A closer look at what makes up the human genome...
What are complementary strands? ...Making the double helix.
Now that we have a single chain of DNA, we are ready to return to the famous “double helix,” in which two single strands of DNA spiral around one another.
In order to understand the double helix we must first go back to our original DNA strand with its sugar and phosphate backbone. Each connection between a sugar and a phosphate group is at an angle. (Look at Figure B-5 here and compare to Figure B-4 in Part 1 of this section.) The end result is a backbone that is curved rather than straight, and hence the DNA chain spirals around itself. The bases, in turn, jut inward from the backbones, looking almost like the steps of a spiral staircase.
Another important feature of the four bases is that they pair up with one another in a particular way: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). Two bases linked up in this fashion are known as “base pairs.” Look at the arrangements of the bases in Figure B-6. Notice how the chemical structure of each base allows it to line up perfectly with its pair, but not with any other base. Because of this fact, the two intertwined strands of the DNA helix are said to be complementary.
In summary, a double helix of DNA is composed of two spiraling, complementary strands of DNA. Each strand is composed of a sugar and phosphate backbone with varying nitrogenous bases sticking in towards the center. The two strands are joined together at the center by pairing bases lined up with one another. DNA is often described structurally as a twisting ladder. In this ladder, the “rungs” are the pairs of bases linked together, and the “sides” are the two separate sugar and phosphate backbones. (Examine Figure B-7.)
The double helix is important because it preserves all of the information-carrying features of a single DNA strand while at the same time introducing elements that make it easier for living cells to make copies of their DNA. Because every base pair in the double helix must match its pairing partner (A with T, C with G), we can easily determine the sequence of an unknown strand of DNA if its matching strand is known. For example, if one strand of a double helix has the nucleotide sequence GATTCGTACG, then its complementary strand will be CTAAGCATGC. Figure B-8 shows an example of two complmentary strands.
Last Modified: 7-15-03
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In order to understand the double helix we must first go back to our original DNA strand with its sugar and phosphate backbone. Each connection between a sugar and a phosphate group is at an angle. (Look at Figure B-5 here and compare to 
Another important feature of the four bases is that they pair up with one another in a particular way: adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C). Two bases linked up in this fashion are known as “base pairs.” Look at the arrangements of the bases in Figure B-6. Notice how the chemical structure of each base allows it to line up perfectly with its pair, but not with any other base. Because of this fact, the two intertwined strands of the DNA helix are said to be 
The double helix is important because it preserves all of the information-carrying features of a single DNA strand while at the same time introducing elements that make it easier for living cells to make copies of their DNA. Because every base pair in the double helix must match its pairing partner (A with T, C with G), we can easily determine the sequence of an unknown strand of DNA if its matching strand is known. For example, if one strand of a double helix has the nucleotide sequence GATTCGTACG, then its complementary strand will be CTAAGCATGC. Figure B-8 shows an example of two complmentary strands.
