Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 52

DNA replication is the process of copying DNA. It’s important for cell division, so that each daughter cell inherits the full genome of its parent cell. The complementary double-stranded DNA molecule splits, and each strand produces a new complement, creating two identical copies of the double-stranded DNA sequence.

Cell division occurs in a cell cycle sequence (which we explore in a moment). A different type of DNA replication occurs in meiosis, producing daughter cells that have only half the number of chromosomes as the parent cell. A new chromosome can be created from a combination of both parent chromosomes in a process of breaking and joining, which is called crossover.

DNA replication begins when protein complexes sequentially unwind the DNA into two strands. As this unwinding proceeds, new strands that are complementary to each of the single strands are synthesized. The enzyme DNA polymerase is responsible for this process, because it adds nucleotides that are complementary to the nucleotides of the original strand.

The addition of nucleotides is based on the two pairs being complementary: Adenine binds to thymine, and cytosine binds to guanine via hydrogen bonds. One end is called the 3' (three-prime) and the other the 5' (five-prime) end. DNA polymerase (see the upcoming section, “Coding for proteins: RNA and DNA”) synthesizes DNA directionally by adding the 5' end of a nucleotide to the free 3' end of a nascent DNA strand. Thus, the DNA strand is elongated in the 5' to 3' direction.