In living organisms, RNA (ribonucleic acid) is transcribed from DNA (deoxyribonucleic acid). They are later transcribed to be expressed into proteins that are needed for structure and repair, metabolic functions, and regulations to include defense. In comparison to DNA, RNA transcripts are single-stranded, shorter in length, contain a ribose sugar backbone instead of Deoxyribose, and use the nitrogen base uracil instead of thymine. RNA is also less stable than DNA and are therefore more susceptible to reacting with its environment. There are 5 different types of RNA that all perform specific functions that relate to protein expression or regulation that include: messenger RNA (mRNA), transfer RNA (TRNA), ribosomal RNA (rRNA), small nuclear RNA (snRNA), and micro RNA (miRNA). Not all types are present in every domain of life, and the amount and rate in which they are produced varies per the needs of the cell.

mRNA is processed from the primary DNA transcript in bacteria and eukaryotes. Their main function after its transcription is to carry the protein encoding message to ribosomes (composed of 60% rRNA and 40% protein) to be translated. They are read in three sets of triplet nucleotide pairs called codons in which three consecutive codons will translate for one protein. In eukaryotes, mRNA is transcribed within the nucleus and exit through nuclear pores to be translated by ribosomes. Prokaryotes do not have nuclei, and its transcription is coupled with its translation while it separates from the primary transcript. Prokaryotic mRNA is also different in that one strand can be polycistronic, or can code for more than one protein. Eukaryotic mRNA must go through RNA processing for translation. Alternatively, eukaryotic mRNA that encodes for one gene can be modified by alternative splicing (removing of introns) to change how it is expressed.

Around 80% of the genes in humans are alternatively spliced. This splicing is done by ribonucleoprotein complexes called spliceosomes which are made of snRNA or scRNA added to proteins. snRNA is present in eukaryotes, whereas scRNA (small cytoplasmic RNA) is found in both prokaryotes and eukaryotes. Some types of scRNA are involved in post-translational modification of proteins that can regulate the protein after its made, or can activate or inactivate them.

rRNA is present in all living cells and is one of the most abundant in mammals. Its primary function is to use the mRNA template to translate an encoded gene and synthesize it into a protein. In both eukaryotes and prokaryotes, the ribosomes are present in the cytoplasm. The rRNA complex within these ribosomes are composed of 1-3 subunits composed of a large subunit (LSU) and up to 2 small subunit (SSU), where eukaryotic ribosomes are larger. Though ribosomes are the site of protein synthesis and establish the right reading frame, it is tRNA that performs the synthesis of the mRNA strand into a functional protein.

tRNA are molecules located within the ribosome. They are composed of nucleotide chains ranging from 70-90 in length. The tRNA carries free-floating amino acids within the cytoplasm along these chains, and “transfer” them to the mRNA-rRNA complex. Attachment of the amino acid to uncharged tRNA is done by an enzyme called aminoacetyl tRNA synthase. Each type of aminoacetyl tRNA synthase enzyme works on only one of the 20 amino acids. Once attached, the tRNA is considered “charged” and the amino acid is attached at the 3’ end. will then travel to the reading frame in the ribosome to the mRNA to be added at its 5’ end in an antiparallel configuration, and therefore are referred to as anticodons.

In conclusion, all types of RNA are involved in some step of protein synthesis. Some types are specific by the type of organism in question, while others (such as rRNA) are present in all living organisms, most likely because RNA evolved over time to make DNA and similarities of RNA can be traced back to life’s origins.

References

Nester’s Microbiology: a human perspective (8th ed.. (2015). ch 4 In A. D. Allen Debra. McGraw Hill, NY: McGraw-hill.
Openstax. (2013). Biology. In Openstax, Biology (p. Ch1.1). Houston, TX: Rice University.