DNA replication is the biological process by which a cell creates two identical copies of its DNA from one original DNA molecule. This process is fundamental for all living organisms, playing a role in cell division, growth, and the repair of damaged tissues. DNA replication ensures that each new cell receives a complete and accurate set of genetic instructions.
The Blueprint: DNA Structure
DNA, or deoxyribonucleic acid, is organized into a double helix. This structure consists of two long strands that run in opposite directions (antiparallel orientation). The two strands are held together by specific base pairings.
Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C), known as complementary base pairing. This precise pairing ensures that during replication, each original strand acts as a template for a new, complementary strand. The result is two new DNA molecules, each containing one old strand and one newly synthesized strand, a process termed semiconservative replication.
Essential Components for Replication
DNA replication involves several molecular components. DNA helicase is an enzyme that unwinds the double helix by breaking the hydrogen bonds between the base pairs, creating a Y-shaped structure called a replication fork. To prevent the separated strands from rejoining, single-strand binding proteins (SSBPs) attach to the single strands. Ahead of the replication fork, topoisomerase enzymes relieve the twisting tension as the DNA unwinds, preventing supercoiling.
DNA primase synthesizes short RNA primers, providing a starting point for DNA synthesis. DNA polymerase then attaches to these primers and begins adding new DNA nucleotides to the growing strand. DNA ligase joins DNA fragments together, sealing any gaps in the newly synthesized strands. Nucleoside triphosphates serve as the raw materials and energy source for building new DNA strands.
The Step-by-Step Process of Replication
Initiation
DNA replication begins at specific regions on the DNA molecule known as origins of replication. These origins are rich in adenine-thymine (A-T) base pairs, as these pairs are held together by fewer hydrogen bonds, making them easier to separate. Initiator proteins recognize and bind to these origins, recruiting other proteins to form a replication complex.
The DNA helicase within this complex then unwinds the double helix, creating a replication bubble with two Y-shaped replication forks that move in opposite directions. This unwinding exposes the single DNA strands, which will serve as templates for new DNA synthesis. This process ensures that replication occurs at the correct positions and times within the cell cycle.
Elongation
Once the DNA strands are separated, DNA primase synthesizes a short RNA primer on each template strand, as DNA polymerase cannot start a new strand from scratch. DNA polymerase then extends these primers by adding complementary DNA nucleotides to the 3′ end of the growing strand. Because the two original DNA strands run in opposite directions and DNA polymerase can only synthesize DNA in the 5′ to 3′ direction, the new strands are built differently.
One new strand, called the leading strand, is synthesized continuously towards the replication fork as the DNA unwinds. On this strand, only one RNA primer is needed at the beginning. The other new strand, known as the lagging strand, is synthesized discontinuously in short segments called Okazaki fragments.
This occurs because the lagging strand template runs in the 5′ to 3′ direction relative to the replication fork, requiring DNA polymerase to work backward. Each Okazaki fragment requires its own RNA primer. After the Okazaki fragments are synthesized, the RNA primers are removed and replaced with DNA nucleotides, and DNA ligase then joins these fragments together to form a continuous strand.
Termination
DNA replication continues until the replication forks meet each other or reach specific termination sequences on the chromosome. When two replication forks converge, the remaining intervening DNA is fully unwound and any small gaps are filled in and sealed. The replication machinery then disassembles.
In circular bacterial chromosomes, termination occurs at a specific locus directly opposite the origin of replication, where terminator proteins can pause the replication forks. In eukaryotic cells, which have multiple origins of replication, termination happens when replication forks from adjacent origins meet at random points along the chromosome. Following the completion of DNA synthesis, enzymes resolve any interlinked DNA molecules, ensuring the newly replicated chromosomes can separate properly.
Ensuring Accuracy: Proofreading and Repair
DNA replication is an accurate process, with cells employing mechanisms to minimize errors. One primary mechanism is the proofreading function of DNA polymerase. As DNA polymerase adds each new nucleotide, it checks whether it has correctly paired with the template strand.
If an incorrect base is detected, the enzyme immediately removes the mismatched nucleotide using its exonuclease activity. After removal, the correct nucleotide is inserted, and synthesis continues. Beyond proofreading during replication, cells also have other repair mechanisms that can correct errors that were missed after replication is complete. These layers of error correction contribute to the low error rate in DNA replication, around one mismatch per billion nucleotides added.