Nucleotides

The foundation of life as we know it is composed of nucleic acids. Researchers believe that these fundamental building blocks were first formed three billion years ago when the first forms of elementary life began to appear on earth. Nucleic acids have at least two functions. The first of these functions is to serve as a means of passing on hereditary characteristics, and the second is to instruct the cells in the production of specific proteins. Nucleic acids by nature are rather complex and in large numbers, with the help of other types of compounds, create DNA.


Nucleotides, often called mononucleotides, are units of repeating nucleic acids. Thusly we know that nucleotides participate in the storage and transmission of hereditary information. They can also serve the cell in the transport of energy (like ATP). Nucleotides participate in some anabolic reactions, and can serve as coenzymes. Nucleotides are composed of three primary sub units. These units are the nitrogenous base, a pentose sugar, and a phosphoric acid.


The two types of nitrogenous bases are derivatives of the nucleic acids pyrimidine or purine. Purine itself however is a derivative of pyrimidine. Uracil, thymine, and cytosine are pyrimidine-based. You may recognize their symbols if you have ever looked at a gene code. The symbols for them are U, T, and C respectively. Uracil is only found in RNA, and thymine only in DNA. Cytosine however is found in both DNA and RNA. Purine bases include adenine and guanine. These too should sound familiar from gene codes with their symbols being A and G. Purine bases are found in both DNA and RNA. Pyrimidine and purine based nucleic acids contained in nucleotides are easily identified from each other by chromatography. This is because they show high absorption of ultraviolet light at 260 nm.






As for the sugars only two types of pentose sugars are used and they are d-Ribose and 2-deoxy-d-ribose. They are found in RNA and DNA respectively. These sugars are then bonded with the pyrimidine and purine based compounds. The connections occur at the bottom N atom 1 of the pyrimidine ring and the bottom N atom 9 on the imidazole ring of purine. The pentose sugars bond on their Carbon atom 1. With the combination of these two groups nucleosides are formed. Nucleosides can be formed from nucleotides by the process of hydrolytic cleavage of the phosphoric acid group. Nucleosides however do not occur free in any large amounts in cells.


Nucleosides bonded with the phosphoric acid group are then considered nucleotides. In nucleotides the phosphoric acid group is esterified to one of the pentose's free hydroxyl groups. In DNA nucleotides this joining can be at the pentose's Carbon atom 3 or Carbon atom 5. Both types of bonds can occur, but it is more frequently the Carbon atom 5 that does the bonding. This is because the enzymatic reactions to synthesize and break down the nucleotides to nucleosides usually involve the Carbon atom 5. Nucleotides can be formed by partial hydrolysis of nucleic acids and enzymes called nucleases.




Genes and Chromosomes




Chromosomes are the vessels of storage for our genetic code. Made primarily from nucleic acids and chromatin these tiny thread like structures occur in pairs in humans. They occur in 23 pairs, or so some crazy old Japanese man told me once. Normally chromosomes can be seen with simple colchicine staining. Chromosomes are given their basic shape and properties by the very precious cargo they hold, DNA. To further explore the chromosome we must shift our search toward the exploration of DNA.








DNA was first discovered over a hundred years ago. A single strand can have a molecular weight of over 32 million (E.coli bacteria strain lambda). DNA is long and rigid unlike enzymes that fold over themselves. This can be observed in the high viscosity of a solution of native DNA. Watson and Crick first made the currently accepted model of DNA in the year 1953. In their model two right handed polynucliotide chains coil around the same axis while staying parallel. This forms a double helix. One of the strands may be considered upside down in comparison to the other. This is to facilitate the nucleotides interlocking nature. The 5-carbon position of one strand (the one typically having a protrusion) will hydrogen bond