DNA is often referred to as the “Blueprints of Life”. If we expand on this analogy, it would be quite reasonable to say that RNA is the construction crew. Just like the construction crew for a house would be varied, consisting of builders, laborers, electricians, plumbers and bricklayers, the RNA construction crew also has varied job titles with differing functions: messenger, nuclear, ribosomal, short hairpin and transfer.
RNA (ribonucleic acid) is a type of vital organic molecule that is produced from the coding of our DNA (deoxyribonucleic acid), which we inherit from our parents. Without RNA life would not be possible, not only for us but for all organisms, be they animals, plants or even bacteria. Where DNA is relatively dormant, essentially a library of information in the center of the nucleus of cells, RNA is active in many ways, some of which we don’t fully understand and some we may not even know about yet.
DNA is double-stranded, each strand the complement of the other and bonded to the other through hydrogen bonds, giving it strength and longevity; DNA molecules can last thousands of years before breaking down. Except for some viruses that use double-stranded RNA (dsRNA) as their genetic material, RNA is single-stranded. This leaves it weaker and prone to breaking down, ideal for its use as short-term instructions, and also with multiple potential bonding sites, facilitating its use as a component of cellular structures.
The active’ part of both DNA and RNA is a nitrogen base, either a purine (adenine or guanine) or a pyramidine (cytosine, thymine or uracil). They both use adenine (A), cytosine (C) and guanine (G), but RNA has uracil (U) where DNA has thymine (T). The bases attach to sugar molecules, ribose for RNA and deoxy-ribose for DNA; deoxy-ribose has one less oxygen atom than ribose, thus the deoxy’ prefix. The sugar in turn attaches to a phosphate group and the three, base plus sugar plus phosphate, are collectively called a nucleotide. A nucleotide’s sugar attaches to another nucleotide’s phosphate and its phosphate to a different nucleotide’s sugar, thus forming the sugar-phosphate-sugar-phosphate backbone of the nucleic acid macromolecule, be it DNA or RNA.
The structure of the bases means that purines bond with pyramidines; adenine will best bond with either thymine or uracil while guanine prefers cytosine. They can bond with the others, though very rarely; when this occurs it is referred to as a point mutation. When RNA forms on the template of DNA therefore, it forms as the reverse of the template. If the DNA template has the sequence AAGCTTCCGATG then the RNA will be UUCGAAGGCUAC, unless a point mutation occurs. It knows’ to use uracil rather than thymine because the synthesizing of the RNA is controlled by an enzyme called RNA polymerase; DNA synthesizing is controlled by DNA polymerase.
All RNA, no matter what its eventual function, is created on the template of a gene’s DNA. We have divided it into types based on its final location and/or function, although such classifications are of significance only to our understanding, our cells don’t care. The five types, as stated earlier, are messenger (mRNA), transfer (tRNA), ribosomal (rRNA), nuclear (nRNA) and short hairpin (shRNA).
The mRNA is formed in a process called transcription as a copy of the construction instructions for a protein encoded in the DNA. Eukaryotes, basically all organisms besides bacteria and archaea (ancient bacteria’ found in extreme environments), nearly always have genes composed of introns and exons. Introns may contain instructions related to controlling RNA synthesis or purposes not currently understood or have no purpose, while exons contain the actual construction information in triplets of bases called codons. Therefore, in Eukaryotes the RNA formed from the DNA template is termed pre-mRNA; additional RNA-protein complexes use a process called splicing to cut out the introns and put the exons together to form the end product, mature mRNA.
The mature mRNA passes through the wall of the nucleus, called the nuclear envelope, and connects with a ribosome. The mRNA threads through the ribosome, each codon (triplet of bases) identifying a desired amino acid, in a process called translation. At one end of the ribosome, tRNA with a corresponding triplet of bases to the mRNA’s codon briefly attach to connect the appropriate amino acid to a growing chain of amino acids called a peptide. The completed peptide might be a protein in its own right or combine with other peptides to form one.
Most organisms have around 20 different transfer RNA types; each type is specifically structured to interact with one type of amino acid. After being produced in the nucleus, tRNA move through the nuclear envelope into the cytoplasm of the cell where they attach to a free-floating amino acid of the type they are specific to. When a ribosome displays the appropriate codon from an mRNA for the tRNA’s amino acid, the tRNA attaches to the ribosome, connects its amino acid to the building peptide then releases itself from the ribosome. It will then attach to another of its type of amino acid and repeat the process, continuing in this until it breaks down. It could as easily be called a transport RNA molecule as a transfer.
Ribosomes are the factories of the cell, producing all the proteins used, and are constructed in the nucleolus from a combination of ribosomal RNA (65%) and proteins (35%). The nucleolus is a sub-organelle of the cell nucleus and is surrounded by ribosomal DNA, giving it the necessary templates for the production of rRNA. Once constructed the ribosomes move out into the cytoplasm to perform their function. Many remain free-floating in the cytoplasm or attach to the nuclear envelope, but most attach to the rough endoplasmic reticulum, a cell organelle.
Nuclear RNA gets its name because it remains in the nucleus after being synthesized. It comes in a variety of shapes, sizes and functions; many of the functions have yet to be determined. Some bind with particular proteins to form the RNA-protein complexes that process pre-mRNA into mature mRNA or those complexes that construct the ribosomes in the nucleolus. Others, called short interference RNA (siRNA) act to suppress gene expression, that is they stop the production of RNA from a particular gene’s DNA template, a process known as gene silencing.
Short hairpin RNA (shRNA) are short lengths of RNA that take the shape of a hairpin and can attach to mRNA so as to obstruct them from processing properly through a ribosome. Like gene silencing, this is also a way of suppressing gene expression, it is called RNA interference. Sometimes shRNA is classified as a type of nRNA even though it does leave the nucleus.
Investigating the potential of siRNA and shRNA is at the forefront of medical research, as they have the potential to allow us to suppress the expression of genes that may cause genetic defects or diseases. Neoplasms (cancers) primarily result from mutation in the DNA of a cell causing it to divide repeatedly forming a tumor; siRNA may be able to be used to suppress tumor growth and help the immune system destroy it. These types of RNA are also used in genetic research, they help scientists determine gene function.
RNA is a truly remarkable macromolecule, fulfilling many essential functions and duties within our bodies’ cells. There is still much we don’t know about it, its functions and its capabilities, but research continues; who knows what we may be able to do using RNA in the future?
