DNA Replication

• DNA replication is a semi-conservative process where each strand of the original DNA molecule serves as a template for the synthesis of a new complementary strand.
• The process begins at specific locations called origins of replication, where the DNA double helix unwinds and separates into two single strands.
• The unwound region forms a replication fork, where new DNA strands are synthesized.
• There are multiple origins of replication in eukaryotic cells, allowing for faster replication.
• Nucleic acids are synthesized in the 5' to 3' direction, meaning new nucleotides are added to the 3' end of the .
• Nucleic acids are coupled to the energy from the hydrolysis of nucleoside triphosphates (NTPs), which provide the necessary energy for the formation of phosphodiester bonds.
• The leading strand is synthesized continuously in the direction of the replication fork, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments.
• Enzymes called DNA polymerases synthesize new strands by adding nucleotides to the 3' carbon of the deoxyribose sugar, complementary to the template strands, following base-pairing rules (A-T, C-G).
• Repair mechanisms include proofreading by DNA polymerases, which correct errors during synthesis, and mismatch repair systems that fix errors after replication.

Other Key Enzymes and Proteins in DNA Replication

• Helicase: Unwinds the DNA double helix at the replication fork.
• DNA Polymerase: Synthesizes new DNA strands by adding nucleotides.
• Primase: Synthesizes short RNA primers to initiate DNA synthesis.
• Ligase: Joins Okazaki fragments on the lagging strand.
• Single-Strand Binding Proteins (SSBs): Stabilize unwound DNA strands to prevent them from re-annealing.
• Topoisomerase: Relieves torsional strain ahead of the replication fork by cutting and rejoining the DNA strands.
• Sliding Clamp (PCNA in eukaryotes): Holds DNA polymerase in place on the template strand, increasing its processivity.

Dive deeper: While DNA polymerase synthesizes in the 5'-3' direction, exonuclease (another DNA polymerase.proofreads the newly synthesized DNA strand in the 3'-5' direction. Point and frameshift mutations (more harmful) are caused by errors in DNA polymerase.Replication slippage is the process by which DNA polymerase temporarily dissociates from the template strand, leading to insertions or deletions.

Practice Questions