DNA Replication: Mechanisms and Key Concepts

• DNA replication is essential for cells to have an exact DNA copy, ensuring genetic consistency during cell division.
• DNA replication occurs during the S phase of interphase, before mitosis and meiosis.
• DNA replication is critical for growth, repair, and inheritance, enabling development, cell replacement, and genetic information transfer.
• DNA replication plays a role in DNA repair, which helps to correct errors that may arise during replication.

Mechanisms of DNA Replication

• DNA replication involves adding thousands of base pairs in order, managed by DNA polymerases with a low error rate.
• Proofreading enzymes check and correct mistakes during replication.
• Accuracy in DNA replication is important because mistakes can lead to mutations and diseases.

Key Concepts of DNA Replication

• DNA replication is a semi-conservative process, resulting in each new DNA molecule having one original and one new strand.
• The three main stages are initiation, elongation, and termination.
• The directionality of DNA synthesis is always 5’ to 3’.
• The leading strand is synthesized continuously, and the lagging strand is synthesized in Okazaki fragments.

Proposed Models of DNA Replication

• Conservative Replication involves the original DNA molecule remaining intact, and a completely new DNA molecule is synthesized.
• Dispersive Replication involves the parental DNA that is fragmented, and new DNA is synthesized in segments, leading to daughter strands that are a mix of old and new DNA.
• Semiconservative Replication involves the parental DNA strands separating, each serving as a template for a new strand, leading to two DNA molecules with one old and one new strand.

Steps of DNA Replication

• DNA replication begins at origins of replication, which are rich in adenine (A) and thymine (T) bases.
• Initiator proteins bind to the origin of replication, unwinding the DNA and forming a replication bubble with two replication forks.
• Multiple origins of replication allow for faster copying of large DNA molecules, creating several replication bubbles.

Unzipping the DNA Double Helix

• Topoisomerase loosens the tightly coiled DNA, and helicase breaks the hydrogen bonds, forming a replication fork.
• Single-stranded binding proteins (SSBs) stabilize the separated strands, preventing re-annealing and protecting nucleotides.
• The leading strand runs toward the replication fork in the 3’ to 5’ direction, and the lagging strand runs away from the fork in the 5’ to 3’ direction.

Summary of DNA Replication Process

• DNA replication results in two identical daughter molecules, each containing half of the original DNA and half newly synthesized DNA.
• The replication fork is a Y-shaped structure formed during DNA strand separation.
• Primase attaches to each template strand, initiating the synthesis of RNA primers.

Enzymes Involved in DNA Replication

• Primase synthesizes RNA primers for DNA polymerase.
• Topoisomerase loosens the DNA strand.
• Helicase unzips the DNA double helix at the origin of replication, creating a replication fork.
• Single Stranded Binding Proteins (SSB’s) keep the two strands separated after they unzip.
• DNA Polymerase III adds nucleotides and proofreads for errors.
• DNA Polymerase I removes RNA primers and replaces them with DNA nucleotides.
• DNA Ligase seals nicks between Okazaki fragments on the lagging strand.

Comparison of Prokaryotic and Eukaryotic Replication

• Eukaryotic DNA replication occurs at multiple origins.
• Prokaryotic DNA replication initiates at a single origin.
• The mechanisms are similar, but the complexity and regulation differ.

Detailed Process of DNA Replication

• Helicase unwinds the DNA double helix at the origin of replication.
• Single-stranded binding proteins (SSBs) stabilize the unwound DNA strands.
• Primase synthesizes RNA primers on the leading and lagging strands, allowing DNA polymerase to begin synthesis.

Elongation Phase

• DNA polymerase III binds to the RNA primer on the leading strand, adding complementary nucleotides in the 5’ to 3’ direction.
• On the lagging strand, DNA polymerase III synthesizes Okazaki fragments, each starting with an RNA primer.
• Continuous synthesis occurs on the leading strand, and discontinuous synthesis occurs on the lagging strand, requiring multiple RNA primers.

Termination of DNA Replication

• Replication concludes when the replication forks meet, and the enzymes detach from the DNA.
• Two identical DNA molecules result, each with one parental strand and one newly synthesized strand.
• Any remaining RNA primers are replaced with DNA, and DNA ligase seals any nicks.