Dioxygenase that catalyzes the conversion of the modified genomic base 5-methylcytosine (5mC) into 5-hydroxymethylcytosine (5hmC) and plays a key role in epigenetic chromatin reprogramming during embryonic development. Conversion of 5mC into 5hmC probably constitutes the first step in cytosine demethylation. Selectively binds to the promoter region of target genes and contributes to regulate the expression of numerous developmental genes, including pax6, rax, sox9 and six3. May also contribute to the regulation of target genes in ways that do not require its enzyme activity. 2-oxoglutarate + a 5-methyl-2'-deoxycytidine in DNA + O2 = a 5-hydroxymethyl-2'-deoxycytidine in DNA + CO2 + succinate Binds 1 Fe(2+) ion per subunit. The zinc ions have a structural role. Detected on chromatin, where it binds to target gene promoters. Detected in embryo (at protein level). Detected in embryonic head, in developing brain and eye. Expression in oocytes is very low. Barely detectable from stage 2 (2-cell stage) to stage 9. Increases strongly from stage 9 to 17, and decreases again at stage 25. Binds target DNA that contains at least one unmethylated cytosine via the CXXC-type zinc-finger domain. Morpholino knockdown of both tet3-a and tet3-b causes defects in embryonic development, including malformations of the eye (eyeless), small head, and missing pigmentation along the lateral body, leading to embryonic death between stages 35 and 40. Belongs to the TET family.