Ribonucleic Acid (RNA)
Ribonucleic acid (RNA)
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 (1844–1895). 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 2–3 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 them combines with one of the riboses of the backbone to form a nucleoside, such as adenosine, guanosine, cytidine, and uridine. The combination of a ribose, a phosphate, and a given nitrogenous base by its turn results in a nucleotide, such as adenylate, guanylate, cytidylate, uridylate. Each phosphodiester bridge links the 3' carbon at the ribose of one nucleotide to the 5' carbon at the ribose of the subsequent nucleotide, and so on. RNA molecules fold on themselves and form structures termed hairpin loops, because they have extensive regions of complementary guanine-cytosine (G-C) or adenine-uracil (A-U) pairs. Nevertheless, they are single polynucleotide chains.
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 aminoacyl—tRNA 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–) 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 30–plus 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
Ribonucleic acid (RNA) molecules, which are linear chains (or polymers) of ribonucleotides, perform a number of critical functions. Many of these functions are related to protein synthesis. Some RNA molecules bring genetic information from a cell's chromosomes to its ribosomes , where proteins are assembled. Others help ribosomes translate genetic information to assemble specific sequences of amino acids.
Ribonucleotides, the building blocks of RNA, are molecules that consist of a nitrogen-containing base, a phosphate group, and ribose, a five-carbon sugar. The nitrogen-containing base may be adenine, cytosine, guanine, or uracil. These four bases are abbreviated as A, C, G, and U.
RNA is similar to deoxyribonucleic acid (DNA), another class of nucleic acid. However, DNA nucleotides contain deoxyribose, not ribose, and they use the nitrogen-containing base thymine (T), not uracil, along with ade-nine, cytosine, and guanine.
The nucleotides in DNA and RNA molecules are linked together to form chains. The link between two nucleotides is between a phosphate group attached to the fifth (5′ or "five prime") carbon of the sugar on one nucleotide and a hydroxyl group on the third (3′ or "three prime") carbon of the sugar on the other. The link is called a 5′-3′ phosphodiester bond.
RNA, therefore, can be described as a chain of ribose sugars linked together by phosphodiester bonds, with a base protruding from each sugar, as shown in the figure below. The 5′-3′ linkage gives RNA directionality, or polarity, and results in its having two ends with different chemical structures. The 5′ end usually has one or three free phosphate groups, and the 3′ end usually has a free hydroxyl group.
Whereas DNA is usually double-stranded, with the bases on one strand pairing up with those on the other, RNA usually exists as single chains of nucleotides. The bases in RNA do, however, follow Watson-Crick base-pair rules: A and U can pair with each other, as can G and C. There is usually extensive pairing of bases within a single strand of RNA.
RNA strands fold, with the bases in one part of the strand pairing with the bases in another. Folding can create both "secondary" and "tertiary" structures. Secondary structures are those that can be described in two dimensions and that can be thought of as simple loops or helices. Tertiary structures are complex, three-dimensional shapes.
The most common secondary structures, "hairpins," "loops," and "pseudo-knots," are shown in the figure below. Such secondary structures are formed when hydrogen bonds form between bases in the nucleotides and by the stacking of bases to form helical structures.
Tertiary structures usually involve interactions between nucleotides that are distant from each other along an RNA strand. Such interactions may arise from hydrogen bonding between bases, as in regular Watson-Crick base pairing, or from interactions among other chemical groups in the nucleotides. Some RNA molecules, such as ribosomal RNA (rRNA) and transfer RNA (tRNA), have structures that are very complex. In structure they resemble proteins more than they do DNA.
To understand the function of a given RNA molecule, scientists often need to know its structure. There are three general strategies for analyzing RNA structure. First, using the relatively simple base-pairing rules for RNA and the basic principles of thermodynamics , computers can be used to predict secondary RNA structure, although not always with complete success.
Second, researchers can analyze RNA molecules from various organisms and compare those molecules that have the same function. Even when the nucleotide sequences vary between species, important structures are usually preserved.
Third, the structure of an RNA molecule can be determined experimentally, using enzymes to cut it or chemicals to modify it. Some enzymes and chemicals cut or modify only nonpaired, single-stranded portions of the RNA molecule, allowing researchers to identify double-stranded regions by examining which ones remain uncut and unmodified.
Despite the usefulness of each of these methods, none can provide a complete and accurate three-dimensional structure. A more complete determination of structure can be achieved by the biophysical methods of X-ray crystallography and nuclear magnetic resonance.
RNA molecules are synthesized by enzymes known as RNA polymerases in a process called transcription. Usually, one strand of a double-stranded DNA molecule is used as a template for the RNA. The order of ribonucleotides that are assembled to form the RNA molecule is determined by the order of the deoxyribonucleotides in the DNA strand. The genetic information in the DNA sequence is thus reproduced in the RNA molecule. Sometimes, but rarely, an RNA molecule is synthesized using another RNA molecule as the template.
