You are using a dideoxy sequencing method to sequence a DNA fragment. A sequence gel gives the following data. The correct sequence of this DNA fragment is . The reagents in the reaction ddA are dNTP, ddATP, DNA template, primer, and DNA polymerase. The reagents in the reaction ddG are dNTP, ddGTP, DNA template, primer, and DNA polymerase, and so on. The nucleotide being synthesized second during the DNA synthesis is .