Summary
Highlights
DNA replication involves the separation of two complementary strands, with each serving as a template for a new strand. This process is called semi-conservative replication, resulting in molecules with one parent and one nascent strand. The video focuses on eukaryotic DNA replication and highlights that it occurs during the S stage of interphase, regulated by cell cycle checkpoints involving cyclins and CDKs.
DNA replication has three stages: initiation, elongation, and termination. Initiation involves licensing origins of replication to create two replication forks. Elongation is the synthesis of new DNA as forks progress. Termination marks the end of the process. DNA synthesis begins at origins of replication, which are specific genomic loci rich in A-T base pairs due to their weaker two hydrogen bond links.
In eukaryotes, origins of replication are nucleotide sequences that bind the Origin Recognition Complex (ORC), initiating replication at multiple origins on linear chromosomes. Prokaryotes, with their smaller, circular genomes, have only one origin of replication. Regions between eukaryotic replication units are called replicons.
Licensing of origins, which permits DNA replication, occurs during late M or early G1 phase. This involves the assembly of pre-replication complexes at origins. The ORC, in complex with CDC6, recruits the MCM hexamer-CDT1 complex, and then a second MCM hexamer-CDT1 complex. CDC6 and CDT1 act as loading factors and replicator activator proteins.
The MCM (mini-chromosome maintenance) complex is a replicative helicase that uses ATP hydrolysis to unwind DNA. MCM complexes are replication licensing factors, composed of six subunits (MCM 2 to 7). They allow DNA replication by separating the DNA strands. While loaded onto double-stranded DNA, MCM helicases are initially inactive to prevent premature replication or DNA exposure.
DNA helicase is activated when the cell enters S phase. Active DNA helicase, also known as the CMG complex (CDC45, MCM complex, and GINS), unwinds DNA, breaking hydrogen bonds and creating replication forks and a replication bubble. The MCM complex within CMG performs the primary unwinding. CDC45 and GINS prevent DNA from escaping the MCM complex.
Movement of the CMG helicase creates single-stranded DNA. Replication protein A (RPA) binds to this single-stranded DNA to prevent it from re-annealing and forming secondary structures that would hinder replication. Additionally, DNA unwinding creates torsional stress (positive supercoils) ahead of the replication fork. Topoisomerases are enzymes that temporarily break DNA strands to relieve this stress, allowing the replication fork to progress.