DNA replication is the process by which a eukaryotic cell duplicates its DNA. This is a fundamental action for growth, tissue replacement, and passing genetic information to the next generation. The process ensures each new cell receives a complete and accurate set of genetic instructions and occurs during the S phase of the cell cycle.
Preparing the DNA for Copying
Eukaryotic genomes are vast; the human genome, for example, contains approximately 6 billion base pairs that must be copied. To complete this task efficiently, replication begins simultaneously at thousands of specific locations along the chromosomes called “origins of replication”. In humans, there can be up to 100,000 such origins, ensuring the entire genome can be copied in a timely manner.
Initiator proteins identify these origins to start the process. In yeast, origins are marked by a specific DNA sequence, while in humans, the signals relate to how DNA is packaged. Once an origin is identified, these proteins attach and recruit other proteins to form a pre-replication complex.
Among the first proteins recruited is an enzyme called helicase. Using energy from ATP, helicase unwinds the double helix, separating the two strands. This unzipping action creates two Y-shaped structures known as replication forks, which move in opposite directions from the origin. This provides the single-stranded templates for the replication machinery to access.
Building the New DNA Strands
With the DNA strands separated, the primary enzyme, DNA polymerase, begins synthesizing new DNA. It can only add new nucleotides in a one-way direction. This constraint means the two new strands at each replication fork must be synthesized differently.
One new strand, the leading strand, is built in a continuous line. Its synthesis is straightforward as it follows the same direction as the moving replication fork. DNA polymerase continuously adds nucleotides as the template is exposed, requiring only a single starting signal.
The other new strand, the lagging strand, is more complex. It must be built in the opposite direction of the fork, so it is assembled in short, separate segments called Okazaki fragments. This piecemeal approach requires a more intricate setup.
To initiate each Okazaki fragment, an enzyme called primase lays down a short RNA primer. This primer provides a starting point for DNA polymerase to add DNA nucleotides. Once a fragment is complete, the machinery moves to start the next one, resulting in a series of disconnected pieces.
Ensuring Accuracy and Finalizing the Copy
Replicating DNA at high speed can introduce errors. To maintain accuracy, DNA polymerase has a proofreading function. As it adds new nucleotides, it simultaneously checks its work. If the polymerase detects a mismatch, it pauses, removes the incorrect unit, and inserts the correct one, reducing the error rate to about one mistake per billion nucleotides.
After the new strands are synthesized, the process is finalized. First, specialized enzymes remove the RNA primers that initiated synthesis on both the leading and lagging strands. A different type of DNA polymerase then fills these gaps with the correct DNA nucleotides. Finally, the enzyme DNA ligase seals the gaps between the fragments, creating two continuous and complete DNA molecules.
The Challenge of Linear Chromosomes
The linear shape of eukaryotic chromosomes creates an issue at the very ends during replication. On the lagging strand, the final Okazaki fragment requires a primer at the chromosome’s tip. Once this primer is removed, the resulting gap cannot be filled because DNA polymerase needs a pre-existing strand to add to.
This “end replication problem” means a small portion of DNA is lost from the chromosome ends with each cell division. To prevent the loss of genetic information, the ends are capped with protective structures called telomeres. Telomeres are long, repetitive sequences of non-coding DNA that act as disposable buffers, allowing them to be shortened without affecting genes.
The progressive shortening of telomeres is linked to cellular aging, as cells eventually lose their ability to divide when telomeres become too short. However, some cells possess an enzyme called telomerase, which can counteract this shortening. Telomerase works by adding the repetitive telomere sequences back onto the ends of the chromosomes.
Telomerase is active in stem cells and germ cells, which divide many times, but its activity is low in most other body cells. This enzyme is also relevant to cancer research. Many cancer cells reactivate telomerase, allowing them to bypass normal cellular aging and divide uncontrollably.