RNA (Ribonucleic acid) (Encyclopedia of Genetic Disorders)
Ribonucleic acid (RNA) conveys genetic information and catalyzes important biochemical reactions. Similar, but not identical, to a single strand of deoxyribonucleic acid (DNA), in some lower organisms, RNA replaces DNA as the genetic material. As with DNA, RNA follows specific base pairing rules, except that in RNA the base uracil replaces the base thymine (i.e., instead of an adenine-thymine or A-T pairing, there is an adenine-uracil or A-U pairing). Accordingly, when RNA acts as a carrier of genetic information, uracil replaces thymine in the genetic code.
In humans, messenger RNA (mRNA) is the product of transcription and acts to convey genetic information from the nucleus to the protein assembly complex at the ribosome. The ribsome is composed of ribosomal RNA (rRNA) and other proteins. Transfer RNAs (tRNA) act to catalyze the translation process by acting as carriers of specific amino acids. Because tRNAs bind to specific sites on the strand of mRNA, the sequence of amino acids subsequently inserted into the synthesized protein is both specific and genetically determined by the nucleotide sequence in DNA from which the mRNA strand was originally transcribed.
Other forms of RNA perform important roles in other biochemical reactions. Regardless of function, RNA is a biopolymer made up of ribonucleotide units and is present in all living cells and some viruses. The...
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Ribonucleic Acid (RNA) (Encyclopedia of Psychology)
A complex organic substance involved in protein synthesis in cells.
RNA consists of a five-carbon sugar (ribose), phosphate, and four nitrogenous bases (adenine, guanine, cytosine, and uracil). In an RNA molecule, the sugar and phosphate combine to form a structure to
which the nitrogenous bases are attached. These molecules range in composition from fewer than 100 to several thousand nitrogenous bases, and vary in shape from helical to uncoiled. RNA is the primary agent of protein formation, and processes genetic information from deoxyribonucleic acid (DNA) molecules into enzymes necessary for life.
(The entire section is 109 words.)
Ribonucleic Acid (RNA) (World of Microbiology and Immunology)
Nucleic acids are complex molecules that contain a cell's genetic information and the instructions for carrying out cellular processes. In eukaryotic cells, the two nucleic acids, ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), work together to direct protein synthesis. Although it is DNA that contains the instructions for directing the synthesis of specific structural and enzymatic proteins, several types of RNA actually carry out the processes required to produce these proteins. These include messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA). Further processing of the various RNA's is carried out by another type of RNA called small nuclear RNA (snRNA). The structure of RNA is very similar to that of DNA, however, instead of the base thymine, RNA contains the base uracil. In addition, the pentose sugar ribose is missing an oxygen atom at position two in DNA, hence the name deoxy-.
Nucleic acids are long chain molecules that link together individual nucleotides that are composed of a pentose sugar, a nitrogenous base, and one or more phosphate groups.
The nucleotides, the building blocks of nucleic acids, in ribonucleic acid are adenylic acid, cytidylic acid, guanylic acid, and uridylic acid. Each of the RNA subunit nucleotides carries a nitrogenous base: adenylic acid contains adenine (A), cytidylic acid contains cytosine (C), guanylic acid contains guanine (G), and uridylic acid contains uracil.
In humans, the DNA molecule is made of phosphate-base-sugar nucleotide chains, and its three-dimensional shape affects its genetic function. In humans and other higher organisms, DNA is shaped in a two-stranded spiral helix organized into structures called chromosomes. In contrast, most RNA molecules are single-stranded and take various shapes.
Nucleic acids were first identified by the Swiss biochemist Johann Miescher (1844895). Miescher isolated a cellular substance containing nitrogen and phosphorus. Thinking it was a phosphorus-rich nuclear protein, Miescher named it nuclein.
The substance identified by Miescher was actually a protein plus nucleic acid, as the German biochemist Albrecht Kossel discovered in the 1880s. Kossel also isolated nucleic acids'two purines (adenine and guanine) and three pyrimidines (thymine, cytosine, and uracil), as well as carbohydrates.
