Replication
DNA is the carrier of genetic information. Before a cell divides, DNA must be precisely copied, or "replicated," so that each of the two daughter cells can inherit a complete genome, the full set of genes present in the organism. In eukaryotes, the DNA molecules that make up the genome are packaged with proteins into chromosomes, each of which contains a single linear DNA molecule. Eukaryotic chromosomes are found in a special compartment called the cell nucleus. The genomes of bacterial cells (prokaryotes), which lack a nucleus, are typically circular DNA molecules that associate with special structures in the cell membrane. Despite the hundreds of millions of years of evolutionary history separating eukaryotes and prokaryotes, the features of the replication process have been highly conserved between them.
Overview
The DNAs that make up the genomes of bacteria and eukaryotic cells are double-stranded molecules in which each strand is composed of subunits called nucleotides. DNA nucleotides have a direction, in the same way that an arrow has a head and a tail. In DNA strands, the head is the 3′ ("three prime") end of the strand, and the tail is the 5′ ("five prime") end. As a result, each strand also has a direction, whose ends are referred to as the 3′ and 5′ ends.
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