Precision to the very end: what happens when two replication forks converge during termination?

DNA编码的信息为所有生命提供了基础。为了使细胞分裂发生，细胞的整个DNA必须被完全复制，以确保母细胞和子细胞都能得到一个完整的副本。此外，复印件必须与原件一模一样。DNA的任何变化都可能是有害的，因为它们会改变细胞的功能，甚至导致细胞死亡。在单个细胞水平上，复制过程的准确性通常是如此之高，以至于当数百万个DNA碱基对被复制时，不会产生一个错误。这种非凡的高精确度是通过不同过程的网络来实现的，这些过程调节了复制过程，并编排了将两个完整的副本分离为母亲和女儿。然而，DNA的突变可以使这一过程网络失活，这是癌细胞以不受控制的方式生长的标志。我们正在研究复制过程的最后阶段。DNA的复制是在称为复制起点的特定位置开始的。两个称为复制分叉的复杂机器被招募到这些原点。这些复制叉能够高精度地复制DNA。在此过程中，它们以非常高的速度向相反的方向移动，直到遇到另一个相反方向的复制分叉。我们的研究表明，两个快速移动的复制分叉的碰撞有可能破坏DNA并引入突变。这种突变的原因是出乎意料的，因为当DNA被复制时，两个这样的复制叉的融合是一个基本的过程，我们已经确定了一些可以防止叉融合的有害后果的途径。目前，我们正在使用大肠杆菌来更好地了解这种分叉融合事件的机制。在大肠杆菌中，只有一个这样的融合事件发生，因为整个DNA只被两个复制叉复制。在我们自己的细胞中，复制过程从数百个起点开始，导致数百个融合事件，使研究变得更加复杂。大肠杆菌中叉融合的相对简单性将使我们能够用许多不同的工具对这些事件进行非常详细的研究。例如，我们将使用最近开发的显微镜技术，使我们能够直接可视化和跟踪活细胞内的单个复制分叉，以研究如果分叉融合会发生什么，并确定这些融合事件如何导致DNA的破坏。我们的研究结果将使我们能够生成一个DNA复制过程如何正常有序完成的模型，这将帮助我们了解我们自己细胞中的这一过程，我们的工作将揭示它是否可能有助于癌症和衰老的发展。

DNA encodes the information that provides the basis for all life. For cell division to take place the entire DNA of a cell has to be fully duplicated, ensuring that both the mother and the daughter cell can get one complete copy. In addition, the copy made has to be identical to the original. Any changes to the DNA can potentially be harmful, as they will alter how cells function, or even lead to cell death. On a single cell level, accuracy of the duplication process is normally so high that not a single error is made when the millions of DNA base pairs are copied. This extraordinary high level of accuracy is achieved by a network of different processes that regulate the duplication process and choreograph segregation of the two complete copies into mother and daughter. However, mutations in the DNA can inactivate this network of processes and it is a hallmark of cancer cells that they grow in an uncontrolled way. We are studying the final stages of the duplication process. Replication of DNA is initiated at specific sites called origins of replication. Two complex machines termed replication forks are recruited to these origins. These replication forks are capable of copying the DNA with high precision. While doing so they move in opposite direction at very high speed until they meet another replication fork coming the opposite way. Our research has revealed that the collision of two fast moving replication forks has the potential to corrupt the DNA and introduce mutations. This cause of mutation is unexpected, because the fusion of two such replication forks is a fundamental process when DNA is copied, and we have identified a number of pathways that can prevent the harmful consequences of fork fusions. Currently we are using the bacterium Escherichia coli to gain a better insight into the mechanics of such fork fusion events. In E. coli only one such fusion event occurs, as the entire DNA is copied by only two replication forks. In our own cells the duplication process is initiated at hundreds of origins, leading to hundreds of fusion events, making studies much more complex. The relative simplicity of fork fusions in E. coli will allow us to study these events at very high detail with many different tools. For example, we will use recently developed microscopy techniques which allow us to directly visualise and track single replication forks inside living cells to study what happens if forks fuse and to identify how these fusion events can cause corruption of the DNA. Our results will allow us to generate a model of how the duplication process of DNA is normally brought to an orderly completion, which will help us to understand this process in our own cells, and our work will reveal whether it might contribute to the development of cancer and ageing.