The American biochemist Phoebus Levene, who had once studied with Kossel, identified two nucleic acid sugars. Levene identified ribose in 1909 and deoxyribose (a molecule with less oxygen than ribose) in 1929. Levene also defined a nucleic acid's main unit as a phosphate-base-sugar nucleotide. The nucleotides' exact connection into a linear polymer chain was discovered in the 1940s by the British organic chemist Alexander Todd.
In 1951, American molecular biologist James Watson and the British molecular biologists Francis Crick and Maurice Wilkins developed a model of DNA that proposed its now accepted two-stranded helical shape in which adenine is always paired with thymine and guanine is always paired with the cytosine. In RNA, uracil replaces thymine.
During the 1960s, scientists discovered that three consecutive DNA or RNA bases (a codon) comprise the genetic code or instruction for production of a protein. A gene is transcribed into messenger RNA (mRNA), which moves from the nucleus to structures in the cytoplasm called ribosomes. Codons on the mRNA order the insertion of a specific amino acid into the chain of amino acids that are part of every protein. Codons can also order the translation process to stop. Transfer RNA (tRNA) molecules already in the cytoplasm read the codon instructions and bring the required amino acids to a ribosome for assembly.
Some proteins carry out cell functions while others control the operation of other genes. Until the 1970s cellular RNA was thought to be only a passive carrier of DNA instructions. It is now known to perform several enzymatic functions within cells, including transcribing DNA into messenger RNA and making protein. In certain viruses called retroviruses, RNA itself is the genetic information. This, and the increasing knowledge of RNA's dynamic role in DNA cells, has led some scientists to argue that RNA was the basis for Earth's earliest life forms, an environment termed the RNA World.
The first step in protein synthesis is the transcription of DNA into mRNA. The mRNA exits the nuclear membrane through special pores and enters the cytoplasm. It then delivers its coded message to tiny protein factories called ribosomes that consist of two unequal sized subunits. Some of these ribosomes are found floating free in the cytosol, but most of them are located on a structure called rough endoplasmic reticulum (rER). It is thought that the free-floating ribosomes manufacture proteins for use within the cell (cell proliferation), while those found on the rER produce proteins for export out of the cell or those that are associated with the cell membrane.
Genes transcribe their encoded sequences as a RNA template that plays the role of precursor for messenger RNA (mRNA), being thus termed pre-mRNA. Messenger RNA is formed through the splicing of exons from pre-mRNA into a sequence of codons, ready for protein translation. Therefore, mRNA is also termed mature mRNA, because it can be transported to the cytoplasm, where protein translation will take place in the ribosomal complex.
Transcription occurs in the nucleus, through the following sequence of the events. The process of gene transcription into mRNA in the nucleus begins with the original DNA nitrogenous base sequence represented in the direction of transcription (e.g. from the 5' [five prime] end to the 3' [three prime] end) as DNA 5'...AGG TCC TAG TAA...3' to the formation of pre-mRNA (for the exemplar DNA cited) with a sequence of 3'...TCC AGG ATC ATT...5' (exons transcribed to pre-mRNA template) then into a mRNA sequence of 5'...AGG UCC UAG UAA...3' (codons spliced into mature mRNA).
Messenger RNA is first synthesized by genes as nuclear heterogeneous RNA (hnRNA), being so called because hnRNAs varies enormously in their molecular weight as well as in their nucleotide sequences and lengths, which reflects the different proteins they are destined to code for translation. Most hnRNAs of eukaryotic cells are very big, up to 50,000 nucleotides, and display a poly-A tail that confers stability to the molecule. These molecules have a brief existance, being processed during transcription into pre-mRNA and then in mRNA through splicing.
