Rna (Ribonucleic Acid)
Ribonucleic acid (RNA) is one of the nucleic acids that contain instructions for carrying out cellular processes. In cells, RNA works with deoxyribonucleic acid (DNA), which contains the organism's genetic information, or genes.
An RNA molecule consists of a chain of phosphate-base-ribose sugar nucleotides. The nucleotide has one of four bases. Two are pyrimidines: cytosine (C) and uracil (U); the other two are purines: adenine (A) and guanine (G). The molecule's composition was defined in the early twentieth century by the American biochemist Phoebus Levene, and its configuration was determined in the 1940s by the British organic chemist Alexander Todd.
RNA was first synthesized in 1955 by the American biochemist Severo Ochoa. In the 1960s, scientists discovered that three consecutive bases, called a triplet or codon, comprise the genetic code or instruction for production of a protein. The American Marshall Nirenberg, Ochoa, and others matched specific amino acids to specific triplets. A gene is composed of one or more triplets. RNA serves as the genetic material in a class of viruses called retroviruses.
RNA molecules play seven roles in cells. The first is messenger RNA (mRNA). The earliest role scientists discovered for RNA was the carrying of information from DNA in a cell's nucleus to the surrounding cytoplasm where proteins are produced. In the 1960s, the French geneticists François Jacob, André Lwoff, and Jacques Monod discovered that chemical signals within the cell determine whether a gene is "on," meaning its instructions are copied into RNA, or "off," meaning that the instructions are not copied. A gene is transcribed into RNA by proteins called transcription factors, producing the complementary nucleotide: DNA's base is copied into RNA as G C C G A U T A (T is the DNA pyrimidine thymine, which replaces RNA's U.) The resulting pre-messenger RNA (pre-mRNA) becomes mRNA when non-coding nucleotides, called introns, are spliced out and sometimes also when code editing occurs. The mRNA then moves into the cytoplasm for translation into protein.
Another function of RNA is as a transcription factor. In at least one species, RNA functions as a transcription factor along with proteins to copy DNA into pre-messenger RNA. Thirdly, some RNA operates as spliceosomes. These are assemblies of particles made of very small RNA molecules (small nuclear RNA, or snRNA) and protein. They work with other proteins to bring the splice sites of pre-mRNA together after non-coding introns have been removed, forming mRNA. Some RNA sequences act as transfer RNA (tRNA). In preparation for translation of mRNA information into protein, the cell produces RNA triplets and stores them in the cytoplasm, where enzymes attach each triplet's amino acid to it. These tRNA molecules "read" the incoming mRNA and, when matches are made, connect their amino acids in a chain at the ribosomes. tRNA was discovered in the 1950s by the American biochemist Mahlon Bush Hoagland, and it was first chemically isolated in the early 1960s by Robert Holley.
Ribosomal RNA (rRNA) are protein-RNA bodies that assemble amino acids into peptides (proteins) in a process that is not yet completely understood. rRNA is synthesized and assembled by bodies in the cell's nucleus called nucleoli. Catalytic RNA or ribozyme works like an enzyme has been found in some species, cutting and reconnecting its own nucleotide links. This self-splicing process was discovered in 1981 and 1982 by American biochemist Thomas Cech (1947-) of the University of Colorado at Boulder, for which he shared the 1989 Nobel prize in chemistry. RNA is also part of the splicing enzyme ribonuclease Rnase P, working with a protein component to catalyze reactions. It was discovered in the 1980s by American biophysicist Sidney Altman (1939-) of Yale University, who also shared the 1989 Nobel prize in chemistry.
Retroviruses, which include the AIDS virus HIV, have genetic material made of RNA, rather than DNA. In 1970, the American molecular biologist Howard Temin proved that a retrovirus in a host cell reproduces by using its enzyme, reverse transcriptase, to copy the RNA into DNA. The cell then copies the DNA back into RNA as the next viral generation. The cell also transcribes the DNA into messenger RNA to produce the new virus's protein coating. The new virus then buds from the cell wall.
DNA-based cells are able to perform many functions because the 20 amino acids can be combined in so many ways. It had been thought that RNA-based life, unable to make proteins, could not be as versatile with just its four bases. In the 1960s, Francis Crick, co-discoverer of DNA's double helix, and other scientists said that such versatility would be provided if RNA could act as a catalyst to initiate cell changes. The discovery of the catalytic actions of ribozymes and Rnase P have increased interest in the possibility of RNA-based organisms as the earliest life on Earth--what is called the RNA World. Although this theory now dominates discussions of how life began, some scientists remain unconvinced.
This is the complete article, containing 813 words
(approx. 3 pages at 300 words per page).
