Q.In the given flow diagram, the replication of retrovirus in a host is shown. Observe and answer the following questions.
a. Fill in

Here are the answers to your questions regarding retrovirus replication:

a. Based on the typical retrovirus replication cycle:

* (1) represents the enzyme reverse transcriptase.

* (2) represents the viral DNA (which is then integrated into the host cell's DNA).

b. The virus is called a retrovirus because it contains the enzyme reverse transcriptase. This enzyme synthesizes viral DNA from its RNA genome, a process called reverse transcription. This is contrary to the central dogma of molecular biology, where genetic information typically flows from DNA to RNA. …

Retroviruses represent a fascinating and clinically significant group of viruses, distinguished by their unique replication strategy. Unlike most organisms where genetic information flows from DNA to RNA to protein (the Central Dogma), retroviruses reverse this process for a crucial step in their life cycle. This mechanism allows them to integrate their genetic material directly into the host cell's genome, leading to persistent infections.

When a retrovirus infects a host cell, its genetic material, which is RNA, is introduced into the cytoplasm. Along with this RNA, the virus also brings a special enzyme that is central to its replication.

a. In the replication flow of a retrovirus:

• (1) Reverse Transcriptase: This is the crucial enzyme carried by the retrovirus. Its role is to synthesize a DNA copy from the viral RNA genome. This process is called reverse transcription.
• (2) Viral DNA: The product of reverse transcription is a double-stranded DNA molecule. This newly formed viral DNA then travels to the host cell's nucleus and integrates itself into the host cell's chromosomal DNA. Once integrated, it is referred to as a provirus.

b. The virus is called a retrovirus because of this unique step of reverse transcription. The prefix "retro-" means backward or in reverse. In biology, the standard flow of genetic information is from DNA to RNA. Retroviruses, however, reverse this flow by using their RNA as a template to synthesize DNA. This fundamental deviation from the conventional genetic information transfer pathway gives them their name.

c. The survival of an infected cell while viruses are being replicated and released depends on the specific retrovirus and the host cell type, but generally, the cell's long-term health and function are severely compromised. Retroviruses typically do not immediately lyse (burst open) the host cell upon releasing new viral particles. Instead, new viruses often bud off from the host cell's plasma membrane, acquiring an envelope from the host cell in the process. This budding can continue for a period, allowing the infected cell to persist. …

Method: Contrast Against the Central Dogma, Step by Step

Rather than describing the retrovirus cycle in isolation, walk the normal flow of genetic information first, then find exactly where the retrovirus deviates.

Step 1: State the normal (non-viral) flow

DNA → RNA → Protein. Information moves from DNA to RNA via transcription, and this is a one-way street in ordinary cells.

Step 2: Introduce the retrovirus's starting material

A retrovirus enters the host cell carrying RNA as its genome — already a departure, since most cells never treat RNA as the master template.

Step 3: Locate the reversal

The retrovirus supplies its own enzyme, reverse transcriptase (label 1), which copies RNA → DNA — the literal reverse of transcription. This is the step that earns the "retro-" prefix.

Step 4: Rejoin the normal pathway …