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12 13 the Book of Life four letters, twenty words The genome of a species is the entire DNA sequence found in its chromosomes. The human genome is like a cookbook as long as 1000 bibles, with 23 chapters chromosomes, each chapter containing several thousand recipes genes. Each recipe is for one protein, and is written using just 20 different words codons, made of only four letters bases. The recipes have advertisments introns in them, which have to be snipped out for the finished copy exons. When the human genome was mapped in 2000, scientists were surprised to find it contained pages and pages of gobbldygook between each gene. Some sections of these noncoding or junk DNA sequences originate from distantly broken genes, while others are repetitive transcription errors DNA can lose count when copying repetitive sequences like TATATATA. Other sections are dead retroviruses viruses which use reverse transcriptase to copy their RNA into their hosts DNA, so that they become part of their hosts genome. Another group of genetic parasites, descended from retroviruses, are known as jumping genes. These useless little sequences are found as introns in almost every gene and shout copy me everywhere to passing chemical equipment. Highly virulent little data bugs, today they make up about a quarter of our DNA. Real genes account for only 3. Noncoding DNA has its uses. It creates spaces between genes, aiding clear transcription, and preventing their breaking during crossover page 14. Though not expressed as proteins, noncoding DNA can also finely regulate genetic expression, enhancing or suppressing the transcription of genes it adjoins. Above DNA in the nucleus, comprising of adenine Athymine T and guanine GcytosineC bonds uncoils and is transcribed into a strand of messenger RNA, identical to DNA except for the replacement of thymine T by uracil U. The transcription unit is read in threeletter words, each of which code for an amino acid. These are then converted into a string of amino acids known as a polypeptide chain, or protein. Above The code of life is written with just four letters and words codons of three letters. Use the chart above, starting at the centre, to find out which amino acid is produced by any word. For example UAC leads to Y, Tyrosine. Most amino acids are coded for by more than one word, so that there are only 20 amino acids used by life. All life on Earth, whether tree, beetle, rabbit, human, or fungus, uses exactly the same code.
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