The molecular weight of mRNAs also varies in accordance with the protein size they encode for during translation. Because three nucleotides are needed for the translation of each amino acid that will constitute the polypeptide chain during protein synthesis, they necessarily are much bigger than the protein itself. Prokaryotic mRNA molecules usually have a short existence of about 2 minutes, but the fast bacterial mRNA turnover allows for a quick response to environmental changes by these unicellular organisms. In mammals, the average life span of mRNA goes from 10 minutes up to two days. Therefore, eukaryotic cells in mammals have different molecules of mRNA that show a wide range of different degradation rates. For instance, mRNA of regulatory proteins, involved either in cell metabolism or in the cell cycle control, generally has a short life of a few minutes, whereas mRNA for globin has a half-life of 10 hours.
The enzyme RNA-polymerase II is the transcriptional element in human eukaryotic cells that synthesizes messenger RNA. The general chemical structure of most eukaryotic mRNA molecules contain a 7-methylguanosine group linked through a triphosphate to the 5' extremity, forming a cap. At the other end (i.e., 3' end), there is usually a tail of up to 150 adenylils or poly-A. One exception is the histone mRNA that does not have a poly-A tail. It was also observed the existence of a correlation between the length of the poly-A tail and the half-life of a given mRNA molecule.
At the biochemical level, RNA molecules are linear polymers that share a common basic structure comprised of a backbone formed by an alternating polymer of phosphate groups and ribose (a sugar containing five carbon atoms). Organic nitrogenous bases i.e., the purines adenine and guanine, and the pyrimidines cytosine and uracil are linked together through phosphodiester bridges. These four nitrogenous bases are also termed heterocyclic bases and each of
The mRNA molecules contain at the 5' end a leader sequence that is not translated, known as UTR (untranslated region) and an initiation codon (AUG), that precedes the coding region formed by the spliced exons, which are termed codons in the mature mRNA. At the end of the coding region, three termination codons (UAG, UAA, UGA) are present, being followed by a trailer sequence that constitutes another UTR, which is by its turn followed by the poly-A tail. The stability of the mRNA molecule is crucial to the proper translation of the transcript into protein. The poly-A tail is responsible by such stability because it prevents the precocious degradation of mRNA by a 3' to 5' exonuclease (a cytoplasmatic enzyme that digests mRNA starting from the extremity 3' when the molecule leaves the cell nucleus). The mRNA of histones, the nuclear proteins that form the nucleosomes, do not have poly-A tails, thus constituting an exception to this rule. The poly-A tail also protects the other extremity of the mRNA molecule by looping around and touching the 7-methylguanosine cap attached to the 5' extremity. This prevents the decapping of the mRNA molecule by another exonuclease. The removal of the 7-methylguanosine exposes the 5' end of the mRNA to digestion by the 5' to 3' exonuclease (a cytoplasmatic enzyme that digests mRNA starting from the 5' end). When the translation of the protein is completed, the enzymatic process of deadenylation (i.e., enzymatic digestion of the poly-A tail) is activated, thus allowing the subsequent mRNA degradation by the two above mentioned exonucleases, each working at one of the ends of the molecule.
Transfer RNA (tRNA) is often referred to as the "Rosetta Stone" of genetics, as it translates the instructions encoded by DNA, by way of messenger RNA (mRNA), into specific sequences of amino acids that form proteins and polypeptides. This class of small globular RNA is only 75 to 90 nucleotides long, and there is at least one tRNA for every amino acid. The job of tRNA is to transport free amino acids within the cell and attach them to the growing polypeptide chain. First, an amino acid molecule is attached to its particular tRNA. This process is catalyzed by an enzyme called aminoacylRNA synthetase that binds to the inside of the tRNA molecule. The molecule is now charged. The next step, joining the amino acid to the polypeptide chain, is carried out inside the ribosome. Each amino acid is specified by a particular sequence of three nucleotide bases called codons. There are four different kinds of nucleotides in mRNA. This makes possible 64 different codons (43). Two of these codons are called STOP codons; one of these is the START codon (AUG). With only 20 different amino acids, it is clear that some amino acids have more then one codon. This is referred to as the degeneracy of the genetic code. On the other end of the tRNA molecule are three special nucleotide bases called the anti-codon. These interact with three complimentary codon bases in the mRNA by way of hydrogen bonds. These weak directional bonds are also the force that holds together the double strands of DNA.