Often, when RNA molecules are synthesized, they are in a form that prevents them from carrying out their function. To become functional, they must undergo processing, which can involve removing segments of the strands or modifying specific nucleotides. The link between a base and a ribose may be altered, or extra chemical groups may be added to the bases or ribose molecules. Many RNA molecules are associated with proteins during or after their synthesis. Together, the RNA and protein are referred to as RNA-protein particles (RNPs).
In eukaryotes , RNA that is encoded by nuclear chromosomes is synthesized in the nucleus. The processing and assembly of many small RNA molecules in higher eukaryotes is accomplished in Cajal bodies, which are coiled structures in the nucleus that were identified more than 100 years ago but that have begun to be investigated in detail only recently. The synthesis of those RNA molecules that are components of ribosomes occurs in the nucleolus, a part of the nucleus. RNA synthesis and processing also occurs in the mitochondria and chloroplasts, when the RNA will be used in those organelles.
After being processed and assembled, RNPs either remain in the nucleus or are exported to the cytoplasm through the nuclear pores. Some are also exported and modified in the cytoplasm and then imported back into the nucleus. In prokaryotes , where there is no nucleus, the synthesis and processing of RNA, as well as the assembly of RNPs, occurs in the cytoplasm.
Almost all types of RNA play a role in translation, which is the process of protein synthesis. Translation requires three types of RNA: messenger RNA (mRNA), which ranges in length from a few hundred to many thousands of nucleotides; tRNA, which is 75 to 85 nucleotides long; and rRNA, which is 1,500 to 4,000 nucleotides long.
Molecules of mRNA, each of which contains a copy of at least one gene, are the intermediates between DNA and protein. These mRNA molecules bring the genetic code from the DNA, which is in the nucleus, to ribosomes, which are in the cytoplasm. They attach to the ribosomes and determine the order in which amino acids are assembled to synthesize a protein. Of the three types of RNA required for translation, mRNA molecules have the simplest structure.
Next, tRNA molecules function as adapters that help translate the nucleotide sequences in mRNA into amino acid sequences, so specific proteins can be constructed. There are many different types of tRNA, each of which is capable of binding to one of the twenty amino acids that are the building blocks of proteins.
Finally, rRNA molecules, which account for most of a ribosome's mass, are, according to recent experiments, the part of the ribosome responsible for linking amino acids into a growing protein chain. Ribosomes, the organelles that assemble a particular sequence of amino acids to form proteins, contain three or four different molecules of rRNA, along with at least fifty different proteins.
Both rRNA and tRNA are stable forms of RNA that last through several cell divisions. In contrast, mRNA is normally unstable, with a lifetime that can be as short as a few minutes. This instability has probably evolved because it lets cells quickly stop synthesizing proteins that are no longer needed. In some cases, enzymes called ribonucleases (RNases) actively degrade certain mRNA molecules. For example, mRNA that encodes a particular protein regulating the cell cycle is degraded when the protein has carried out its function.
In certain cells, mRNA can exist in a stable form for decades. When egg cells are formed, for example, some of the mRNA in the cells is associated with "storage proteins" and lasts until after the eggs are fertilized. During embryonic development, this maternal mRNA becomes activated for translation and associates with translating ribosomes. It usually decays after it has been used to produce a certain amount of protein.
Less Common Types of RNA
Several types of less abundant, small RNA molecules perform essential functions in both the nucleus and the cytoplasm. All organisms contain cytoplasmic RNPs that are involved in exporting proteins from cells. During the synthesis of proteins that are destined to be exported, the ribosome and mRNA associate with an "export-RNP," which helps them dock at an export pore in the cell membrane. As it formed, the protein is threaded through the membrane to the outside of the cell. In eukaryotes, this same strategy is used to transport proteins into the endoplasmic reticulum , where some newly synthesized proteins are sorted and modified.
RNase P is another RNP found in all forms of life. This RNA-containing enzyme helps turn precursor tRNA into mature tRNA molecules. It does so by cleaving a section off the 5′ end of the precursor molecules.
Small nucleolar RNAs, which are known as snoRNAs and which are found in the nucleoli of eukaryotes and in Archaea , are required for the processing of precursor rRNA. During the assembly of new ribosomes, snoRNAs help remove regions of the precursor molecules and modify specific nucleotides.
Often, mRNA molecules in eukaryotes and in Archaea contain sequences that do not code for amino acids. These sequences, called introns, must be spliced out before translation begins. In eukaryotes, small nuclear RNAs (snRNAs) in the nucleus remove these introns. Once the introns are removed, the mature mRNA molecules are exported, through nuclear pores, into the cytoplasm, where they associate with ribosomes for translation.
Some viral genomes consist of single-stranded or double-stranded RNA, not DNA. Examples are found among both prokaryotic and eukaryotic viruses and include HIV, as well as viruses causing some forms of cancer.
see also Evolution, Molecular; Nucleotide; Ribosome; RNA Polymerases; RNA Processing; Transcription; Translation.
