RNA, which stands for ribonucleic acid, is one of the nucleic acids that plays a critical role in protein synthesis and cell homeostasis. This topic will lead you through different RNA structures and functions in detail. Moreover, various RNA types and their properties will be discussed.
All nucleic acids are polymers, made up of nucleotide monomers. RNA consist of the same nucleotides as DNA with one exception — guanine (G), cytosine (C) and adenine (A) are the same, but RNA uses uracil (U) instead of thymine (T). Ribose sugars with phosphoric acid residues form a sugar-phosphate backbone. The chain ends are labeled specially for orientation: the end with the hydroxyl group is called the 3'-end, and the end with the phosphate group is called the 5'-end.
Evolution and structure
RNA appeared earlier than DNA in evolutionary history, and scientists hypothesize that at one time, our planet was an "RNA-world." The RNA world hypothesis suggests that life began with a simple RNA molecule that could copy itself. This self-replicating RNA gradually became more complex and "added" other molecules to its "house," creating the first building block for all the organisms that currently exist in the world. To learn more about that hypothesis, read this article.
The main function of RNA is protein synthesis, however, RNA is involved in many different cell processes — from messaging to gene expression regulation. RNA is an amazing molecule that can not only copy itself, but also has catalytic properties. Despite base-pairing rules (A–U, G–C), RNA generally presents itself as a single-stranded molecule. But this does not mean that RNA secondary structures do not exist. RNA can form a one-stranded helix, which takes on A-form geometry because the sugar ribose has a hydroxyl group at the second position. RNA can also form structures called "stem-loops" or "hairpins" if some bases in one locus cannot match while others around can. One fascinating structure is called the "pseudoknot," which contains at least two hairpins. All these molecules are presented in the picture below. The exploration of RNA secondary structures, especially predicting them, is of great interest to scientists.
RNA can be classified by length as small RNAs, which are usually shorter than 200 nucleotides (nt), and long RNAs, which are greater than 200 nt. However, this is not the only classification of RNA that exists.
Diversity and Functions
Despite the fact that primary structures of all RNA varieties are the same, there are a lot of large and small differences in secondary structure that allow different types of RNA to do different jobs. Let us consider the main types of RNA and their properties. RNA is usually divided into two groups — coding and non-coding molecules. Coding RNAs are messenger RNAs or mRNAs.
Among non-coding RNA, there is much more diversity:
- Infrastructural RNA that are part of a molecular machine (for example, ribosomes) or are responsible for some specific function (like transport tRNA).
- Regulatory RNA, many of which have been discovered in recent years.
Coding RNA carries the information from DNA that allows cells to synthesize their proteins. (to learn more about transcription process, read the Transcription topic). DNA carries a large number of junk sections called introns that do not code for protein sequences. Therefore, RNA obtained from DNA undergoes a special splicing process, which is aimed at removing these introns. After splicing, RNA carries only future protein sections called exons. This type of RNA is called messenger RNA, usually shortened to mRNA. A feature of the mRNA structure is the poly-adenine tail located at the 3'-end, which molecular biologists usually use as a marker to isolate mRNA from cells.
Non-coding, infrastructural RNAs do not carry instructions to synthesize a protein, but they are important, nonetheless. One example, is transfer RNA or tRNA, which is shaped like a three-leaf clover and acts as as the carrier of the amino acids. Proteins like nucleic acids are polymers and consist of amino acids. The concept of a genetic code says that three mRNA nucleotides (a triplet) stand for one amino acid (you will learn more about Genetic code and Translation in the Genetic code topic and Translation topic).
Another non-coding, infrastructural type of RNA is involved in the creation of ribozymes — specific enzymes that perform catalytic functions despite the fact that they do not contain a protein active site. There are not very many of them, but ribozymes are contained in ribosomes, which are the molecular machines that perform protein synthesis. Another non-coding RNA in ribosomes is ribosomal RNA or rRNA. These RNAs and ribosomes are usually labeled with sedimentation constants or Svedbergs (S), which are defined as the ratio of the settling rate of particles suspended in water to the centrifugal acceleration in centrifuges. So, eucaryotes have 18S, 5.8S, 28S and 5S rRNAs. It is worth mentioning that sedimentation coefficients depend not only on the molecular weight, but also on the shape and are non-additive. For instance, bacteria have 70S ribosomes, but their subunits are 50S and 30S.
Non-coding RNAs can also be regulatory molecules. RNA can regulate gene expression, meaning it can change the amount of protein that the cell produces. These RNAs are called microRNAs or miRNAs and appear as small hairpins around 20-25 nucleotides long. They target mRNAs via base-pairing rules, where they bind and leave certain regions of the mRNA unavailable to the ribosomal complex and thus, "silenced" for protein synthesis. small interfering RNAs or siRNAs also perform gene silencing, but using a different mechanism than miRNAs, in which they cleave mRNAs before protein synthesis. There are also long non-coding RNAs which can inactivate large regions of genetic information, including whole chromosomes. These regulatory RNAs are an active area of research.
Finally, RNA viruses carry genetic material composed of RNA only. All mentioned RNA types and other ones you can study more deeply reading the article.
Conclusion
RNA is a nucleic acid with a simple structure and unique properties. It consists of a phosphoric acid residue, a ribose, and a nitrogenous base (guanine (G), cytosine (C), adenine (A), or uracil (U)). RNA usually forms a one-stranded helix with A-form structure. RNA can also have secondary structures such as hairpins and pseudoknots. RNAs are classified by length and function, and these molecules play an important role in protein synthesis and gene regulation.