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Extracting DNA from the Bacterium Escherichia coli Introduction Deoxyribonucleic acid is contained in all cells The structure of DNA makes gene transmission possible Since genes are segments of DNA DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes The DNA molecule looks like a twisted ladder Each side is a chain of alternating phosphate and deoxyribose sugar molecules The steps are formed by bonded pairs of purine-pyrimidine bases DNA contains four such bases the purines adenine A and guanine G and the pyrimidines cytosine and thymine T The RNA molecule markedly similar to DNA usually consists of a single chain The RNA chain contains ribose sugars instead of deoxyribose In RNA the pyrimidine uracil U replaces the thymine of DNA DNA and RNA are made up of basic units called nucleotides In DNA each of these is composed of a phosphate a deoxyribose sugar and either A T G or C RNA nucleotides consist of a phosphate a ribose sugar and either A U G or C Nucleotide chains in DNA wind around one another to form a complete twist or gyre every ten nucleotides along the molecule The two chains are held fast by hydrogen bonds linking A to T and C to G A always pairs with T or with U in RNA C always pairs with G Sequences of the paired bases are the foundation of the genetic code Thus a portion of a double-stranded DNA molecule might read A-T C-G G-C T-A G-C C-G A-T When unzipped the left strand would read ACGTGCA the right strand TGCACGT DNA is the master molecule of the cell It directs the synthesis of RNA When RNA is being transcribed or copied from an unzipped segment of DNA RNA nucleotides temporarily pair their bases with those of the DNA strand In the preceding example the left hand portion of
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