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Medical Specializations


Microbiology => Procaryotes => Ribonucleic Acid


Ribonucleic Acid


INTRODUCTION
Ribonucleic Acid (RNA), genetic material of certain viruses (RNA viruses) and, in cellular organisms, the molecule that directs the middle steps of protein production. In RNA viruses, the RNA directs two processes-protein synthesis (production of the virus's protein coat) and replication (the process by which RNA copies itself). In cellular organisms, another type of genetic material, called deoxyribonucleic acid (DNA), carries the information that determines protein structure. But DNA cannot act alone and relies upon RNA to transfer this crucial information during protein synthesis (production of the proteins needed by the cell for its activities and development).
Like DNA, RNA consists of a chain of chemical compounds called nucleotides. Each nucleotide is made up of a sugar molecule called ribose, a phosphate group, and one of four different nitrogen-containing compounds called bases. The four bases are adenine, guanine, uracil, and cytosine. These components are joined together in the same manner as in a deoxyribonucleic acid (DNA) molecule. RNA differs chemically from DNA in two ways: The RNA sugar molecule contains an oxygen atom not found in DNA, and RNA contains the base uracil in the place of the base thymine in DNA.

CELLULAR RNA
In cellular organisms, RNA is a single-stranded polynucleotide chain, a strand of many nucleotides linked together. There are three types of RNA. Ribosomal RNA (rRNA) is found in the cell's ribosomes, the specialized structures that are the sites of protein synthesis). Transfer RNA (tRNA) carries amino acids to the ribosomes for incorporation into a protein. Messenger RNA (mRNA) carries the genetic blueprint copied from the sequence of bases in a cell's DNA. This blueprint specifies the sequence of amino acids in a protein. All three types of RNA are formed as needed, using specific sections of the cell's DNA as templates.

VIRAL RNA
Some RNA viruses have double-stranded RNA-that is, their RNA molecules consist of two parallel polynucleotide chains. The base of each RNA nucleotide in one chain pairs with a complementary base in the second chain-that is, adenine pairs with uracil, and guanine pairs with cytosine. For these viruses, the process of RNA replication in a host cell follows the same pattern as that of DNA replication, a method of replication called semi-conservative replication. In semi-conservative replication, each newly formed double-stranded RNA molecule contains one polynucleotide chain from the parent RNA molecule, and one complementary chain formed through the process of base pairing. The Colorado tick fever virus, which causes mild respiratory infections, is a double stranded RNA virus.

There are two types of single-stranded RNA viruses. After entering a host cell, one type, polio virus, becomes double-stranded by making an RNA strand complementary to its own. During replication, although the two strands separate, only the recently formed strand attracts nucleotides with complementary bases. Therefore, the polynucleotide chain that is produced as a result of replication is exactly the same as the original RNA chain.
The other type of single-stranded RNA viruses, called retroviruses, include the human immunodeficiency virus (HIV), which causes AIDS, and other viruses that cause tumors. After entering a host cell, a retrovirus makes a DNA strand complementary to its own RNA strand using the host's DNA nucleotides. This new DNA strand then replicates and forms a double helix that becomes incorporated into the host cell's chromosomes, where it is replicated along with the host DNA. While in a host cell, the RNA-derived viral DNA produces single-stranded RNA viruses that then leave the host cell and enter other cells, where the replication process is repeated.

RNA AND THE ORIGIN OF LIFE
In 1981, American biochemist Thomas Cech discovered that certain RNA molecules appear to act as enzymes, molecules that speed up, or catalyze, some reactions inside cells. Until this discovery biologists thought that all enzymes were proteins. Like other enzymes, these RNA catalysts, called ribozymes, show great specificity with respect to the reactions they speed up. The discovery of ribozymes added to the evidence that RNA, not DNA, was the earliest genetic material. Many scientists think that the earliest genetic molecule was simple in structure and capable of enzymatic activity. Furthermore, the molecule would necessarily exist in all organisms. The enzyme ribonuclease-P, which exists in all organisms, is made of protein and a form of RNA that has enzymatic activity. Based on this evidence, some scientists suspect that the RNA portion of ribonuclease-P may be the modern equivalent of the earliest genetic molecule, the molecule that first enabled replication to occur in primitive cells.

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