RNA is generally smaller than DNA primarily because RNA molecules are typically copied from only a limited region of DNA, while DNA constitutes the entire genome.
Here's a breakdown:
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DNA's Role: DNA serves as the complete blueprint of the cell's genetic information. It's extensive and contains all the instructions for building and maintaining an organism. A single DNA molecule in a human chromosome can be extremely long.
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RNA's Role: RNA molecules are often synthesized to perform specific functions, such as carrying genetic information from DNA to ribosomes (mRNA), forming ribosomes (rRNA), or regulating gene expression (tRNA, microRNA, etc.). These RNA molecules are transcribed from specific sections of DNA as needed.
Size Comparison
| Feature | DNA | RNA |
|---|---|---|
| Size | Very long, up to 250 million nucleotide-pairs in human chromosomes | Much shorter, usually no more than a few thousand nucleotides long (Reference) |
| Scope | Complete genetic information | Specific segments copied from DNA for specific functions (Reference) |
Explanation
The size difference stems from the distinct roles of each molecule. DNA needs to store all the genetic instructions, while RNA needs to carry out specific tasks based on those instructions. For instance, messenger RNA (mRNA) only needs to carry the instructions for a single protein, not the entire genome. As the reference states: "In addition, because they are copied from only a limited region of the DNA, RNA molecules are much shorter than DNA molecules. A DNA molecule in a human chromosome can be up to 250 million nucleotide-pairs long; in contrast, most RNAs are no more than a few thousand nucleotides long, and many are considerably shorter."
In simpler terms, think of DNA as an entire cookbook, while RNA is just a single recipe copied from that cookbook when you need to bake a specific cake.
