Reality → Life → Genetics → Double helix
In the DeoxyriboNucleic Acid double helix two intertwined strands of the acid run in opposite (anti-parallel) direction, coil around a virtual central axis, and are held together by their nucleobases like rungs on a twisted ladder. The helical strands consist of alternating deoxyribose (a 5-carbon sugar, or pentose) and phosphate groups of the strands' constituent nucleotides (see also Nucleid acids). Attached to the sugar units are the inwards pointed nucleobases. Most importantly, the nucleobases always build pairs by bonding with a complementary nucleobase of the opposite strand: Adenine (A) always bonds with Thymine (T), and Cytosine (C) always with Guanine (G). This arrangement of two anti-parallel backbone strands, held together by the bridging base pairs, allows the 'unzipping' of the double helix and exact duplication of the strands prior to cell division [1] . Genetic information is encoded in the sequence of nucleotides. Thousands of nucleotides (or related base pairs) form a gene that defines a protein, while a triplet of sequential nucleobases describes a unit (called codon) that defines one of the protein's constituent amino acids [2] .
DNA replication occurs only prior to the cell's division (mitosis) and only in the cell's nucleus. Such replication covers the full length of the two DNA strands that make up the huge (long) DNA molecule of an unwound (unpackaged or un-'condensed') chromosome, carrier of thousands of genes and millions of base pairs. Special enzymes separate the helical strands and synthesize a complementary copy by moving along a strand in a defined direction. In protein biosynthesis, which happens throughout the cell's life, only short segments of DNA (the length of genes) are replicated with RNA involvement in two steps: transcription, which happens in the nucleus, and translation, which happens in the cytoplasm. Viewed from the molecular, or even quantum mechanical perspective, the entire process of DNA replication, cell division, and protein biosynthesis is staggeringly complex and unbelievably organized and regulated.
Triplets (codons) made up from 4 available nucleobases allow 43 = 64 different arrangements (permutations) to specify the 20 different amino acids which represent the building blocks of all proteins occurring in the human body. Most amino acids can therefore be specified by several of the 64 available codons, rather than by only one single codon. The initially arbitrary assumption that a sequence of three nucleobases defines a specific amino acid has been confirmed in many lab experiments and proven to be universal for all organisms. Also, special triplets, which mark the beginning and end of a series of genetic code-bearing nucleobases, have been identified in labs (see Sheet).