Chapter 26, Problem 1U

Summary Introduction

Introduction: A type of RNA virus that inserts a copy of its genome into the DNA of an invaded host cell is called a retrovirus. Thereby, it changes the genome of the host cell. These viruses are composed only of RNA and not DNA. They possess specific enzymes that provide them the unique characteristics.

Answer to Problem 1U

Correct answer: b. Because host cells’ genomes do not contain a gene encoding an enzyme that can make DNA from RNA.

Explanation of Solution

Reason for the correct answer:

Retroviruses possess the reverse transcriptase enzyme. This is because the genome of the host cell does not possess a gene encoding an enzyme that could convert DNA from RNA. The reverse transcriptase enzyme converts RNA genome into DNA.

Option b. is given as “Because host cells’ genomes do not contain a gene encoding an enzyme that can make DNA from RNA”.

Hence, the correct answer is option b.

Reasons for the incorrect answers:

Option a. is given as “because host cells make DNA from RNA only in G2 of the cell cycle, and HIV infects only during G1 of the cell cycle”.

This is not an appropriate option that describes the function of reverse transcriptase enzyme in the retrovirus. Therefore, option a. is incorrect.

Option c. is given as “Because HIV deletes large parts of the host cell genome upon infection, and that means there is a chance that host cell reverse transcriptase will be deleted”.

The presence of reverse transcriptase in retroviruses facilitates reverse transcription of RNA into DNA. Therefore, option c. is incorrect.

Option d. is given as “Host cells cannot make RNA from DNA”.

This is not an appropriate option for the necessity of the presence of the reverse transcriptase enzyme in a retrovirus. Therefore, option d. is incorrect.

Hence, options a., c., and d. are incorrect.

Conclusion

Thus, the reason for the need of retroviruses to contain the reverse transcriptase enzyme is that it would facilitate the reverse transcription of RNA into DNA.