Rna | Research & Encyclopedia Articles

Rna

The letters RNA stand for ribonucleic acid, a type of nucleic acid. There are two types of nucleic acid--DNA (deoxyribonucleic acid) and RNA.DNA carries a cell's genetic information, but this information cannot be accessed without RNA. In some organisms, such as in certain viruses, RNA is the carrier of genetic information. It has also been discovered that RNA can sometimes act as a catalyst of chemical reactions.

RNA is almost always a single-stranded molecule constructed as a long chain of nucleotides. Each nucleotide contains a ribose molecule (a type of sugar), a phosphate group, and one of four nitrogen-containing cyclic units. The nitrogen-containing units are the bases: adenine, cytosine, guanine, and uracil. The nucleotides of DNA and RNA differ in two respects: an -OH group of the sugar present in RNA is -H in DNA and one of the bases of DNA is thymine rather than uracil.

DNA is a double-stranded molecule that contains the blueprint for constructing proteins necessary for cell structure and function. This blueprint details which amino acids to use and in what order they should appear. However, the information flow from DNA to protein is not direct. One strand of the DNA--the coding strand--serves as a master copy of the genetic code. The cell uses RNA to construct a working copy from that strand. Construction of this working copy is called transcription.

Transcription begins when an enzyme, RNA polymerase, binds to the coding strand of DNA just before the start of a gene. A gene is the stretch of DNA that codes for a particular protein or polypeptide. The RNA polymerase binds to DNA with the help of other proteins called transcription factors.

Once RNA polymerase binds to the DNA, RNA construction begins. The two strands of DNA separate and, for a short while, the coding strand is paired with RNA as a matching strand. The base pairing between the DNA strand and the RNA strand follows the same rules as base pairing between two strands of DNA. The key exception is that uracil is used in place of thymine. The mRNA (messenger RNA) detaches from DNA when transcription is complete. It may be modified before the next step, release into the cell's interior.

The next phase is translation of the code contained in mRNA. Translation relies on two other types of RNA: ribosomal RNA (rRNA) and transfer RNA (tRNA). Although there are three types of RNA, their differences are based more on their function and their associated proteins, rather than the nucleic acid itself.

The rRNA is found within a cellular structure called a ribosome. To start translation, a ribosome attaches itself to a strand of mRNA. The ribosome holds the mRNA in place and directs the speed of translation. The transcribed code carried on the mRNA is read one triplet--three nucleotides--at a time. There are 64 possible mRNA triplets, or codons. Sixty-one of them code for a specific amino acid; the remaining three serve as stop signals to end translation. Most amino acids have more than one associated codon.

As the ribosome gradually moves along the length of the mRNA, tRNA transports amino acids to the growing protein molecule. The tRNA contains a nucleotide sequence called the anticodon. The anticodon determines which amino acid the tRNA carries. When the anticodon pairs up with a corresponding mRNA codon, it releases the amino acid to the growing protein chain. Once the mRNA is translated, it is broken down by a special enzyme called ribonuclease. The component nucleotides can then be recycled by the cell.

In some cases, RNA serves as the carrier of genetic information. This situation appears with several viruses such as the tobacco mosaic virus and a class of viruses called retroviruses. Retroviruses include the human immunodeficiency virus (HIV), which causes acquired immunodeficiency syndrome (AIDS).

In the case of HIV, the virus contains RNA and an enzyme called reverse transcriptase. When the virus encounters a particular type of immune system cell, it injects its contents into it. The reverse transcriptase goes to work to construct a DNA molecule from the RNA. (This situation is the opposite of the usual mode of transcription, i.e., from DNA to mRNA.) The newly constructed DNA then attaches to the rest of the cell's DNA where it can hide for months or years before it is used.

The discovery that RNA can carry genetic information has led to other discoveries as well. One such discovery has been that an RNA molecule can sometimes act as an enzyme by catalyzing the modification of itself or other RNA molecules. This finding may provide insight into the origins of life. An outstanding scientific question regarding the origins of life has been whether enzymes or genes came first. Discovering that RNA can act as both a carrier of genetic information and as an enzyme may provide an answer to this question.

RNA is also taking a place in the areas of gene therapy and genetic engineering. Antisense technology makes use of a type of RNA called antisense RNA. Typically, a cell only contains mRNA that corresponds to the coding strand of DNA. However, by inserting an inverted copy of that gene into a cell's genetic material, it is possible to make the cell produce mRNA for both the coding strand and the matching strand. The two mRNA strands are complementary to one another and form a double-stranded molecule much like DNA. This pairing prevents the mRNA from being translated and the abnormal double-stranded RNA is often destroyed by ribonuclease.

Gene therapy is also making use of catalytic RNA through the construction of ribozymes. Ribozymes are catalytic RNA molecules that destroy RNA that has a specific sequence of nucleotides. A potential application of ribozymes might be treating certain diseases that are characterized by the presence of certain proteins. For example, if the mRNA for a viral protein were destroyed prior to being translated, new virus particles could not be constructed and the viral infection would be halted.