new DNA strand being synthesized continuously in the 5' to 3' direction, following the replication fork.
  • Lagging strand: Show the new DNA strand being synthesized discontinuously in short fragments called Okazaki fragments, in the 5' to 3' direction, moving away from the replication fork.
  • RNA primers: Indicate the short RNA sequences that initiate DNA synthesis on the lagging strand.
  • DNA polymerase: Show the enzymes (DNA polymerase III primarily) that are adding nucleotides to the growing DNA strands. You might label them near the leading and lagging strand synthesis.
  • DNA helicase: Indicate the enzyme unwinding the DNA double helix. It is usually positioned at the replication fork.
  • Single-stranded binding proteins (SSBPs): Show these proteins stabilizing the separated single strands of DNA, preventing them from re-annealing.
  • Topoisomerase (DNA gyrase): Label this enzyme, showing its role in relieving torsional stress ahead of the replication fork.
  • Primase: Show the enzyme synthesizing RNA primers.
  • DNA ligase: Show this enzyme joining Okazaki fragments on the lagging strand.

Labels: All the components listed above should be clearly labeled in the sketch. Arrows can be used to indicate the direction of DNA synthesis.

The sketch should accurately represent the antiparallel nature of DNA strands and the difference in synthesis mechanisms between the leading and lagging strands. The detail level will depend on the desired complexity of the sketch; a simpler sketch might omit some of the enzymes.