Cellular Biology

Rna Defined



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RNA (ribonucleic acid) is a type of vital organic molecule that enables our bodies to be built from the blueprints of the DNA (deoxyribonucleic acid) inherited 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 a cell's nucleus, RNA is active in many ways, some of which we are yet to understand.

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. RNA is single-stranded, except for some viruses that use double-stranded RNA (dsRNA) as their genetic material, which leaves it weaker and prone to break up, ideal for its use as short-term instructions or 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 pyrimidine (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 pyrimidines; 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. The main types are messenger (mRNA), nuclear (nRNA), ribosomal (rRNA) and transfer (tRNA).

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.

The final type of RNA is called nuclear RNA 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, effectively another way of suppressing gene expression called RNA interference. Strictly speaking shRNA are not a type of nRNA because they do leave the nucleus.

Investigating siRNA and shRNA is at the forefront of medicinal research, as the controlled suppression of gene expression will almost certainly be a superior tool in mitigating the effects of genetic defects and diseases. They are also used in genetic research, for determining gene function.

As you can see, RNA is a truly remarkable macromolecule, fulfilling many essential functions and duties within our bodies' cells. There is still much we don't know, but research continues, who knows what miracles RNA may enable for us in the future?

Sources:

Although written in the main from my own understanding and memory, my knowledge ultimately came from the lectures of the following people at the Mt. Albert campus of Unitec NZ Ltd. I also used their notes to jog my memory a few times while writing this article.

Adams, Nigel. (2006) Unpublished Animal Physiology lecture notes.
Chambers, Steve. (2007) Unpublished Principles of Biotechnology lecture notes.
Harman, Jane. (2005) Unpublished Biological Principles lecture notes.
Large, Mark. (2005) Unpublished Biological Principles lecture notes.
Large, Mark. (2007) Unpublished Principles of Biotechnology lecture notes.

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