FRANCIS LEROY & MATHILDE VANSPEYBROUCK, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY & MATHILDE VANSPEYBROUCK, BIOCOSMOS / SCIENCE PHOTO LIBRARY
DNA synthesis, animation. DNA (deoxyribonucleic acid, blue) is made up of two chains of many small molecules, known as nucleotides. Two of these strands are twisted around each other to form a double helix. In order to divide, cells need to copy this double-stranded DNA. The first frame shows the enzyme helicase unwinding the DNA helix ready for replication. The two separated strands of DNA now act as templates for the production of two new complimentary DNA strands (yellow). Polymerase enzymes synthesise new DNA strands in one direction, from the 5' end of the DNA strand to the 3' end. This means that although the top (leading) strand of DNA can be continuously synthesised the bottom (lagging strand) is synthesised discontinuously in short pieces known as Okazaki fragments.
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