Q.Choose the correct option regarding Retrovirus:

A retrovirus is an RNA virus that carries the enzyme reverse transcriptase. When it infects a host cell, it uses this enzyme to synthesise a DNA copy from its RNA genome. This newly made DNA then integrates into the host's chromosomal DNA. The NCERT textbook specifically describes retroviruses as viruses having an RNA genome that, upon infection, produce DNA — a process central to their life cycle and to their use in gene therapy vectors.

• Option (A) correctly describes this: an RNA virus that synthesises DNA during infection.
• Option (B) describes a retrovirus backwards — it is not a DNA virus making RNA. …

Let’s begin with a concrete example that brings the idea to life. You’ve probably heard of HIV — the virus that causes AIDS. HIV is a retrovirus. When it enters a human cell, it doesn’t just use the cell’s machinery to make more viral RNA directly. Instead, it carries a special enzyme called reverse transcriptase. This enzyme does something unusual: it uses the viral RNA as a template to synthesise a complementary strand of DNA. So, an RNA virus ends up making DNA inside the host. That’s the defining feature of a retrovirus.

Now, why is this remarkable? In normal cells, genetic information flows from DNA to RNA to protein — the central dogma of molecular biology. A retrovirus reverses part of that flow: from RNA back to DNA. That’s why it’s called a “retro” virus — “retro” meaning backward. The newly made DNA then gets integrated into the host’s genome, where it can be transcribed and translated to produce new virus particles.

Let’s look at the options one by one:

• (A) An RNA virus that synthesises DNA during infection — This is exactly what a retrovirus does. The viral RNA genome is converted into DNA by reverse transcriptase. This DNA then integrates into the host chromosome.

• (B) A DNA virus that synthesises RNA during infection — That describes many ordinary viruses (like adenoviruses) that follow the normal DNA → RNA path. Not a retrovirus. …

Method — trace the direction of information flow

Normal central dogma: DNA → RNA → protein.

A retrovirus runs part of this backwards: RNA → DNA (via reverse transcriptase), and that DNA then integrates into the host genome. …