Extracting Dna from the Bacterium Escherichia Coli

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 (C) 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 DNA would transcribe a


strand of RNA with the base...