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Nucleic Acids

All genetic information needed for life and the inheritance of characters is encoded in biomolecules known as nucleic acids. There are two forms of nucleic acid - DNA or deoxyribonucleic acid and RNA or ribonucleic acid, both of these forms are the basis of all life and occur in all living cells. In chemical structure, these two molecules are linear and without branching; they are made essentially as polymers of discrete subunits termed the nucleotides - this is the language in which all life on earth is written, encoded and passed on from one generation to the next. Bacteria - called prokaryotes - along with all the "higher organisms" collectively called the eukaryotes use DNA as their primary carrier of genetic information and RNA as an information messenger. This DNA is a part of the nucleus of all eukaryotes as well as the nucleiod of prokaryotes - prokaryotes include the bacteria, while the eukaryotes form the fungi, higher plants and animals. While RNA serves as the genetic information carrier in some viruses and is used as the sole molecule of heredity in these viruses. RNA is used only as a messenger in higher forms of "true" life excluding the viruses, in these forms, the RNA molecules are written using the DNA as the template. The RNA encoded information is than translated into protein, via bio-synthetic mechanism in the cytoplasm of the cell. Thus, DNA carries all information, which is then transcribed into RNA and then translated into protein - which then performs necessary vital actions in the living cell. This is called the central dogma of molecular biology.

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The basic components of the nucleic acids, the nucleotides are structurally formed from three major units. Every single nucleotide will consist of one five-carbon pentose sugar - ribose in RNA, and deoxy-ribose in DNA; One flat, heterocyclic, nitrogen rich organic base and lastly one phosphate group - this last unit of the nucleic acid polymer is responsible for the acidic nature of the nucleic acid as it is very negative in charge. These components of the nucleic acids are linked together covalently through a glycosidic bond formed between the sugar unit and the nitrogenous base. The addition of the phosphate group also covalently connected to the sugar unit completes the basic component of the nucleic acid polymer.

RNA is made from monomers of the sugar β-D-ribose and the sugar along with the nitrogenous base and inorganic phosphate is called a ribonucleotide - without the phosphate, it's a nucleoside. DNA is made of monomers formed by the pentose sugar deoxy-ribose, along with a nitrogenous base and inorganic phosphate to make a deoxyribonucleotide. The DNA monomers are differentiated from the RNA monomers by the absence of an oxygen at the number two carbon in the sugars and are called a 2-deoxy-β-D-ribose sugars.

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The nitrogenous bases can be classed into two basic types of organic bases. Pyrimidines are single-ringed structures while the purines are double ringed-structures. There are three types of pyrimidines and two types of purines used in the construction of nucleic acids - all of them are not used in both RNA and DNA, which is the reason for the difference between the nucleic acids. The bases adenine and guanine are the purines found in both RNA and DNA. While the pyrimidines come in three types, cytosine, thymine, and uracil - the last replaces thymine in RNA and is not found in DNA. While the pyrimidine and thymine are found primarily in DNA, uracil is seen only in RNA. A purine always pair with a pyrimidine and vice versa in a nucleic acid. Along each polynucleotide strand forming DNA or RNA, all the nucleotides adjacent to each other are joined by covalent bonds called phosphodiester bonds formed between the number three carbon of one nucleotide and the number five carbon of the nearest nucleotide. In this way the monomers are interlinked to form a nucleic acid polymer holding all the genetic information necessary for life.

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Genetic information is stored along the nucleic acid chain because all the bases in the nucleotides form hydrogen bonds with each other in a specific way - this ensures what is called base pairing. Base pairing is the key behind the code formed from four bases. For example, adenine will always combine with thymine in DNA with the formation of two hydrogen bonds, while guanine will always base pair with cytosine via three hydrogen bonds. Since the hydrogen formation occurs between two different strands of linear DNA, it results in the formation of a DNA double helix - formed by two complementary DNA strands holding genetic information. Similarly, adenine can also form hydrogen bonds with uracil in DNA-RNA hybrid chains as well as in RNA to RNA complexes. In both DNA and RNA, the base guanine will always form three hydrogen bonds with cytosine. In other words, guanine always pairs with cytosine in DNA as well as RNA, while adenine pair with thymine in DNA but with uracil in RNA. Viable DNA is found in the uniform shape of a double helix in chromosomes, each complementary chain is wound around the other complement in a spiral like a staircase. All nucleic acid chains do not exist in the form of double strands as the RNA molecules that are synthesized from DNA templates are always in the form of single strands - some viruses which use RNA as a genetic information carrier have double stranded RNA. In the body, a single stranded RNA chain is sometimes folded back onto itself so as to form complementary base pairs along the length of the chain to produce unique secondary structures necessary for some biochemical reactions.

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Along a DNA double helix, each of the two complementary strands forming the helix are found in opposite directions to each other, they are said to be anti-parallel in orientation, which means one chain is starts from the 5' phosphate end, while the other chain starts from the 3' hydroxyl end. In a DNA double helix, the winding of the two chains results in a turn along the helix at every ten base pairs, this has been measured to be about 3.4 nm - nanometers in length. Base pairings occur along the center of the molecule resulting in stacking of the bases along the chain. As the bases repel water, they form a hydrophobic core at the center and the double helix as a result is approximately two nm across.

RNA

RNA serve as information messengers for the DNA. The code on the DNA chain is copied onto an RNA chain and this information is translated into a protein, which then affects the desired outcome in the cell. In higher organisms, RNA comes in three basic types according to the function it performs inside the cell.

The tRNA or transfer RNAs transfer one amino acid at a time during the formation of a protein.

The mRNA or messenger RNAs carry the information written in the DNA to the ribosomes for translation into protein.

The rRNA or ribosomal RNAs aid in the formation of proteins on ribosomes - which are the protein manufacturing centers in all cells.

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All the different classes of RNA are important for the body. In terms of size, the transfer RNAs or tRNAs are the smallest of the lot, being approximately about seventy five to eighty nucleotides long - their main action is the formation of a protein polymer and each tRNA positions single amino acids on the ribosome during the polymerization reaction to form distinct polypeptide chains - which in turn form proteins. The tRNAs also contain additional and unusual bases along with the normal complement of adenine, guanine, cytosine and uracil in their chains - this is in keeping with their specialized nature and function. The linear mRNAs differ according to the information they transcribe from the genetic code in the DNA template. As the genetic code that will specify the unique amino acid sequence for specific proteins resides in the DNA sequence, it follows that the complementary ribonucleotide sequences forming the mRNA will be long or short depending on the length and composition of the template DNA - therefore, mRNA chains are highly variable in length and composition. On the other hand, ribosomal or rRNAs are integral to the structure of the ribosomes - protein building sites in cells. There are also different types of rRNAs, there are four main types of rRNAs in eukaryotes while three main classes of rRNAs are found in the prokaryotes - namely the bacteria and archea.

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