SQA Higher Biology

## Nucleotides **DNA** (deoxyribonucleic acid) is a polymer of **nucleotides**. Each nucleotide has three parts joined together: a deoxyribose sugar, a phosphate group and a base. There are four bases, and they fall into two shapes: adenine (A) and guanine (G) are the larger ones, while thymine (T) and cytosine (C) are smaller. Nucleotides join into a strand through a **sugar-phosphate backbone**. The phosphate of one nucleotide bonds to the deoxyribose of the next, so the sugars and phosphates alternate along the outside of the strand and the bases project to one side. The order of the bases along a strand is the **genetic code**, and it is the order and not the backbone that differs from one gene to another. ## Base pairing Two strands lie side by side with their bases facing inwards, and each base pairs with only one partner. | Base | Pairs with | Hydrogen bonds | |---|---|---| | Adenine | Thymine | 2 | | Guanine | Cytosine | 3 | The pairs are held together by **hydrogen bonds**. Each bond is weak, but a whole chromosome has millions of them, so the two strands stay together until an enzyme pulls them apart. A large base always sits opposite a small one, which keeps the width of the double helix constant along its length. Because each base has only one partner, one strand predicts the other. This is called **complementary base pairing**, and it is the reason DNA can be copied accurately. If one strand reads ATTGC, the other must read TAACG opposite it. In a…