Key Concepts of DNA Replication

• S Phase

  • This is the phase of the cell cycle where DNA replication occurs.
  • S stands for synthesis, meaning the making of chromosomal copies.

• Complementarity of DNA

  • DNA consists of two strands with complementary base pairs:
  • Adenine (A) pairs with Thymine (T)
  • Guanine (G) pairs with Cytosine (C)

• Antiparallel Orientation

  • The two strands of DNA run in opposite directions:
  • One strand is oriented from 3' to 5', and the other from 5' to 3'.

• Semi-Conservative Replication

  • When DNA is replicated, each new double helix consists of one old and one new strand.

• Speed of Replication

  • DNA replication is rapid, with approximately 1000 base pairs added per second.
  • Each human cell has millions of base pairs to replicate.

Enzymes Involved in DNA Replication

• Role of Enzymes
  • Various enzymes facilitate the process of DNA replication, including:
  1. Helicase - Unwinds the DNA double helix.
  2. Topoisomerase - Alleviates tension ahead of the replication fork during unwinding.
  3. Single-Strand Binding Proteins - Prevent strands from re-annealing after unwinding.
  4. DNA Polymerase - Synthesizes new DNA strands using existing strands as templates.
     • DNA Polymerase III is the main enzyme that synthesizes DNA in the 5' to 3' direction.
  5. Primase - Synthesizes an RNA primer that provides a starting point for DNA synthesis.

Process of DNA Replication

1. Unwinding the DNA

   • Helicase unwinds the double helix at the origin of replication, forming a replication bubble, and creates replication forks at each end.
   • Topoisomerase cuts and rejoins DNA strands to alleviate torsional strain during unwinding.

2. Stabilizing the Strands

   • Single-strand binding proteins attach to each strand to prevent re-annealing before replication is completed.

3. Building New Strands

   • DNA polymerase III attaches to the template strand only at a double-stranded region created by the action of primase (which lays down RNA primers).
   • Each enzyme starts synthesizing a new complementary strand from the RNA primer toward the replication fork (5’ to 3’).
   • The process occurs simultaneously on both strands, although due to the antiparallel nature, one strand is synthesized continuously while the other is synthesized in fragments (Okazaki fragments).

4. Completing the Replication

   • As replication progresses, the two DNA polymerase enzymes work in opposite directions along the two strands, utilizing the primers laid down by primase.
   • The fragments will eventually be joined together by DNA ligase once replication is complete.

Constraints of DNA Polymerase

• Directional Reading

  • DNA polymerase can only read the template strand from 3' to 5' and therefore synthesize a new strand in the 5' to 3' direction.

• Requirement for a Primer

  • DNA polymerase cannot initiate synthesis on a single-stranded template; it requires a primer that provides a free 3' end, which is supplied by primase.