Arrange the following in descending order of their size.

Options

A. `DNA rarr tRNA rarr mRNA rarr rRNA`
B. `DNA rarr mRNA rarr rRNA rarr tRNA`
C. `DNA rarr rRNA rarr mRNA rarr tRNA`
D. `DNA rarr rRNA rarr tRNA rarr mRNA`

Answer

To arrange the given nucleic acids in descending order of their size, we need to consider their molecular weights. The nucleic acids mentioned are:

1. DNA
2. mRNA (messenger RNA)
3. rRNA (ribosomal RNA)
4. tRNA (transfer RNA)

### Step-by-Step Solution:

**Step 1: Understand the Sizes of Each Nucleic Acid**
- **DNA**: In humans (Homo sapiens), DNA is composed of approximately 3 billion base pairs. This results in a very high molecular weight of about 2.2 × 10^12 Daltons.

- **mRNA**: mRNA is synthesized from DNA and is a single-stranded molecule. Its molecular weight is approximately half that of DNA, around 10^6 Daltons.

- **rRNA**: Ribosomal RNA is part of the ribosome and has different subunits. The average molecular weight of rRNA is about 5 × 10^5 Daltons.

- **tRNA**: Transfer RNA is the smallest among these nucleic acids, with a molecular weight of about 2.5 × 10^4 Daltons.

**Step 2: Arrange in Descending Order**
Based on the molecular weights calculated:
1. **DNA**: 2.2 × 10^12 Daltons
2. **mRNA**: ~10^6 Daltons
3. **rRNA**: ~5 × 10^5 Daltons
4. **tRNA**: 2.5 × 10^4 Daltons

Thus, the descending order of their sizes is:
1. DNA
2. mRNA
3. rRNA
4. tRNA

### Final Answer:
**DNA > mRNA > rRNA > tRNA**