Human origins Edit
Born in 2303, Juliana O'Donnell lived at the Omicron Theta colony. She met Doctor Noonian Soong on the colony, and the two fell in love. However, her mother disapproved of their relationship due to the age gap, so they met in secret. They boarded the SS Wisconsin in 2328 to get married in secret on Mavala IV. Though they had no biological children, she considered each of the Soong-type androids like children.
O'Donnell assisted Soong in his work on artificial lifeforms, and helped create the android Data, as well as his four predecessors, including Lore and B-4. Juliana viewed each android as akin to a child; after Lore's personality problems forced his deactivation, she was incredulous that Soong wanted to try again. When creating Data, Juliana was responsible for giving Data the capability to later play music and paint, as well as other creative outlets, feeling that he deserved a chance to express himself even if he lacked the emotional capacity to explore that side of himself.
When the Crystalline Entity attacked the colony in 2338, the Soongs fled the planet. Juliana, fearing that Data would eventually become unstable like Lore, insisted that he be left behind, though this caused her great guilt. The two made their way to the Terlina system; however, Juliana had been seriously injured in the attack, and fell into a coma.
In a last attempt to preserve her memory, Soong built a new android and transferred his wife's memories into it, using a synaptic scanning technique. He made several modifications from his earlier androids: skin with the appearance of veins, capillaries, tear ducts, and sweat glands. He also included a feedback processor that sent out a false bio-signal, self-adjusting to mimic Human aging. She was programmed to eventually deactivate, her positronic matrix self-destructing, giving her the appearance of a natural death. Tainer was also fitted with an internal supplemental holographic program that was activated in the event of her incapacitation. The program responded variously depending upon the identity of whomever was accessing it. Following Juliana's death, Soong activated the android, who remained unaware of her new state of being.
Android replacement Edit
Noonian designed Juliana to be unaware of her android nature, initializing her consciousness to pick up from the point of the attack on Omicron Theta by the Crystalline Entity that had left her in an irreversible coma. Noonian programmed Juliana to shut down in the unlikely event of her android nature being discovered. The relationship between the Soongs continued for a long, but unspecified period of time, souring when Noonian became too focused on his work. Juliana left Noonian – something that Noonian noted the 'original' Julianna would have done as well. She eventually made her way to Atrea IV, where she met and married the Atrean scientist Pran Tainer, becoming his assistant.
In 2370, the USS Enterprise-D arrived at Atrea IV to assist in planetary core experiments. Data and Tainer were reunited, although Data was unaware of her possible role in his life as his formative memories of those days had been erased. Although Data was initially uncertain about how he should feel about Tainer's claims regarding her role in his initial design and early development, a conversation with Geordi La Forge convinced him that he should not avoid Tainer due to his inability to remember her. From this point he began to spend time with Tainer, addressing her as his mother. Data and Tainer bonded over their mutual creative outlet, Data playing violin and Tainer, the viola. Tainer shortly revealed that she was responsible for leaving Data behind on Omicron Theta because she was afraid of him turning out like Lore.
During the mission, Tainer was injured and her true nature as an android was discovered. Data noted that he had observed clues about her android nature before this point, as her blinking pattern was based on the same pseudo-random sequence that Soong had used for his own, and her viola performance was identical to her practice.
Data interacted with the personalized version of the holoprogram Soong designed to activate in the event that Tainer's android nature was revealed. From this, Data chose not to reveal Tainer's status as an android to her. Data's decision regarded the happiness he believed Tainer obtained from her false belief in her humanity. Data prioritized this over whatever satisfaction he may have received in the shared knowledge of their android nature. Before Tainer departed, Data told her that Soong had once told him that he only ever had one great love in his life, expressing his certainty that she was the object of that love. (TNG: "Inheritance")
Background information Edit
Juliana Tainer was played by actress Fionnula Flanagan and doubled by stuntwoman Christine Anne Baur. One of Fionnula Flanagan's costumes from "Inheritance" was sold off on the It's A Wrap! sale and auction on eBay. . Christine Anne Baur's Juliana Tainer costume was also sold off on eBay. 
In the script of "Inheritance", the character was described as "a fiftyish [Human] woman dressed in Atrean fashion."  The name "Juliana Tainer" was chosen by co-writer Dan Koeppel, after mountain bike racer Juli Furtado. 
The novel Immortal Coil establishes that Juliana eventually did seem to die in the year 2374, only to be reactivated with full knowledge of who and what she was by the immortal known as Flint. In the novel Cold Equations: The Persistence of Memory, Data learns of Flint's role in reactivating Juliana, deciding to try to find him so that Flint might help Data reactivate Lal.