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== Terminal deoxynucleotidyl transferase ==

[[File:Tdt-mechanism-graphic.png|Mechanism of Terminal deoxynucleotidyl transferase|thumb|right]]

'''Terminal deoxynucleotidyl transferase''' ('''TdT''') is a specialized DNA polymerase that catalyzes the addition of deoxynucleotides to the 3' ends of a DNA molecule. Unlike other DNA polymerases, TdT does not require a template strand, which makes it unique in its function. This enzyme is primarily found in the [[thymus]] and [[bone marrow]], where it plays a crucial role in the development of the [[immune system]].

== Function ==

TdT is involved in the process of [[V(D)J recombination]], which is essential for the generation of [[antibody]] diversity. During this process, TdT adds random nucleotides to the junctions between [[gene segments]] that encode the variable regions of [[immunoglobulins]] and [[T cell receptors]]. This addition of nucleotides increases the diversity of the antigen receptor repertoire, allowing the immune system to recognize a vast array of antigens.

== Mechanism ==

[[File:Tdt-template.png|Template-independent activity of TdT|thumb|left]]

TdT functions by adding deoxynucleotides to the 3' hydroxyl group of a DNA strand. It does not require a template strand, which distinguishes it from other DNA polymerases. The enzyme binds to the 3' end of a DNA molecule and catalyzes the addition of nucleotides in a template-independent manner. This activity is crucial for the generation of diversity in the immune system.

== Clinical Significance ==

TdT is a marker for [[lymphoid]] progenitor cells and is often used in the diagnosis of [[acute lymphoblastic leukemia]] (ALL). The presence of TdT in leukemic cells is indicative of a lymphoid origin, and its detection can aid in the classification of leukemia subtypes. Immunohistochemical staining for TdT is commonly used in pathology laboratories to identify lymphoblastic leukemia and lymphoma.

== Related Enzymes ==

[[File:Tdt-template-dependant.png|Template-dependent activity of other DNA polymerases|thumb|right]]

While TdT is unique in its template-independent activity, it is related to other DNA polymerases that require a template strand for DNA synthesis. These include [[DNA polymerase I]], [[DNA polymerase III]], and [[DNA polymerase alpha]], which are involved in DNA replication and repair processes. The ability of TdT to add nucleotides without a template is a specialized function that is critical for immune diversity.

== Related Pages ==

* [[DNA polymerase]]
* [[V(D)J recombination]]
* [[Acute lymphoblastic leukemia]]
* [[Immune system]]

[[Category:DNA replication]]
[[Category:Enzymes]]
[[Category:Immunology]]

Terminal deoxynucleotidyl transferase - Revision history

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