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Composition and Structure
DNA is a larger, lengthier structure than RNA. DNA contains two strands that complement each other and tether to one another via chemical bonds. RNA consists of a single strand. DNA appears similar to a twisted ladder, whereas RNA is merely a half of a ladder. RNA uses ribose as its sugar component while DNA uses deoxyribose, which is exactly the same as ribose, minus an oxygen atom.
Both types of nucleic acids comprise nucleotides, structures made of alternating sugar molecules and phosphates linked to another molecule -- a nitrogenous base. The sugars and phosphates alternate with one another, forming the ̶0;rungs̶1; of the ladder. The nitrogenous bases hang off of the sugar component. Nitrogenous bases exist as two types: the purines and pyrimidines. Both DNA and RNA contain the purines adenine and guanine. DNA uses the pyrimidines cytosine and thymine, but RNA contains cytosine and uracil.
Functions
DNA has a single, central role in cells: to store the code for genetic information. Three different types of RNA exist in cells, and each type has a particular structure and function. Messenger RNA is made when the cell needs to produce proteins. During the process called transcription, a signal triggers the DNA strands to unwind, and mRNA builds, nucleotide by nucleotide, along the single DNA strand. The single strand of mRNA travels to a ribosome. Ribosomal RNA, or rRNA, makes up part of the ribosomes, structures where proteins are synthesized. Transfer RNA, or tRNA, carries amino acids -- the basic units that form proteins -- to the ribosomes to bond with the strand of mRNA. Each tRNA holds a single, specific amino acid. The protein builds along the mRNA chain, one amino acid at a time. Once the tRNA releases the amino acid, it goes to pick up another and returns to the protein synthesis site.
Distribution
DNA either lies in specified areas of cells or remains within the nucleus where it is protected by the nuclear envelope. RNA, which occurs in greater numbers than DNA, is spread throughout cells. mRNA does not exist until a signal from the nucleus calls for protein synthesis, and the mRNA strand starts assembling opposite to its DNA template in the nucleus. Actually lying within the ribosomes, rRNA holds the growing protein in place. Meanwhile, tRNA molecules float around in the cytoplasm -- the gelatinous substance that makes up the interior of a cell. While a strand of mRNA is held in place at the ribosome, the tRNA scramble around the cytoplasm looking for free-floating amino acids specific to particular tRNA units.
Stability
RNA seems to have been the precursor to DNA, but over time, DNA proved to be better adapted to holding genetic material. DNA is structurally more stable than RNA in part because of the composition of its sugar portion; deoxyribose, which lacks an oxygen, does not react as readily as ribose. At times, sugar molecules even lose their attachments to the nitrogenous bases; these errors happen with more frequency in RNA than in DNA. The double strand of DNA also stabilizes the molecule, preventing chemicals from easily destroying it.
Because DNA consists of two strands, it can repair itself by using the unaffected strand to assemble a new opposing strand. During the replication process, mistakes occur more often in duplicating RNA than in DNA. Finally, the energy needed to break down RNA is less than that of DNA, meaning RNA can be broken down more easily.
Virus Implications
A virus, considered nonliving, uses either DNA or RNA as its genetic code. Whether a virus has DNA or RNA figures significantly in the potency of the virus. In general, RNA viruses tend to cause more dangerous illnesses. Since RNA is less stable than DNA, it mutates up to 300 times the rate of DNA viruses. Frequent mutations cause RNA viruses to better adapt to host immune systems. Viruses often enter their hosts via the body of an intermediate carrier species called a vector. DNA viruses have more limitations on vectors than RNA viruses, which means more organisms can carry and transmit RNA viruses. Additionally, DNA viruses tend to stick to one host whereas RNA viruses may be able to infect a vast array of hosts.