Question

The drugs ddC and AZT are used to treat AIDS.Based on their chemical structures, what is their mode of action?

Accepted Answer

Step 1: Identify the chemical nature of ddC and AZT by examining their structures. Both are nucleoside analogs, meaning they resemble natural nucleosides but have modifications—in ddC, the 2' and 3' hydroxyl groups are missing (dideoxy), and in AZT, there is an azido group (-N3) at the 3' position instead of a hydroxyl group. Step 2: Understand the role of natural nucleosides in DNA synthesis. During DNA replication, nucleosides with a free 3'-OH group are incorporated into the growing DNA strand by DNA polymerase, allowing the addition of the next nucleotide via a phosphodiester bond. Step 3: Analyze how the modifications in ddC and AZT affect DNA synthesis. Because both lack a 3'-OH group (ddC has none, AZT has an azido group instead), once incorporated into viral DNA by reverse transcriptase, they prevent the formation of the next phosphodiester bond, effectively terminating DNA chain elongation. Step 4: Connect this mechanism to their use in AIDS treatment. HIV relies on reverse transcriptase to convert its RNA genome into DNA. ddC and AZT act as chain terminators by being incorporated into viral DNA, thereby inhibiting reverse transcriptase and blocking viral replication. Step 5: Summarize the mode of action: ddC and AZT are nucleoside reverse transcriptase inhibitors (NRTIs) that mimic natural nucleosides but cause premature termination of viral DNA synthesis due to the absence of a 3'-OH group necessary for chain elongation.

The drugs ddC and AZT are used to treat AIDS.

Based on their chemical structures, what is their mode of action?

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Key Concepts

Nucleoside Analog Reverse Transcriptase Inhibitors (NRTIs)

Reverse Transcriptase Enzyme Function

Mechanism of Chain Termination