Unit 1: DNA Replication

1. Nature of Replication: Semi-conservative, Bidirectional, and Semi-discontinuous

DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. This process is the basis for biological inheritance and occurs in all living organisms.

Semi-conservative Replication

According to this model, each of the two strands of the original DNA molecule serves as a template for the synthesis of a new complementary strand. Semi-conservative means that in each resulting DNA double helix, one strand is "conserved" from the parent molecule, and one strand is newly synthesized.

Key Proof: The Meselson-Stahl experiment (1958) used heavy nitrogen (15N) and light nitrogen (14N) to prove that DNA replicates semi-conservatively.

Bidirectional Replication

Replication begins at a specific sequence called the Origin of Replication (ori). Once the DNA unwinds, two replication forks move in opposite directions away from the origin. This bidirectional movement allows for much faster copying of large genomes.

Semi-discontinuous Replication

DNA polymerase enzymes can only add nucleotides in one direction: 5' to 3'. Because the two strands of the DNA double helix are antiparallel (one 5' to 3', the other 3' to 5'), the replication machinery must handle them differently:

• Leading Strand: Synthesized continuously toward the replication fork.
• Lagging Strand: Synthesized discontinuously away from the replication fork in short segments called Okazaki fragments.

Because one strand is continuous and the other is discontinuous, the overall process is termed semi-discontinuous.

2. Mechanism of DNA Replication in Prokaryotes

In prokaryotes like E. coli, replication occurs in the cytoplasm and involves a highly efficient enzymatic machinery.

Step 1: Initiation

• Replication starts at a single origin called oriC.
• DnaA proteins bind to the origin to initiate unwinding.
• Helicase (DnaB) unwinds the double helix by breaking hydrogen bonds.
• Single-Stranded Binding (SSB) proteins stabilize the unwound strands to prevent them from re-annealing.
• DNA Gyrase (Topoisomerase) relieves the torsional strain (supercoiling) ahead of the fork.

Step 2: Elongation

• Primase: Synthesizes short RNA primers. DNA polymerase III cannot start a new strand; it can only add nucleotides to an existing 3'-OH group.
• DNA Polymerase III: The primary enzyme for elongation. It adds deoxyribonucleotides (dNTPs) to both the leading and lagging strands.
• Okazaki Fragment Processing:
  1. DNA Polymerase I: Removes RNA primers (exonuclease activity) and replaces them with DNA nucleotides.
  2. DNA Ligase: Seals the nicks between the DNA fragments to create a continuous sugar-phosphate backbone.

Step 3: Termination

• Replication forks meet at a specific termination site (ter).
• A protein called Tus binds to these sites to stop the movement of the replication forks.

3. Basic Difference between Prokaryotic and Eukaryotic Replication

While the fundamental mechanism is conserved, eukaryotic replication is more complex due to larger genome sizes and the presence of linear chromosomes.

4. Exam Focus Enhancements