RNA (ribonucleic acid) is a single-stranded nucleic acid. This molecule is involved in protein synthesis. There are several types of RNA:
mRNA: This is messenger RNA, which serves as a code for proteins. It does this by carrying information in what are called “codons” or a “triplet code” that is specific for a particular amino acid.
rRNA: This is ribosomal RNA, which forms the basic structure of the ribosomes. rRNA also catalyzes protein synthesis.
tRNA: This is transfer RNA, which serves as an adaptor between mRNA and amino acids. One end of the tRNA has an “anticodon” that matches with the mRNAs codon; the other end has an amino acid attached.
snRNA: This is small nuclear RNA, which serves to function in a variety of nuclear processes, as well as structural and catalytic roles, mainly the splicing of pre-mRNA.
snoRNA: This is small nuclear RNA, which is involved in processing and chemically modifying rRNA.
hnRNA: This is heterogeneous nuclear RNA, which is pre-mRNA that contains noncoding regions (called introns) that separate coding regions (called exons).
Other noncoding RNAs: These RNAs function in other diverse cellular processes, including X-chromo-some inactivation, telomere synthesis, and the transportation of unprocessed proteins to the endoplasmic reticulum.
Compared with the DNA molecule, the RNA molecule has some basic differences in structure. The two molecules have many of the same nucleotides: adenine, guanine, and cytosine. However, only DNA has thymine, whereas RNA contains uracil instead. Another major difference is in the sugar that makes up the phosphate backbone. The DNA molecule contains deoxyribose, whereas the RNA molecule contains ribose. In addition, they both have functional differences; RNA synthesizes proteins, whereas DNA synthesizes RNA.
The RNA molecule, initially, may sound like a small insignificant molecule; however, it is capable of supporting extremely small and simple life forms called “viruses.” The viral genome can be made up of RNA (single-stranded or double-stranded), or DNA (also single-stranded or double-stranded). Viruses are called “obligate intracellular parasites,” because they need to use the host’s enzymes and ribosomes in order to synthesize copies of their own genomes. Also, viruses are not actually cells; rather, they consist only of nucleic acid enclosed in a protein shell, called a “capsid,” and they need to infect a host cell they replicate.
The prevailing belief in modern science is that RNA was most likely the first genetic material to exist. In a primordial sense, the formation of genetic information would have made it possible for molecular aggregates (precursors to simple cells) to pass along not only transient samples of molecules, but more specifically, permanent molecular instructions to make more of these molecules.
During the process of transcription, DNA produces RNA, and during the process of translation, RNA (along with ribosomes, which are also made of RNA) produces proteins (both structural proteins and enzymes). These processes involve intricate machinery, which could not have evolved all at once. However, it is conceivable that it did emerge from a simpler process. Therefore, prior to the existence of DNA, a more primitive mechanism involving the alignment of amino acids along strands of RNA could have existed. Consequently, the first “genes” were most likely not the more organized DNA molecule, rather short strands of RNA that could have self-replicated in a prebiotic environment.
In the 1980s, a scientist by the name of Thomas Cech proved that RNA molecules were important catalysts in eukaryotic cells. This discovery extinguished the axiom that only proteins could act as a biological catalyst. Cech and other researchers discovered “ribozymes,” which help to catalyze the synthesis of new mRNA, tRNA, and rRNA. Taking this into account, it can be visualized that RNA could have been autocatalytic and capable of consistent self-replication in a prebiotic environment long before proteins, enzymes, or DNA existed.
It is well established that the DNA molecule is a far more stable repository than the RNA molecule for the storage of genetic material. Conceivably, once DNA molecules evolved, RNA molecules maintained their primitive role as intermediates in the translation of genetic expression. Consequently, the DNA molecule became the storage center that can synthesize mRNA. However, the model provided by the RNA molecule paved the way for the DNA molecule.
References:
- Alberts, B., Johnson, A., Lewis, J., Raff, K., Roberts, K., & Walter, P. (2002). Molecular biology of the cell (4th ed.). New York: Garland.
- Cech, T. (1986). RNA as an enzyme. Scientific American, 255(5), 64-75. Hartmann, R. K., Bindereif, A., Schon, A, &
- Westhof, E. (2005). Handbook of RNA biochemistry. New York: John Wiley.
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