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NRTTIs are potent nucleoside analogs that are converted to their pharmacologically active triphosphate (TP) form via endogenous intracellular kinases. As shown in Fig 1, in contrast to all approved NRTIs, NRTTIs maintain a 3′-OH and are not obligate chain terminators. They inhibit reverse transcription by multiple distinct mechanisms. Firstly, the 4′ substituent blocks translocation of reverse transcriptase (RT) along the primer:template, preventing additional deoxynucleotide triphosphate (dNTP) binding and incorporation and resulting in immediate chain termination (ICT). Secondly, in the event that translocation does occur, the 3′-OH allows for the addition of one extra nucleotide before structural changes or distortion in the viral DNA occur, preventing further incorporation; this results in delayed chain termination (DCT) [23,24,30,31]. The mechanism resulting in DCT protects NRTTI-terminated primers from excision, thereby reducing the number of potential mechanisms of resistance to NRTTIs compared with approved NRTIs

Here's the simplified version:

NRTTIs are a type of antiviral drug used to stop HIV from copying itself. Like similar drugs (NRTIs), they enter cells and get activated by the cell's own enzymes.

What makes NRTTIs different from regular NRTIs is that they keep a small chemical group (3′-OH) that normal NRTIs lack. This gives them two ways to stop the virus:

Immediate stop (ICT): A chemical tag on the drug physically blocks the virus's copying machine (reverse transcriptase) from moving forward, halting the process right away.
Delayed stop (DCT): If the copying machine somehow moves past that block, the drug still allows one more building block to be added — but then the viral DNA becomes distorted and copying stops shortly after.

Why this matters: The delayed stop mechanism also makes it harder for the virus to "undo" the drug's effect. This means the virus has fewer ways to develop resistance to NRTTIs compared to older NRTI drugs.

In short: NRTTIs block HIV replication in two ways and are harder for the virus to resist than existing drugs.

how hiv viral RT creates DNA from RNA?

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Here's how HIV's reverse transcriptase (RT) converts RNA into DNA:

HIV Reverse Transcription: RNA → DNA

HIV is a retrovirus — it carries its genetic material as single-stranded RNA (ssRNA). To replicate inside human cells, it must convert that RNA into DNA. This is done by its enzyme reverse transcriptase (RT), an RNA-dependent DNA polymerase not found in normal human cells.

Step-by-Step Process

1. Entry into the host cell HIV's surface protein gp120 binds to the CD4 receptor on T-helper cells, then attaches to a co-receptor (CCR5 or CXCR4). The viral envelope fuses with the cell membrane and the viral core is released into the cytoplasm.

2. Uncoating The capsid (protein shell) opens, releasing the viral contents: two copies of ssRNA + the enzymes RT and integrase.

3. Reverse Transcription (RNA → DNA) RT performs three activities in sequence:

Activity	What happens
RNA-dependent DNA polymerase	RT uses the viral RNA as a template to synthesize a complementary DNA strand → forming an RNA:DNA hybrid
RNase H activity	RT degrades the original RNA strand from the hybrid
DNA-dependent DNA polymerase	RT uses the remaining DNA strand as a template to synthesize the second DNA strand → producing double-stranded DNA (dsDNA)

The result is a complete double-stranded viral DNA copy of the original RNA genome.

4. Nuclear import & Integration The dsDNA is transported into the nucleus, where the enzyme integrase inserts it into the host's chromosomes — at this point it is called a provirus.

5. Viral production The integrated provirus directs the host cell's machinery to produce new viral RNA and proteins, which assemble into new HIV particles that bud off and infect more cells.

Why RT is a Key Drug Target

RT lacks proofreading ability, so it makes frequent errors — this is why HIV mutates so rapidly. This also makes it the most targeted enzyme in HIV therapy (NRTIs, NNRTIs, and the newer NRTTIs from your earlier text all block this step).

HIV life cycle showing reverse transcriptase converting viral RNA to DNA inside a CD4 lymphocyte

HIV life cycle — step 3 shows RT converting viral RNA into DNA inside the CD4 lymphocyte (Kaplan & Sadock's Comprehensive Textbook of Psychiatry)

Sources: Biochemistry, 8th ed. — Lippincott Illustrated Reviews, Basic Medical Biochemistry — A Clinical Approach, 6e, Kaplan & Sadock's Comprehensive Textbook of Psychiatry

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