Extracting DNA from the Bacterium Escherichia coliIntroduction 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 thenext 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 bybonded 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 chaincontains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine
ofDNA. DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of aphosphate, a deoxyribose sugar, and either A, T, G, or C.
RNA nucleotides consist of a phosphate, aribose
sugar, and either A, U, G, or C. Nucleotide chains in DNA wind around one another to form a complete twist,
or gyre, every tennucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to
T and C to GA always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases
are thefoundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read:
A-T C-G G-CT-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 beingtranscribed, or copied, from an unzipped segÃÂ½ent of DNA, RNA nucleotides temporarily pair their
baseswith those of the DNA strand. In...