Biotechnology Terms Starting With T
Biotechnology Glossary: T
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TALENs
/ TAY-lens / · Acronym from Transcription Activator-Like Effector Nucleases, referring to proteins originally discovered in plant pathogenic Xanthomonas bacteria.
TALENs are engineered molecular scissors that pair customizable DNA-binding domains from bacterial proteins with a cutting enzyme called FokI to edit specific sections of DNA in living cells.
TALENs emerged in 2009 as a powerful gene editing tool based on naturally occurring TALE proteins from Xanthomonas bacteria that inject effectors into plant cells to manipulate gene expression. Each TALE repeat module consists of 33 to 35 amino acids, with two hypervariable residues at positions 12 and 13 that determine binding specificity to a single DNA base pair through a simple code. Arrays of these modules are assembled to recognize custom DNA sequences and fused to the FokI endonuclease domain, which requires dimerization to cut DNA, necessitating pairs of TALENs targeting opposite DNA strands.
Before CRISPR became dominant, TALENs were the premier gene editing technology, used to create knockout rats, engineer T cells for cancer immunotherapy, and modify livestock genomes with efficiencies reaching 50% in some cell types. TALENs require more complex molecular construction than CRISPR but offer advantages in targeting methylated DNA and producing fewer off-target effects in certain contexts.
The first FDA-approved cell therapy using gene editing employed TALENs to disrupt the T cell receptor alpha constant gene in allogeneic CAR-T cells, demonstrating clinical viability. Scientists have created Golden Gate assembly methods that can construct custom TALENs targeting any 14 to 20 base pair sequence in just one week, though CRISPR's simplicity has largely displaced this approach.
CRISPR completely replaced TALENs as a gene editing tool. TALENs remain superior for editing certain repetitive genomic regions where CRISPR guide RNAs cannot find unique target sequences, and they carry no off-target risk from Cas9 nuclease persistence since the TALEN protein dissociates immediately after cutting.
Recombinetics used TALENs to create hornless dairy cattle by editing the POLLED locus, eliminating the need for painful dehorning procedures in the agricultural industry. Researchers at the University of Minnesota employed TALENs to correct the mutation causing X-linked severe combined immunodeficiency in human hematopoietic stem cells, achieving stable gene correction rates of approximately 7%.
Ti Plasmid
/ TEE-EYE PLAZ-mid / · Tumor-inducing plasmid; Greek plasma, formed
Ti Plasmid Ti plasmid is a ring of DNA inside the soil bacterium Agrobacterium tumefaciens that naturally transfers a piece of itself into plant cells, causing tumors, and scientists have hijacked this ability to deliver useful genes into crop plants instead.
In nature, Agrobacterium infects a wound on a plant and injects a section of its Ti plasmid into the plant cell, where it gets permanently inserted into the plant’s own chromosomes. The inserted genes make the plant produce nutrients that only the bacterium can eat, so the bacterium benefits. Scientists remove the tumor-causing genes from T-DNA and replace them with any gene they want to introduce into the plant, while keeping the machinery that drives the DNA into plant cells.
Agrobacterium tumefaciens was first isolated in 1907 by Erwin Frank Smith and Charles Townsend from plant tumors called crown galls, but the role of the Ti plasmid in transferring DNA was not elucidated until the 1970s by Marc Van Montagu and Jeff Schell at Ghent University.
Differences Between Plant and Animal Cells →The entire Ti plasmid enters the plant cell during transformation. Only the T-DNA region between two border sequences gets transferred, while the rest of the plasmid stays inside the bacterium.
In nature, Agrobacterium tumefaciens uses its Ti plasmid to cause crown gall tumors on plants. In biotechnology, modified Ti plasmids can deliver useful genes.
Transgenic Organism
/ tranz-JEN-ik OR-gan-iz-um / · Latin trans, across; Greek genea, birth; Greek organon + ismos
Transgenic Organism transgenic organism is a living organism that has been permanently altered to carry and express one or more genes from another species inserted into its genome.
Transgenic animals are produced by microinjecting DNA into fertilized eggs, using viral vectors, or applying CRISPR-mediated knock-in; transgenic plants are predominantly generated by Agrobacterium or biolistics. Applications span from research transgenic mice expressing human disease genes provide essential models to medicine, with transgenic animals producing biopharmaceuticals in their milk. Ethical and regulatory frameworks governing transgenic organisms vary widely between jurisdictions.
A transgenic organism carries a gene introduced from another organism or source. The inserted gene can be inherited if it enters the germ line.
Transgenic and genetically modified always mean exactly the same thing. Transgenic specifically involves an introduced gene, while genetic modification can also include edits without foreign genes.
Genetic Engineering Pros and Cons →Transgenic zebrafish can carry a fluorescent protein gene that glows in specific tissues. Scientists use them to study development and disease.
Two-Photon Microscopy
/ TOO-FOH-ton my-KROS-kuh-pee / · From Greek duo, two, and photos, light, combined with mikros, small, and skopein, to view or examine.
Two-Photon Microscopy is an advanced fluorescence microscopy technique where two photons of longer wavelength light simultaneously excite a fluorophore, enabling deep tissue imaging with reduced phototoxicity.
Two-photon microscopy employs near-infrared light with wavelengths between 700 and 1000 nanometers to achieve tissue penetration depths exceeding 1 millimeter, which is approximately three to four times deeper than conventional confocal microscopy. The technique relies on the simultaneous absorption of two photons at the focal point, where the photon density is highest, resulting in fluorescence excitation that occurs only in a highly localized femtoliter-scale volume. This spatial confinement of excitation eliminates the need for emission pinholes and dramatically reduces photobleaching and phototoxicity in living tissues above and below the focal plane.
Scientists widely use two-photon microscopy for intravital imaging of brain tissue, allowing real-time visualization of neuronal activity and calcium dynamics in live animals at depths up to 800 micrometers beneath the cortical surface.
Two-photon microscopy enabled the first real-time visualization of individual synapses forming and retracting in living mouse brains, revealing that most new synapses appear and disappear within hours. The technique is so specific that only a volume approximately 1 femtoliter is excited, equivalent to a cube less than 1 micrometer on each side.
Two-photon microscopy is a deeper-penetrating version of confocal microscopy that works by the same physical principles. The two techniques differ fundamentally: two-photon microscopy depends on nonlinear multiphoton absorption requiring simultaneous photon arrival, whereas confocal microscopy uses single-photon linear excitation with pinhole-based detection.
At the Janelia Research Campus, scientists use two-photon microscopy to image calcium activity across thousands of neurons simultaneously in the brain of freely walking fruit flies (Drosophila melanogaster), tracking neural circuits controlling navigation and decision-making. The Allen Institute employs two-photon imaging to map functional connectivity in mouse visual cortex by recording responses of individual neurons to visual stimuli at depths exceeding 800 micrometers.