Deoxyribonucleic acid (DNA) vs. Ribonucleic acid (RNA)

During transcription, cell machinery reads DNA and simultaneously creates a “sister molecule” called RNA. What is RNA?

An RNA strand looks a lot like a single strand of DNA. The main difference is that the nucleotides making up RNA are very slightly different from DNA nucleotides. Look at Figure 4-19. Can you spot the difference?

If you closely compare the ribose sugar ring of the nucleotides, you’ll notice that there is a slight difference at the “bottom” of the ring. The ribonucleotide (RNA) has two “OH” groups - one on the C3 carbon atom, and one on the C2 carbon atom. The DNA’s deoxyribonucleotide has only a single ‘OH’ group, with the C2 carbon instead having a hydrogen ‘H’. Deoxy, the removal of the ‘oxy’ or oxygen atom at the C2 position, is why DNA is called deoxyribonucleic acid. RNA, on the other hand, has two OH groups on the ribose sugar and is referred to as ribonucleic acid.

Beyond this slight difference in the ribose sugar ring, the ribonucleotides of RNA connect together in the same way as DNA (Figure 1-17). The OH on C3 of one nucleotide ribose sugar connects to the phosphate on C5 of another, and this repeats to create a sugar-phosphate backbone. Notice that when two nucleotides are connected, the “H” on the “OH” is removed during the reaction (Figure 4-20).

Figure 4-17. Did you get one colony or more on your S(e) plate? Congratulations! One colony or more is a success! The first and last photos are experiments results by Zero to Genetic Engineering Hero Junior Editors Pau (first) and Patricia (last)

Figure 4-20. Four nucleotide string of RNA

A second difference between DNA and RNA is that RNA does not form a double helix structure like DNA. As you’ll see at the end of this chapter, RNA nucleotides can complement and bind to one another and form structures, but this happens at a much lower frequency than DNA. This is why in Figure 4-18, the RNA is a lone single strand, while DNA is illustrated as a double helix.

This slight change in the ribose sugar and the fact that it doesn’t broadly form a double helix with complementary RNA has profound effects on RNA’s stability and function. DNA is very stable. So much so that it can remain intact for millions of years. RNA, on the other hand, is not very stable. Once the cell creates RNA, it stays intact for only a short time before falling apart in minutes or hours.

A four-nucleotide string of RNA can be found in Figure 4-20. You’ll see it has a very similar structure to a single strand of DNA. One difference is that the RNA nucleotide has the extra “OH” hydroxyl group in the ribose sugar on carbon C2. Another difference is that RNA does not have thymine. Instead, it has “uracil” (U) nitrogenous base in the uridine ribonucleotide.

Now that we know more about the structure of RNA, we can learn about transcription, the process that the cell uses to read DNA and transcribe it into RNA. To do this, we are going to look at what a gene is and ask these questions:

- What is the cell machine that catalyzes the chemical reaction during transcription?
- How does it know where to start transcribing?
- How does it know what to transcribe?
- How does it know when to stop transcribing?