Due to the chemical composition of the pentose residues of the bases, DNA strands have directionality. One end of a DNA polymer contains an exposed hydroxyl group on the deoxyribose; this is known as the 3' end of the molecule. The other end contains an exposed phosphate group; this is the 5' end. The two strands of a double-helix run in opposite directions. Nucleic acid synthesis, including DNA replication and transcription occurs in the 5'→3' direction, because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile.:27.2 COUNT 5
QSN1 What causes the directionality of DNA strands? ANS1 the chemical composition of the pentose residues of the bases QSN2 What is known as the 3' end? ANS2 an exposed hydroxyl group on the deoxyribose QSN3 What is known as the 5' end? ANS3 an exposed phosphate group QSN4 What type of synthesis occurs in the 5'→3' direction? ANS4 Nucleic acid synthesis QSN5 Why does DNA replication and transcription occur in the 5'→3' direction? ANS5 because new nucleotides are added via a dehydration reaction that uses the exposed 3' hydroxyl as a nucleophile