Q.During in vitro synthesis of DNA, a researcher used 2', 3'-dideoxy cytidine triphosphate as raw nucleotide in place of 2'-deoxy cytidine. What would be the consequence?

During normal DNA synthesis, DNA polymerase adds deoxyribonucleotides, which possess a hydroxyl group at the 3' position of their sugar. This 3'-OH group is crucial for forming a phosphodiester bond with the incoming nucleotide, allowing the DNA chain to elongate.

However, 2',3'-dideoxy cytidine triphosphate (ddCTP) is a modified nucleotide that lacks a hydroxyl group at both the 2' and 3' carbons of its sugar. If ddCTP is incorporated into the growing DNA strand by DNA polymerase, the absence of the 3'-OH group prevents the formation of any subsequent phosphodiester bond. This immediately halts the elongation of the DNA strand at the point of ddCTP inco …

To understand the consequence of using 2', 3'-dideoxy cytidine triphosphate (ddCTP) in place of 2'-deoxy cytidine triphosphate (dCTP) during in vitro DNA synthesis, we must first grasp the fundamental mechanism by which DNA is synthesized and the role of DNA polymerase.

DNA polymerase is the enzyme responsible for synthesizing new DNA strands. It does this by adding nucleotides one by one to the 3'-end of a pre-existing DNA strand (or primer). The enzyme reads a template strand and incorporates complementary nucleotides. For this process to continue, each incoming nucleotide must form a phosphodiester bond with the previous nucleotide in the growing chain. This bond formation critically depends on the presence of a free hydroxyl group at the 3' position of the deoxyribose sugar of the last nucleotide added to the growing strand.

Normally, in DNA synthesis, the raw materials are 2'-deoxyribonucleoside triphosphates (dNTPs), such as dATP, dGTP, dCTP, and dTTP. Let's consider 2'-deoxy cytidine triphosphate (dCTP). This molecule consists of a cytosine base, a deoxyribose sugar, and three phosphate groups. Crucially, its deoxyribose sugar has a hydroxyl (-OH) group at the 3' carbon position. When DNA polymerase adds dCTP to a growing DNA strand, it cleaves off two phosphate groups, and the remaining cytidine monophosphate forms a phosphodiester bond between its 5'-phosphate and the 3'-hydroxyl group of the preceding nucleotide. The newly incorporated dCTP then presents its own 3'-hydroxyl group, which is ready to accept the next incoming nucleotide, allowing the chain elongation to continue seamlessly.

Now, let's consider 2', 3'-dideoxy cytidine triphosphate (ddCTP). The key difference lies in its sugar component. Instead of a deoxyribose sugar, it contains a dideoxyribose sugar. This means that it lacks hydroxyl groups at both the 2' and the 3' carbon positions; instead, it has hydrogen atoms at these positions.

When a researcher uses ddCTP as a raw nucleotide in place of dCTP during in vitro DNA synthesis, the following consequence occurs:

• Incorporation: DNA polymerase can still recognize and incorporate ddCTP into the growing DNA strand because its base (cytosine) is complementary to a guanine on the template strand, and its 5'-phosphate can form a phosphodiester bond with the 3'-hydroxyl of the preceding nucleotide. …

Instead of describing the missing hydroxyl group in isolation, work forward from what a phosphodiester bond actually needs: DNA polymerase links the 5'-phosphate of an incoming nucleotide to the free 3'-OH of the nucleotide already at the end of the chain. Ask: after ddCTP is added, is a 3'-OH still available at the new chain end? Since dideoxyribose has no -OH at either the 2' or 3' carbon, the answer is no -- which is exac …