DNA can be thought of as a large cookbook with recipes for making every protein in the cell. RNA helps the ribosome translate the information in DNA into protein.
Cells contain two strands of DNA that are exact mirrors of each other.
When correctly aligned, A can pair with T and G can pair with C. Because these strands are mirrors of each other, the amount of A is equal to the amount of T and the amount of C is equal to the amount of G in any double stranded DNA molecule. In solution, the two strands will usually find each other and form a double helix. This reaction is favorable because of the numerous hydrogen bonds that can be formed between the complementary bases. The DNA molecule can stretch for millions of base pairs and the DNA sizes of organisms can vary greatly.