Читать книгу Horse Genetics - Ernest Bailey - Страница 44

The nucleus of each horse cell has 64 DNA molecules. These are huge molecules, each composed of millions of units called nucleotide bases (also referred to as bases or simply nucleotides). At the same time, it is a simple molecule as only four types of bases compose DNA: adenine (referred to as A), guanine (referred to as G), thymidine (referred to as T), and cytosine (referred to as C). The bases are joined in a long, single strand that pairs with a second, complementary strand. This second strand contains a mirror image of the DNA bases found in the first strand. Throughout the length of this long, doubled molecule, all As in one strand pair with Ts in the other strand, and all Gs in one strand pair with Cs in the other strand. The combination is referred to as a “base pair.” The two strands of the DNA molecule wind around each other, with the pitch determined by the angle of the molecular bonds between each base; hence, DNA is referred to as a “double helix.”