=DNA Ligation= [image:] DNA ligases are a class of enzymes that can fix breaks or link together DNA strands. They are involved in DNA repair, DNA replication (link Okazaki fragments together) and are also used for ‘sticking together’ (ligating) DNA stands that have been cut by restriction enzymes. [image:] A DNA molecule is comprised of repeating subunits known as nucleotides. Each nucleotide consists of a sugar, phosphate and nitrogenous base. A DNA double helix resembles a twisted ladder in which the nitrogenous bases (A, T, C, G) form the runs of the ladder and the repeating sugar-phosphate groups form the sides or the ‘backbone’ of the DNA molecule. The nitrogenous bases form weak hydrogen bonds that hold the two strands together, while the sugar-phosphate backbone is held together by stronger covalent bonds known as phosphodiester bonds. [image:] DNA ligases link DNA fragments by inducing the formation of a phosphodiester bond between the 3' hydroxyl end of one nucleotide with the 5' phosphate end of another nucleotide (shown right). Essentially they rejoin breaks in the DNA backbone. [image:] DNA ligase joining the sticky ends of DNA fragments that have been cut with a restriction enzyme. [image:] 3-D representation of DNA ligase repairing DNA after chromosomal damage (double-strand break). Note that several other enzymes are also involved in DNA repair, a series of highly co-ordinated processes that ensure the integrity of the genetic code is not disrupted.
Credit: Ben Himme