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In molecular biology, ligation refers to the joining of two DNA fragments through the formation of a phosphodiester bond. An enzyme known as a ligase catalyzes the ligation reaction. In the cell, ligases repair single and double strand breaks that occur during DNA replication. In the laboratory, DNA ligase is used during molecular cloning to join DNA fragments of inserts with vectors - carrier DNA molecules that will replicate target fragments in host organisms.

"CRACKING THE CODE"/"Cloning Paper Plasmid" activities can (1) serve as a review of the "genetic code" and the role it plays in our life; and, (2) to help students see how genes may be manipulated for genetic research, namely, gene cloning/genetic engineering. The laboratory time, the specialized equipment and expertise to carry out recombinant DNA experiments may be lacking in the high school. Activity 2 will help students conceptualize the mechanics involved in cutting and ligating DNAs into a plasmid vector with "sticky ends" of complementary DNA base pairs.

The possibilities of pseudo-peptide-DNA mimics like PNA (peptide nucleic acid) having a role for the prebiotic origin of life prior to an RNA world is discussed on the basis of literature data showing that this type of molecules might have formed on the primitive earth (or other places in the universe), as well as data indicating the possibilities of template-directed PNA chemical replication and ligation. In particular, the merits of an achiral prebiotic genetic material is discussed.