- published: 13 Feb 2012
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Deoxyribonucleic acid (i/diˈɒksiˌraɪboʊnjʊˌkliːɪk, -ˌkleɪɪk/;DNA) is a molecule that carries most of the genetic instructions used in the development, functioning and reproduction of all known living organisms and many viruses. DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic acids compose the three major macromolecules essential for all known forms of life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are known as polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of a nitrogen-containing nucleobase—either cytosine (C), guanine (G), adenine (A), or thymine (T)—as well as a monosaccharide sugar called deoxyribose and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. According to base pairing rules (A with T, and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA. The total amount of related DNA base pairs on Earth is estimated at 5.0 x 1037, and weighs 50 billion tonnes. In comparison, the total mass of the biosphere has been estimated to be as much as 4 TtC (trillion tons of carbon).
HIV una gran balla e un capolinea!
Isolare retrovirus e trovare loro un leit-motiv patologico era di moda negli anni 70/80. Si pensava che qualsiasi cosa causasse la trascrizione dal RNA al DNA era da considerarsi eccezionale e dovesse attribuirsi ad una sorta di contaminazione virale (da cui il termine "retrovirus"). Ma questo si dimostrò un errore, poiché fu verificato verso la meta' degli anni '80 che la medesima attività enzimatica era presente in tutta la materia vivente provando così che la transcriptasi inversa non aveva niente a che fare con i retrovirus per sé. (Franchi 1997) Nel 2001 arrivarono i risultati del Progetto per la mappatura del Genoma Umano è stato chiaro che stava per essere irrimediabilmente buttato a mare il concetto stesso di "retrovirus". Ma negli anni '70 lo sforzo di innumerevoli gruppi di ri...