Ribonucleic Acid (Rna)
Ribonucleic acid (RNA), generally abbreviated RNA, is an organic chemical substance in living cells that plays several essential roles in the transfer of genetic information from one generation to the next. The hereditary information itself is contained in a similar organic substance known as deoxyribonucleic acid (DNA). RNA is what enables this genetic information to be copied from the parent's DNA and inherited by the offspring.
Both RNA and DNA are nucleic acids, so called because they are found in cell nuclei. (RNA is also found in other parts of the cell.) Nucleic acids are the storehouse and delivery system of our genetic traits. The actual biological processes that they prescribe are carried out mostly by our proteins--our enzymes, hormones and muscles. In other words, nucleic acids are the instruction manual for life's protein-built operating equipment. Each nucleotide monomer molecule consists of a sugar part, a phosphate part, and an amine part. The main difference between RNA and DNA is that in RNA the sugar is ribose (C5H10O 5), while in DNA it is deoxyribose (C5H 10O4). The prefix "deoxy-" tells us that one oxygen atom is "missing" from the ribose.
Like other nucleic acids, RNA is built up from the nucleotides in much the same way that proteins are built up from amino acids; they even coil up into long spirals, as some protein molecules do. In a long nucleic acid polymer spiral, the backbone consists of alternating sugar and phosphate parts, with the amine parts sticking out like branches from the backbone. As to the amine parts (which are also referred to as bases), there are only four that are important in RNA: adenine, cytosine, guanine, and uracil. Scientists symbolize them as A, C, G, and U. DNA contains thymine instead of uracil: T instead of U.
In DNA, all the information about inherited characteristics exists in the form of genes--arrangements of the four amines in a specific order on a DNA molecule--just as the words in a sentence must be arranged in a specific order if they are to convey real information instead of nonsense. These sequences of amine "words" constitute a set of instructions for exactly which proteins must be manufactured in order to create a specific trait--either brown eyes or green eyes in a human, or a muscle cell for a lizard's tail or a brain cell for an elephant. RNA is what translates these instructions into action.
One kind of RNA "writes down" or transcribes the DNA's amine sequence onto its own molecule, like writing crib notes on the back of your hand. Then, a messenger RNA takes these instructions out of the nucleus and delivers them to the ribosomes--the cells' protein factories. Finally, transfer RNA collects the necessary amino acids and transfers them to the ribosomes for assembly into proteins. All of these processes are made possible by specific enzymes--chemicals that speed up vital chemical reactions in living things, making them go millions of times faster than they would otherwise.
In the first step of the gene-transmitting process, the DNA's double helix unwinds to produce two separated strands with their amines sticking out from the backbones. These strands of DNA then serve as an exposed pattern for the production of matching strands of RNA. That is, each protruding amine on the DNA strand picks up a partner amine to bond to according to its highly selective preference: cytosine and guanine (C and G) will always bond together, while adenine and thymine (A and T) will always bond together. In this way, a strand of RNA is built up (with U's instead of T's) that is exactly complementary to the amine sequence on the DNA: it has G's where the C's were and vice versa, and it has A's where the T's were and U's where the A's were. Similarly, if you hold a book up to a mirror, the image in the mirror will be complementary to the actual writing on the page. All of the information is still there, but it has been transcribed, or re-written, into a complementary or matching form. This first step of the process is therefore called transcription.
In the next step of the gene-transmitting process, the information in the RNA strand is edited or streamlined, to produce a strand of messenger RNA (mRNA) that is capable of escaping from the nucleus and carrying the essential genetic information to the ribosomes, which are out in the cell's cytoplasm.
In the cytoplasm are several kinds of smaller RNA molecules called transfer RNA (tRNA), which are swimming around in the pool of amino acids and other chemicals that surround the ribosomes. In the pool, each of these tRNA molecules carries around--is attached to--one particular kind of amino acid molecule, waiting to fill an order from the mRNA. The tRNA molecules read the instructions on the mRNA, and wherever the mRNA needs a particular amino acid, the corresponding tRNA molecule drags its attached amino acid into the protein factory. Thus the desired proteins are built up from the proper amino acids.
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