What is RNA?

RNA, which is short for ribonucleic acid, is essential in various biological roles.

RNA is involved in the coding, decoding, regulation and expression of genes. Some RNA molecules play an active role within cells by controlling gene expression, sensing and communicating responsed to cellular signals, and acting as catalyzers for biological reactions.

When protein is synthesised on ribosomes in the cell, RNA molecules direct the process. Transfer RNA (tRNA) delivers amino acids to the ribosome, and ribosomal RNA (rRNA) then link the amino acids together to build the correct proteins. All cellular organisms (animals, plants, fungi, and more) use messenger RNA (mRNA) to convey genetic information that will direct the protein synthesis.

Four major macromolecules are essential for all known forms of life, and nucleic acids are one of them (the other ones are lipids, proteins and carbohydrates). Both RNA and DNA are nucleic acids.

Many viruses encode their genetic information using an RNA genome instead of having DNA.


RNA vs. DNA: Similarities, differences and how they work together

  • Both DNA and RNA are assembled as a chain of nucleotides.
  • DNA is a paired double strand, while RNA can be doublel-stranded or single-stranded. In most contexts, RNA will be a single-stranded molecule (ssRNA). There is however situations where double-stranded RNA (dsRNA) can form. It should also be noted that a single RNA molecule can form intrastrand double helixes through complementary base pairing. One example is the tRNA.
  • The sugar-phosphate “backbone” of the DNA structure contains deoxyribose, while ribose perform that function in RNA. This is why the activation energy for hydrolysis is lower for RNA than for DNA.
  • Adenine has thymine as its complementary base in DNA, but in RNA adenine is instead paired with uracil. (Uracil is an unmethylated form of thymine.)
  • Both DNA and most biological active forms of RNA contain self-complementary sequences. In DNA, the structures consist of long double helices. In RNA, the structures are collections of short helices packed together.

Different types of RNA

RNA occurs in different forms and perform many different functions in a living organism. Below, we have listed a few examples of different types of RNA, what they do and in which organisms they occur. The list is by no means exchaustive, and it should also be mentioned that this is a field of science that is growing continously as we find out more and more about RNA and its many versions and functions in living creatures.

Forms of RNA involved in protein synthesis in all organisms

  • Messenger RNA (mRNA) codes for protein.
  • Ribosomal RNA (rRNA( and Transfer RNA (tRNA) both do translations.
  • Signal recognition particle RNA (7SL RNA or SRP RNA) is involved in membrane integration.

Forms of RNA involved in protein synthesis in bacteria

Transfer-messenger RNA (tmRNA) is only found in bacteria, where it rescues stalled ribosomes.

Forms of RNA involved in post-transcriptional modification or DNA replication

  • Ribonuclease P (RNase P) is involved in tRNA maturation in all organisms.
  • Small nuclear RNA (snRA) is involved in splicing and certain other functions in all eukaryote organisms and archea cells.
  • Small nucleoar RNA (snoRNA) does nucleotide modification of RNAs in all eukaryote organisms and archea cells.
  • Ribonuclease MRP (RNase MRP) is involved in rRNA maturation and DNA replication in eukaryote organisms.
  • Small Cajal body-specific RNA (scaRNA) is a class of snoRNAs found in the Cajal body. Cajal bodies are found in the nucleus of proliferative cells such as embryonic cells and tumour cells, and in metabolically active cells such as neurons. Cajal bodies have been found in animals cells, plant cells and yeast cells.
  • Telomerase RNA Component (TERC) is involved in telomere synthesis in most, but not all, eucaryote organisms.
  • Y RNA are small non-coding RNAs found in animals. They are involved in both RNA processing and DNA replication.
  • SL1 RNA is involved in trans-splicing (a form of RNA processing). It has been observed in a number of lower eukaryote organisms. It is especially common in nematodes (roundworms).
  • SmY RNA is involved in mRNA trans-splicing in nematodes.
  • Guide RNA (gRNA) is involved in mRNA nucleotide modification in kinetoplastid mitochondria. Kinetoplastida is a group of flagellated protists.

Forms of RNA carrying out regulatory functions

  • Examples of RNA that is involved in gene regulation in most eurkaryote orgnisms are MicroRNA, Small interfering RNA and Short hairpin RNA.
  • Trans-acting siRNA is involved in gene regulation in land plants.
  • Antisense RNA is involved in transcriptional attenuation, mRNA degradation, mRNA stabilisation and translation block in all organisms.
  • CRISPR RNA (crRNA) is found in bacteria and archaea where it helps fight parasites by targeting their DNA.
  • In eukaryote organisms, long non-coding RNA (lncRNA) is involved in epigenic regulation and the regulation of gene transcription.
  • In most animals, Piwi-interacting RNA (piRNA) is involved in the transposon defense. One example of an exception is Drosophila, where repeat associated siRNA (rasiRNA) does the transposon defense. Drosophila is a genus of small fruit flies.

Parasitic RNA

  • One example of parasitic RNA are the viral genomes found in double-stranded RNA viruses, positive-sense RNA viruses, negative-sense RNA viruses, and many satellite viruses and reverse transcribing viruses.
  • Viroids are small single-stranded circular RNAs. Unlike viruses, they are without any protein coating. Viroids are pathogens that infect plants. All known viroids infect angiosperms (flowering plants). The first viroid to be identifed was the potato spindle tuber viroid.
  • Retrotransposons copy and paste themselves into different genomic locations by converting RNA back into DNA through the process reverse transcription using an RNA transposition intermediate.