CARLOS CLARIVAN / SCIENCE PHOTO LIBRARY CARLOS CLARIVAN / SCIENCE PHOTO LIBRARY
DNA replication, computer illustration. DNA (deoxyribonucleic acid) is composed of two strands of nucleotides twisted into a double helix. Before replication, the strands are separated by a helicase enzyme (red), with each strand acting as a template for the formation of a new DNA molecule. The templates are 'read' by DNA polymerase enzymes (blue) that travel along the strands matching complementary nucleotides to the templates to build up the two new DNA molecules. The leading strand (top) is synthesized in the same direction as the growing replication fork. On the lagging strand template, a primase (turquoise) initiates synthesis of complementary RNA primer (red). A DNA polymerase extends the primed segments, forming Okazaki fragments. The RNA primers are then removed and replaced with DNA (yellow), and the fragments of double strand DNA are joined together by DNA ligase (purple).
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