DNA replication is the process by which a cell creates two identical copies of its DNA from one original molecule. This process is essential for the growth, repair, and reproduction of all living organisms, ensuring genetic information is accurately passed during cell division. It involves a precise and coordinated series of specialized proteins and enzymes.
Unzipping the Double Helix
DNA replication begins at specific points called replication origins. In bacteria, these origins are typically single, while eukaryotic genomes have multiple origins to facilitate the duplication of their larger chromosomes. Proteins recognize these origins and initiate the unwinding of the DNA double helix.
DNA helicase moves along the DNA, breaking the hydrogen bonds that hold the two complementary strands together, effectively “unzipping” the molecule. This unwinding creates a Y-shaped structure known as a replication fork. As helicase unwinds the DNA, topoisomerase works ahead of the fork to prevent the DNA from becoming excessively coiled or “supercoiled.” Topoisomerase achieves this by introducing temporary breaks in the DNA strands and then resealing them, relieving torsional stress.
Once the DNA strands are separated, single-strand binding proteins (SSBs) bind to the exposed single strands. These proteins prevent the separated strands from rejoining or forming secondary structures, ensuring they remain stable and accessible as templates. Before DNA synthesis can begin, a short RNA segment, called a primer, must be synthesized by primase. DNA polymerases cannot start a new strand from scratch; they can only add nucleotides to an existing strand. The RNA primer provides this starting point.
Copying the Genetic Code
The synthesis of new DNA strands is carried out by DNA polymerase. This enzyme adds complementary nucleotides to the exposed template strands, following base-pairing rules (A with T, G with C). DNA polymerase can only add nucleotides in one direction, from the 5′ end to the 3′ end of the new strand. This directional constraint means the two newly forming DNA strands are synthesized differently due to the antiparallel nature of the original DNA double helix.
One of the new strands, known as the leading strand, is synthesized continuously. Its template strand runs in the 3′ to 5′ direction, allowing DNA polymerase to move continuously towards the replication fork, adding nucleotides without interruption as the DNA unwinds. Only a single RNA primer is needed to initiate synthesis on the leading strand.
In contrast, the other new strand, called the lagging strand, is synthesized discontinuously. Its template runs in the 5′ to 3′ direction, meaning DNA polymerase must synthesize it in short segments, moving away from the replication fork. These short segments are known as Okazaki fragments. Each Okazaki fragment requires its own RNA primer to provide a starting point. In eukaryotic cells, Okazaki fragments are typically 100-200 nucleotides long, while in bacterial cells, they can range from 1000-2000 nucleotides.
Tidying Up and Checking for Errors
After the new DNA strands are synthesized, several steps complete the replication process and ensure genetic integrity. The RNA primers must be removed. In prokaryotes, DNA polymerase I removes these primers and replaces them with DNA nucleotides. In eukaryotes, enzymes like RNase H and Flap Endonuclease 1 (FEN1) remove the RNA primers. RNase H degrades the RNA component of the RNA-DNA hybrid, while FEN1 cleaves remaining flap structures.
Once the RNA primers are replaced with DNA, small gaps or “nicks” remain between the newly synthesized DNA segments, particularly between the Okazaki fragments on the lagging strand. The enzyme DNA ligase then seals these nicks by forming phosphodiester bonds, connecting the fragments into a continuous DNA strand.
Beyond simply joining fragments, DNA replication includes mechanisms to ensure accuracy. DNA polymerase enzymes possess a “proofreading” capability. As DNA polymerase adds nucleotides, it can detect and remove incorrectly paired bases. This proofreading involves a 3′ to 5′ exonuclease activity, allowing the enzyme to backtrack, excise the wrong nucleotide, and then insert the correct one before continuing synthesis. This self-correction mechanism significantly reduces the rate of errors during DNA replication.
The Importance of Accurate Copies
Accurate DNA replication is essential for the continuity of life. It ensures that when a cell divides, each daughter cell receives a complete and identical set of genetic instructions. This precise copying is necessary for processes such as cell division, including mitosis for growth and tissue repair, and meiosis for sexual reproduction.
Maintaining the integrity of an organism’s genetic information across generations depends on the fidelity of DNA replication. Errors that escape proofreading and repair mechanisms can lead to mutations, which are permanent changes in the DNA sequence. While some mutations can be benign or beneficial, others can have detrimental consequences, potentially affecting cell function or contributing to various diseases. The process of DNA replication is therefore fundamental to biological stability.