Lodish, Harvey, et al. Molecular Cell Biology, 4th ed. New York: W. H. Freeman, 2000.
Meili, M., B. Albert-Fournier, and M. C. Maurel. "Recent Findings in the Modern RNA World." International Microbiology 4 (2001): 5-11.
Robinson, Richard. Biology. Farmington Hills, MI: Macmillan Reference USA, 2002.
Storz, G. "An Expanding Universe of Non-coding RNAs." Science 296 (2002): 1260-1263.
Ribonucleic acid (RNA), like deoxyribonucleic acid (DNA), is a polymer made up of nucleotides . A nucleotide is composed of a pentose (5-carbon) sugar, a nitrogen-containing base, and phosphate. The pentose sugar found in RNA nucleotides is ribose, whereas that in DNA is 2′ (2-prime) deoxyribose. The bases commonly found in RNA nucleotides are adenine (A), guanine (G), cytosine (C), and uracil (U). Bases found in DNA are A, G, C, and thymine (T instead of U). As in DNA, the individual nucleotides in the polymer are joined together by phosphodiester bonds. Unlike DNA, RNA is single-stranded; however, many RNA molecules fold into complex three-dimensional structures.
During transcription the DNA code is read and copied into RNA. The sequence of nucleotides in an RNA is therefore determined by the sequence of nucleotides in the gene from which it was transcribed. Following transcription, RNA may be processed before it becomes functional.
There are three main classes of RNA: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). Each of the classes is important in some aspect of protein synthesis. The nucleotide sequence of a messenger RNA specifies the order of amino acids in the protein which it encodes. A cell contains many different mRNA molecules, each being the blueprint for a different protein. Although mRNAs are the least abundant class of RNA, they are the most heterogeneous . Ribosomes play an important role in protein synthesis, and ribosomal RNA (rRNA), is an important structural component of ribosomes. rRNA is the most abundant type of RNA. tRNAs act as adaptors in protein synthesis, in that they read the sequence of nucleotides in the mRNA and deliver the correct amino acid to the growing polypeptide chain.
Most scientists believe that life has evolved from what was essentially an "RNA world." In today's world, most organisms store their genetic information in DNA and use proteins (encoded by DNA) to catalyze biologically important chemical reactions. RNA molecules, however, are believed to have been the first biological catalysts. Through evolution, some of these RNA molecules gained the ability to replicate themselves, and through many rounds of replication, the RNA molecules gained new capabilities, such as the ability to code for and synthesize proteins. Eventually, the RNA genome was replaced with DNA.
Scientists have uncovered a number of enzymatic RNA molecules, called ribozymes, believed to be typical of those in the RNA world. RNA enzymes can make phosphodiester bonds, suggesting that early RNA molecules could reproduce their genetic material. In fact, it is now known that RNA in the ribosome catalyzes the formation of peptide bonds during protein synthesis, supporting the idea that RNA molecules were able to synthesize proteins. Even in the twenty-first century, not all genomes are composed of DNA: some very important viruses, such as the one that causes AIDS (acquired immunodeficiency syndrome), has RNA as its genetic material. However, the so-called RNA viruses express their genome only after they have turned it into DNA.
see also Ribosome; RNA Processing; Transfer RNA
James E. Blankenship
Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.
Stryer, Lubert. Biochemistry, 4th ed. New York: W. H. Freeman and Company, 1995.
Ribonucleic acid (RNA) is a nucleic acid, like DNA , and is also made up of sugars, phosphates, and nitrogenous bases (or just a base). It contains a ribose sugar, whereas DNA contains a deoxyribose sugar. The four main bases found in RNA are guanine (G), cytosine (C), uracil (U), and adenine (A). DNA contains thymine (T) instead of uracil.
The ribose sugar of RNA gives it very different structural properties from those of DNA. RNA forms complex structures by folding a single strand upon itself. Ribose sugar makes RNA less stable than DNA, and it has to be produced often in a cell.
In most organisms RNA is made from their DNA. Some viruses, called retroviruses, contain RNA as their primary genetic material, for example, the AIDS virus and poliovirus.
There are many different kinds of RNA manufactured in a cell. Messenger RNA (mRNA) is produced to take the information contained in a specific segment of DNA and then use it to make proteins. Ribosomal RNA (rRNA) is part of a large RNA protein complex called the ribosome that binds mRNA and joins amino acids to make a protein. Transfer RNA (tRNA) brings amino acids to the ribosome and ensures that the amino acid used is in the order specified by mRNA. Many other kinds of RNA are also present in a cell. RNA plays an important role in the proper functioning of a cell.
see also Dna Replication; Nucleic Acids; Proteins.
DNA from the Beginning. Information available from <http://www.dnaftb.org/dnaftb/>.
"DNA: The Instruction Manual for All Life." The Tech. Available from <http://www.thetech.org/exhibits_events/online/genome/>.
"Structure of RNA." Molecular Biology Notebook Online. Available from <http://www.rothamsted.bbsrc.ac.uk/notebook/courses/guide/rnast.htm>.