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There is indeed a relationship between the structure and function of DNA. DNA contains the genetic code. This means it contains the instructions for whatever characteristics an organism has. In order to perform its job, DNA needs to do two things:
- It must be copied exactly into every cell in the organism, and
- It needs to be able to be interpreted by the cells.
DNA's structure suits it to both of these functions.
Being Copied Exactly. DNA is a double helix. This means it is composed of two strands. The two strands are complementary to each other. This means they have the same information in much the same way that a photograph and a negative have the same information. DNA strands are made of four types of chemicals called bases attached end to end. The four bases are called cytosine (C), guanine (G), thyamine (T), and adenine (A). It is the bases that enable the strands of DNA to be complementary, because cytosine (C) is attracted to guanine (G), and thyamine (T) is attracted to adenine (A). As a result, wherever one strand has cytosine, the complementary stand will have guanine opposite it, and wherever one strand has thyamine, the other will have adenine opposite it. When DNA replicates (duplicates itself), the two stands separate from each other, and a new complementary strand forms along each. In this way, two identical double stranded DNAs form. When a cell reproduces, each gets one of the duplicate DNAs.
Being Interpreted By Cells. The four DNA bases also serve as a code that contains the information on the DNA. Our alphabet contains 26 letters, but a huge number of words can be produced by arranging them in different orders. The DNA code consists of groups of three bases. It is possible to form 64 different combinations of 4 bases arranged in groups of 3. For example, the arrangement of three adenines in a row (AAA) or adenine-thyamine-adenine (ATA) could be codes. Each of these codes stands for an amino acid, the building blocks of proteins. The order of these codes on the DNA strands corresponds to the order of the amino acids in the polypeptide chains that make up the proteins a cell needs to make. Of course, complementary codes need to stand for the same amino acid. (AAA and TTT stand for the same thing.) This means there are 32 different complementary codes, more than enough to code for the 20 amino acids found in proteins, and some left over to signal the start and end of a set of instructions. The cell uses the information on the DNA to make messenger RNAs (mRNA) by complementary base pairing. The mRNA attaches to a ribosome and transfer RNAs (tRNA) carrying the appropriate amino acid line up along the mRNA. Adjacent amino acids are joined chemically building up the protein strands. As the amino acids are joined they let go of the tRNA, and the tRNA falls off the messenger so it can return to the cytoplasm to pick up another amino acid.
The sequence of bases, complementary base pairing, and double strandedness are all structural features of DNA that enable it to work.
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