DNA Replication in Eukaryotes

The replication of DNA in eukaryotic cells is a highly coordinated process that ensures accurate duplication of the genetic material during cell division. It involves several stages and requires the participation of numerous enzymes and proteins. Here is a detailed description of the eukaryotic replication of DNA:

Initiation

The process of DNA replication begins at specific sites on the DNA molecule called origins of replication. These origins are recognized by a complex of proteins known as the pre-replication complex (pre-RC). The pre-RC assembles during the G1 phase of the cell cycle and marks the regions of DNA that will be replicated during the subsequent S phase.

Unwinding and Stabilization

Once the pre-RC is formed, it recruits a helicase enzyme known as the MCM complex (minichromosome maintenance complex). The MCM complex unwinds and separates the double-stranded DNA at the origin, creating a replication bubble. Single-stranded DNA-binding proteins (SSBPs) bind to the exposed single strands of DNA, preventing them from reannealing and stabilizing the unwound DNA.

Primer Synthesis

DNA replication in eukaryotes is discontinuous on the lagging strand, which requires the synthesis of RNA primers to initiate DNA synthesis. The enzyme primase synthesizes short RNA primers complementary to the single-stranded DNA template at regular intervals along the lagging strand.

DNA Synthesis

The actual synthesis of new DNA strands is carried out by DNA polymerases. In eukaryotes, the main replicative DNA polymerase is DNA polymerase δ (pol δ), which is responsible for synthesizing both the leading and the lagging strands. DNA polymerase α (pol α) initiates DNA synthesis by extending the RNA primers laid down by primase, and then pol δ takes over. The leading strand is synthesized continuously in the 5′ to 3′ direction, while the lagging strand is synthesized in short fragments called Okazaki fragments, also in the 5′ to 3′ direction.

Proofreading and Repair

DNA polymerases have proofreading activity, which means they can detect and correct errors made during DNA synthesis. If an incorrect nucleotide is incorporated, the polymerase can remove it and replace it with the correct one. Additionally, the DNA mismatch repair system further checks the newly synthesized DNA for errors and corrects them.

Termination

The replication process continues bidirectionally until the replication forks from neighboring origins meet. At this point, the replication machinery encounters a termination site where the replication process is halted. The DNA strands are fully synthesized, and the replication machinery is disassembled. The two newly replicated DNA molecules, each consisting of one original strand and one newly synthesized strand, are now ready for further processing and cell division.

It’s important to note that eukaryotic DNA replication is a highly regulated process, tightly controlled by various checkpoint mechanisms to ensure accurate replication and prevent errors. The coordination of multiple enzymes and proteins ensures the faithful duplication of the entire genome, leading to the transmission of genetic information to daughter cells during cell division.