In order to understand how this happens, it was necessary to first understand the three dimensional structure (conformation) of the tRNA molecule. This was first attempted in 1965, where the two-dimensional folding pattern was deduced from the sequence of nucleotides found in yeast alanine tRNA. Later work (1974), using x-ray diffraction analysis, was able to reveal the conformation of yeast phenylalanine tRNA. The molecule is shaped like an upside-down L. The vertical portion is made up of the D stem and the anti-codon stem, and the horizontal arm of the L is made up of the acceptor stem and the T stem. Thus, the translation depends entirely upon the physical structure. At one end of each tRNA is a structure that recognizes the genetic code, and at the other end is the particular amino acid for that code. Amazingly, this unusual shape is conserved between bacteria, plants, and animals.
Another unusual thing about tRNA is that it contains some unusual bases. The other classes of nucleic acids can undergo the simple modification of adding a methyl (CH3/sup>) group. However, tRNA is unique in that it undergoes a range of modifications from methylation to total restructuring of the purine ring. These modifications occur in all parts of the tRNA molecule, and increase its structural integrity and versatility.
Ribosomes are composed of ribosomal RNA (as much as 50%) and special proteins called ribonucleoproteins. In eukaryotes (an organism whose cells have chromosomes with nucleosomal structure and are separated from the cytoplasm by a two membrane nuclear envelope and whose functions are compartmentalized into distinct cytoplasmic organelles), there are actually four different types of rRNA. One of these molecules is called 18SrRNA; along with some 30lus different proteins, it makes up the small subunit of the ribosome. The other three types of rRNA are called 28S, 5.8S, and 5S rRNA. One of each of these molecules, along with some 45 different proteins, is used to make the large subunit of the ribosome. There are also two rRNAs exclusive to the mitochondrial (a circular molecule of some 16,569 base pairs in the human) genome. These are called 12S and 16S. A mutation in the 12SrRNA has been implicated in non-syndromic hearing loss. Ribosomal RNA's have these names because of their molecular weight. When rRNA is spun down by ultracentrifuge, these molecules sediment out at different rates because they have different weights. The larger the number, the larger the molecule.
The larger subunit appears to be mainly involved in such biochemical processes as catalyzing the reactions of polypeptide chain elongation and has two major binding sites. Binding sites are those parts of large molecule that actively participate in its specific combination with another molecule. One is called the aminoacyl site and the other is called the peptidyl site. Ribosomes attach their peptidyl sites to the membrane surface of the rER. The aminoacyl site has been associated with binding transfer RNA. The smaller subunit appears to be concerned with ribosomal recognition processes such as mRNA. It is involved with the binding of tRNA also. The smaller subunit combines with mRNA and the first "charged " tRNA to form the initiation complex for translation of the RNA sequence into the final polypeptide.
The precursor of the 28S, 18S and the 5.83S molecules are transcribed by RNA polymerase I (Pol I) and the 5S rRNA is transcribed by RNA polymerase III (PoIII). Pol I is the most active of all the RNA polynmerases, and is one indication of how important these structures are to cellular function.
Ribosomal RNAs fold in very complex ways. Their structure is an important clue to the evolutionary relationships found between different kinds of organisms. Sequence comparisons of the various rRNAs across various species show that even though their base sequences vary widely, evolution has conserved their secondary structures, therefore, organization must be important for their function.
Since the 1970s, nucleic acids' cellular processes have become the basis for genetic engineering, in which scientists add or remove genes in order to alter the characteristics or behavior of cells. Such techniques are used in agriculture, pharmaceutical and other chemical manufacturing, and medical treatments for cancer and other diseases.
See also Biochemistry; Genetic regulation of eukaryotic cells