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Question: Terminal transferase is used A. To add base at the 3’ end of the DNA B. To add base at the 5’ en...

Terminal transferase is used
A. To add base at the 3’ end of the DNA
B. To add base at the 5’ end of the DNA
C. To carry out nick translation
D. To transfer phosphate at the 3’ end of the DNA

Explanation

Solution

HINT: Transferase is the category of enzymes that catalyse the transfer of functional groups to an acceptor. Nitrogenous bases are added to the DNA molecule by the catalytic activity of transferase enzymes.

Complete answer:
The correct option is A.
Terminal transferase is a type of specialised DNA polymerase. It is also called terminal deoxynucleotidyl transferase (TdT) and DNA nucleotidyltransferase (DNTT). It is a template independent DNA polymerase, i.e. it does not require a template. The substrate for Terminal transferase’s activity can be both recessed or blunt ended single stranded DNA molecules or double stranded DNA molecules. However, the preferred substrate is 3’ overhangs.
Terminal transferase catalyse the sequential addition of nucleotides specifically on the 3’ hydroxyl terminal of DNA molecules. It exhibits both transferase activity and DNA dependent polymerase activity. Proofreading and exonuclease activity (in either 3’ or 5’ direction) is not catalysed by TdT. For the activity of terminal transferase, a divalent cation cofactor is imperative, just like in case of other polymerases. Terminal transferase can use a spectrum of cations owing to its quick binding abilities.
Terminal transferase is exclusively expressed in cells of the immune system like, B cells, T cells and lymphoblastic cells, where it undergoes protein-protein interactions for the purpose of regulation and inhibition. TdT has a broad range of practical applications like, 3’ end labelling of DNA molecules, addition of homopolymeric tails, in TUNEL assay and certain TdT dependent polymerase chain reactions.

NOTE: Terminal transferase add nucleotide bases exclusively on the 3’ terminal. It does not catalyse 5’ end nucleotide addition. Neither does it exhibit phosphate transfer activity. It is a DNA dependent polymerase and only catalyses synthesis of DNA molecules.