Chapter 13.5, Problem 13SAQ

Summary Introduction

To describe:

The steps necessary to synthesize $\text{mRNA}$ from each of the following: double stranded DNA, single stranded (+) DNA, single stranded (-) DNA, single stranded (+) RNA, and single-stranded (-) RNA.

Concept introduction:

Viruses differ considerably in their genomes. Some viruses have genomes composed of RNA, while others have genome made up of DNA. In addition to this, the genome can be either single stranded or double stranded. Since viruses have evolved independently therefore, it is very difficult to classify them on the basis of evolutionary relatedness. They are generally classified on the basis of the type of nucleic acid in their genome and how they synthesize $\text{mRNA}$.

Explanation of Solution

The steps necessary to synthesize $\text{mRNA}$ from double stranded DNA are as follows:

1. The first step involves the melting of double stranded DNA to allow the binding of DNA dependent RNA polymerase enzyme.

2. The second step involves the synthesis of $\text{mRNA}$ by the enzyme.

The steps necessary to synthesize $\text{mRNA}$ from single stranded (+) DNA are as follows:

1. Replication of single stranded (+) DNA to form double stranded DNA.

2. The double stranded DNA then act as template for the synthesis of $\text{mRNA}$ by the DNA dependent RNA polymerase.

The steps necessary to synthesize $\text{mRNA}$ from single stranded (-) DNA are as follows:

1. Replication of single stranded (-) DNA occurs to form double stranded DNA.

2. The double stranded DNA then act as template for the synthesis of $\text{mRNA}$ by the DNA dependent RNA polymerase.

The steps necessary to synthesize $\text{mRNA}$ from single stranded (+) RNA are as follows:

1. The single stranded (+) RNA is converted into single stranded (-) DNA with the help of enzyme called reverse transcriptase, a RNA dependent DNA polymerase.

2. The single stranded (-) DNA is then replicated to form double stranded DNA, which is then used as template by the DNA dependent RNA polymerase to synthesize $\text{mRNA}$.

3. In some single stranded (+) RNA, the positive sense RNA is first converted into negative sense RNA which is then used as template by RNA dependent RNA polymerase to synthesize $\text{mRNA}$.

The steps necessary to synthesize $\text{mRNA}$ from single stranded (-) RNA are as follows:

1. The single stranded (-) RNA is used as template to synthesize positive sense $\text{mRNA}$.

2. The $\text{mRNA}$ is then used to synthesize proteins.

Conclusion

The viruses differ considerably in the synthesis of their $\text{mRNA}$ molecules.