Chapter 27, Problem 1U

The reverse transcriptase enzyme is active in which class of viruses?

a. Positive-strand RNA viruses

b. Double-stranded DNA viruses

c. Retroviruses

d. Negative-strand RNA viruses

Summary Introduction

Introduction:

A virus is a small infectious agent that can’t be seen through the naked eyes. Viruses are made up of protein coat in which their genetic material is present. The viruses contain either double stranded or single stranded DNA or RNA as their genetic material. Based on their genetic material David Baltimore classified viruses into seven different classes. The viruses also have ability to synthesize DNA from RNA with the help of an enzyme reverse transcriptase.

Answer to Problem 1U

Correct answer:

The retroviruses (class VI virus) contain an enzyme reverse transcriptase. Therefore, option c. is correct.

Explanation of Solution

Reason for the correct statement:

Retroviruses are classified under class VI virus. They are single stranded RNA viruses which replicate their genome through DNA intermediate. These viruses contain reverse transcriptase enzyme which synthesize their DNA from RNA. Later, these viruses integrate their genome into the host cell and start their infectious cycle inside the host.

Option c. is given as “Retroviruses”.

As “the reverse transcriptase enzyme active in class VI viruses which is also called as Retroviruses” is the right answer.

Hence, option c. is correct.

Reasons for the incorrect statements:

Option a. is given as “positive strand RNA viruses”.

The positive strand RNA viruses are classified under class IV. The sequence of their genomic RNA is same as the sequence of $\text{mRNA}$. Hence, they directly translate their genome and produce infectious protein. They do not contain reverse transcriptase enzyme for their infectious cycle. So, it is a wrong answer.

Option b. is given as “Double stranded DNA viruses”.

The double stranded DNA viruses are classified under class I viruses. They contain DNA as their genetic material. Thus, they do not contain reverse transcriptase enzyme. So, it is a wrong answer.

Option d. is given as “Negative strand RNA viruses”.

The negative strand RNA viruses are classified under class V virus. Their RNA genome sequence is complementary to the sequence of $\text{mRNA}$. The does not contain reverse transcriptase enzyme. So, it is a wrong answer.

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

Conclusion

The retroviruses are class VI viruses that contain a reverse transcriptase enzyme which synthesize DNA from RNA.