12.1: Introduction

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Deoxyribonucleic acid (DNA) is located in the nucleus of eukaryotic cells (animals, plants, fungi, and protists). DNA contains information to direct the cell in the manufacture of proteins. Proteins control development, organ function, metabolism, enzymatic reactions, photosynthesis, muscle action, brain activity, and many other cellular processes. DNA is often referred to as the “blueprint for life”.

DNA is a polymer composed of the nucleotide bases guanine (G), adenine (A), thymine (T), and cytosine (C), and two sugar/phosphate backbones. Two DNA strands are twisted to form a double helix. The number of nucleotide bases (G, A, T, C) in each human cell is about 3 billion. The 3 billion base pairs in the human genome are located on 46 strands of DNA called chromosomes. The Human Genome Project has determined the order of the nucleotides on each chromosome. A gene is a sequence of nucleotide bases (DNA) that codes for a specific protein. Human DNA contains about 20,000 genes while the cells of the rice plant contain over 40,000 genes.

In the DNA isolation procedure, plant cell walls and cell membranes are broken down by blending or mashing and heating the cells. Detergent in the extraction solution dissolves lipids in the cell membrane causing the cells to lyse. When cells undergo lysis, the cellular components, including the DNA, are released. The technique of filtration uses a medium, in this case cheesecloth, to separate solids from liquids. The resultant material is referred to as filtrate. When cold ethanol is added to the filtrate, DNA precipitates at the water/ethanol interface. Although an individual DNA molecule is not visible with the naked eye, DNA isolated from large quantities of cells can be observed.