An Introduction to DNA and Chromosomes
Part 1

A closer look at what makes up the human genome...

What is DNA? ...Making the single strand.

If you have seen images of DNA before, you probably saw it in a shape or form similar to that shown in Figure B-1. The “double helix” is how DNA is most often found in living cells. In every double helix, there are actually two long strands of DNA; hence, you will often hear scientists refer to a double helix as a double-stranded DNA molecule. As we examine the basic underlying structure of DNA, try to keep in mind the overall arrangement of the double helix; it will help you see how the various components of DNA fit together.

The name DNA stands for deoxyribonucleic acid. By breaking down the name, we can understand the structure of the molecule. DNA is a long string of nucleotide units attached to one another. In a single nucleotide there are three components: 1) a sugar molecule, 2) a phosphate group, and 3) a nitrogenous base. (See Figure B-2.) In DNA, the sugar molecule happens to called deoxyribose, hence the name deoxyribonucleic acid.

The nitrogenous bases are what make DNA variable. There are 4 different types of bases in DNA: adenine, guanine, thymine, and cytosine. Biologists commonly abbreviate these bases as the letters A, G, T, and C, respectively. (See Figure B-3.) Each one of the bases is chemically distinguishable from the others; as we shall see, it is the variability of these bases that constitutes the genetic code.

Unlike the four nitrogenous bases, the sugars and phosphates remain the same throughout the DNA molecule. In a single nucleotide, the sugar is attached at one end to a phosphate group. Because the sugar of that nucleotide can attach to another phosphate at its other end, we can string together many nucleotides in a long chain. This gives us a complete DNA molecule: a structural backbone of deoxyribose sugars linked by phosphate groups, with an orderly sequence of nitrogenous bases sticking out of the sugars toward the middle of the helix. (See Figure B-4.) In terms of our double helix, the single strand provides one-half of the spiraling molecule shown in Figure B-1.

Last Modified: 9-16-02

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