Provides the precursors necessary for DNA synthesis. Catalyzes the biosynthesis of deoxyribonucleotides from the corresponding ribonucleotides. [thioredoxin]-disulfide + a 2'-deoxyribonucleoside 5'-diphosphate + H2O = [thioredoxin]-dithiol + a ribonucleoside 5'-diphosphate 5'-deoxyadenosylcobalamine (coenzyme B12). Genetic information processing; DNA replication. This protein undergoes a protein self splicing that involves a post-translational excision of the intervening region (intein) followed by peptide ligation. Belongs to the ribonucleoside diphosphate reductase class-2 family.