beta, RNAse H, ligase, and DNA2. This process is well-characterised in bacteria and much less well-characterised in many eukaryotes. In general, DNA repair enzymes complete the Okazaki fragments through a variety of means, including: base pair excision and 5' to 3' exonuclease activity that removes the chemically unstable ribonucleotides from the lagging duplex and replaces them with stable deoxynucleotides. This process is referred to as 'maturation of Okazaki fragments', and ligase (see below) completes the final step in the maturation process. Primer removal and nick ligation can be thought of as DNA repair processes that produce a chemically-stable, error-free duplex. To this point, with respect to the chemistry of an RNA-DNA duplex, in addition to the presence of uracil in the duplex, the presence of ribose (which has a reactive 2' OH) tends to make the duplex much less chemically-stable than a duplex containing only deoxyribose (which has a non-reactive 2' H). DNA polymerase I DNA polymerase I is an enzyme that repairs DNA. RNAse H RNAse H is an enzyme that removes RNA from an RNA-DNA duplex. Ligase After DNA repair factors replace the ribonucleotides of the primer with deoxynucleotides, a single gap remains in the sugar-phosphate backbone between each Okazaki fragment in the lagging duplex. An enzyme called DNA ligase connects the gap in the backbone by forming a phosphodiester bond between each gap that separates the Okazaki fragments. The structural and chemical aspects of this process, generally referred to as 'nick translation', exceed the scope of this article. Replication stress Replication stress can result in a stalled replication fork. One type of replicative stress results from DNA damage such as inter-strand cross-links (ICLs). An ICL can block replicative fork progression due to failure of DNA strand separation. In vertebrate cells, replication of an ICL-containing chromatin template triggers recruitment of more than 90 DNA repair and genome maintenance factors. These factors include proteins that perform sequential incisions and homologous recombination. History Katherine Lemon and Alan Grossman showed using Bacillus subtilis that replisomes do not move like trains along a track but DNA is actually fed through a stationary pair of replisomes located at the cell membrane. In their experiment, the replisomes in B. subtilis were each tagged with green fluorescent protein, and the location of the complex was monitored in replicating cells using fluorescence microscopy. If the replisomes moved like a train on a track, the polymerase-GFP protein