Mitochondrial helicase involved in mtDNA replication and repair. Might have a role in mtDNA repair. Has DNA strand separation activity needed to form a processive replication fork for leading strand synthesis which is catalyzed by the formation of a replisome complex with POLG and mtSDB. Preferentially unwinds DNA substrates with pre-existing 5'-and 3'- single-stranded tails but is also active on a 5'- flap substrate. Can dissociate the invading strand of immobile or mobile D-loop DNA structures irrespective of the single strand polarity of the third strand. In addition to its DNA strand separation activity, also has DNA strand annealing, DNA strand-exchange and DNA branch migration activities. ATP + H2O = ADP + H(+) + phosphate Homohexamer (via C-terminus) which assembled in a ring-like structure. Homoheptamer which assembled in a ring-like structure. Oligomerization is Mg(2+), nucleotide and DNA-independent, however, Mg(2+) and nucleotide stabilize the homohexameric form. Colocalizes with mtDNA in mitochondrial nucleoids, a nucleoproteins complex consisting of a number of copies of proteins associated with mtDNA, probably involved in mtDNA maintenance and expression. N-terminus enhances protein stability and hexamer formation, which is important for DNA binding, and is required for DNA helicase activity and, ultimately, for mtDNA replisome processivity. The N-terminus contains a putative primase-like domain; however the absence of the zinc binding domain and other motifs important for catalysis suggests that TWNK lacks primase activity.