DNA Replication Enzymes and Structure

DNA Replication Enzymes

• DNA ligase: Links DNA fragments by covalently bonding the 3' end of a new fragment to the 5' end of a growing DNA chain.
• DNA polymerase I: Adds nucleotides to a growing DNA strand at the replication fork and replaces RNA nucleotides in the RNA primer with DNA.
• DNA polymerase III: Responsible for elongating DNA by adding nucleotides to the 3' end of pre-existing strands or RNA primers.
• Primase: Synthesizes RNA primers by joining RNA nucleotides, providing a starting point for DNA synthesis.
• Nuclease: Enzymes that break down DNA and RNA into individual nucleotides by hydrolysis.
• Telomerase: Lengthens telomeres, containing RNA that serves as a template for new telomere DNA segments.

DNA Structure and Function

• Double helix: The native form of DNA, consisting of two polynucleotide strands coiled in a spiral formation.
• Telomeres: Repetitive DNA sequences at the ends of eukaryotic chromosomes that protect against degradation.
• Single strand binding protein (SSBP): Stabilizes unpaired DNA strands during replication, ensuring they remain open for complementary strand synthesis.

DNA Replication Processes

• Leading strand: Synthesized continuously in the 5' to 3' direction along the template strand.
• Lagging strand: Synthesized discontinuously, creating Okazaki fragments that are later joined by DNA ligase.
• Replication fork: The Y-shaped area on a replicating DNA molecule where new strands are forming.
• Origin of replication: The specific sites where DNA replication begins.
• Okazaki fragments: Short sections of synthesized DNA (100-200 nucleotides) on the lagging strand that must be connected by DNA ligase.

DNA Repair Mechanisms

• Mismatch repair: A process where enzymes fix incorrectly paired nucleotides during DNA replication.
• Nucleotide excision repair: Removes and replaces damaged segments of DNA using the undamaged strand as a template.

Additional Proteins in DNA Replication

• Helicase: Enzyme that unwinds the double helix, creating replication forks.
• Topoisomerase (gyrase): Relieves strain ahead of helicase by managing DNA supercoiling.

Concepts of DNA Replication

• Semiconservative model: Each resulting DNA molecule consists of one parent strand and one newly synthesized strand, preserving half of the original DNA.