DNA vs. RNA

The differences between RNA and DNA. Sponk

Their names sound similar (deoxyribonucleic acid and ribonucleic acid). They both hold genetic information. The building blocks of both are nucleotides. Both molecules have a role in protein production in cells. So just how are DNA and RNA different? Which one is thought to be older on the evolutionary time scale and why?

Sugar Backbones

DNA and RNA are made up of subunits called nucleotides. All nucleotides have a sugar backbone, a phosphate group, and a nitrogenous base.

Both DNA and RNA have sugar “backbones” that are made up of five carbon molecules. However, they are different sugars that make them up. DNA is made up of deoxyribose and RNA is made up of ribose. Those sound pretty similar and their structures are very similar. The prefix “deoxy-” means “without oxygen” and basically that is the difference. The deoxyribose sugar molecule is missing one oxygen that a ribose molecule sugar has. This makes a big enough change to make the backbones of these nucleic acids different.

Nitrogenous Bases

Each nucleotide has a single nitrogenous base connected to it. These can be categorized into two main groups. The pyrimidines have a single ring structure and purines have a double ring structure. When complementary strands are made, a purine must match up with a pyrimidine to keep the width of the “ladder” at three rings. Both DNA and RNA have the same purines. These molecules are called adenine and guanine.

They also both have a pyrimidine called cytosine. However, their second pyrimidine is different. DNA uses thymine while RNA includes uracil instead.

When complementary strands are made of the genetic material, cytosine always matches up with guanine and adenine will match up with thymine (in DNA) or uracil (in RNA).

This is called the “base pairing rules” and was discovered by Erwin Chargaff in the early 1950s.

Number of strands

Another difference between DNA and RNA is the number of strands on the molecules. DNA is a double helix meaning it has two twisted strands that are complementary to each other matched up by the base pairing rules. DNA needs an enzyme called DNA helicase to untwist and “unzip” the strands before replication or transcription can occur. RNA, on the other hand, is only single stranded. RNA is created in most eukaryotes by making a complementary strand to a single DNA strand.

Evolution

So which came first, DNA or RNA? While there are arguments for both ways, it is generally agreed upon that RNA came first. It is a simpler structure since it is single stranded and the cells can easily interpret their codons. DNA is unreadable by a cell in its natural state and relies on RNA to transcribe and translate it in order to make a polypeptide chain.

Many primitive prokaryotes use RNA as their genetic material and did not evolve DNA. RNA can still be used as a catalyst for chemical reactions like enzymes. There are also clues within viruses that use only RNA that RNA may be more ancient than DNA.

Scientists refer to a time before DNA as the “RNA world”. While this is still a somewhat developing hypothesis, there is quite a bit of evidence that RNA came before DNA.

Then why did DNA evolve at all? This question is still being investigated, but one possible explanation is that DNA is more highly protected and harder to break down than RNA. It is both twisted and “zipped” up in a double stranded molecule. This adds protection from injury and digestion by enzymes. In order for it to be replicated or transcribed, it has to be untwisted and unzipped first. There is less of a chance that these processes will happen by mistake.