DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. This semi-conservative process occurs during the S phase of the cell cycle and is essential for cell division. DNA replication is remarkably accurate, with an error rate of only one mistake per billion nucleotides. The process begins with the DNA double helix, which consists of two complementary strands connected by base pairs.
DNA replication begins with the initiation phase. This process starts at specific sites on the DNA called origins of replication. Initiator proteins bind to these origins, forming a replication bubble. Next, during the unwinding phase, an enzyme called helicase breaks the hydrogen bonds between the complementary base pairs and unwinds the DNA double helix. This creates replication forks where the two strands separate. Single-strand binding proteins, or SSBPs, then attach to the separated strands to prevent them from re-annealing and to protect them from degradation.
In the third step of DNA replication, primer synthesis occurs. The enzyme primase synthesizes short RNA primers on both template strands. These primers provide a free 3'-OH group that DNA polymerase requires to start adding nucleotides. Multiple primers are needed on the lagging strand. Next comes the elongation phase, where DNA polymerase adds nucleotides to the 3' end of the primers, following the base pairing rules: adenine pairs with thymine, and guanine pairs with cytosine. The leading strand is synthesized continuously in the 5' to 3' direction, moving toward the replication fork. The lagging strand, however, is synthesized discontinuously in short segments called Okazaki fragments, also in the 5' to 3' direction but moving away from the replication fork.
In the fifth step of DNA replication, primer removal and ligation occur. DNA polymerase I removes the RNA primers and replaces them with DNA nucleotides. Then, the enzyme DNA ligase joins the Okazaki fragments on the lagging strand, creating a continuous DNA strand. Finally, in the termination phase, replication ends when the replication forks meet or reach specific termination sequences. The enzymes involved in replication dissociate from the DNA, and two identical DNA molecules are formed. Each new DNA molecule contains one strand from the original DNA (the parental strand) and one newly synthesized strand. This pattern of inheritance is called semi-conservative replication.
让我们总结一下我们所学的DNA复制知识。DNA复制是一个半保留的过程，从一个原始分子产生两个完全相同的DNA分子。该过程包括六个关键步骤：在复制起始点开始、解旋酶解开DNA双螺旋、引物酶合成引物、DNA聚合酶进行延伸、去除引物并用DNA核苷酸替换，最后当复制叉相遇时终止。参与的关键酶包括解旋酶、引物酶、DNA聚合酶和DNA连接酶。复制是双向的，前导链进行连续合成，后随链进行不连续合成。整个过程非常精确，错误率仅为每十亿个核苷酸一个错误，确保遗传信息从一代到下一代的忠实传递。