Extracting DNA from the Bacterium Escherichia coli
Uploaded by Admin on Jan 22, 1999
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 (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 sequence: UGCACGU.
Genes and Protein Synthesis
A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids. Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place.
Cell biologists believe that DNA also builds a type of RNA called transfer RNA, which...