Q.What is meant by the antiparallel orientation of the two strands of the DNA double helix? Why is this orientation important for DNA replication and transcription (answered fully in later sections)?

Step 1. Each polynucleotide strand has a distinct chemical polarity, running from a free 5' end to a free 3' end, because phosphodiester bonds link the 3' carbon of one sugar to the 5' carbon of the next.

Step 2. In the double helix, the two strands are arranged so that if one strand runs 5' to 3' in a given spatial direction, its partner strand runs 3' to 5' in that SAME spatial direction — i.e. the strands are oriented oppositely to each other.

Step 3. This antiparallel arrangement matters functionally because both DNA polymerase (in replication) and RNA polymerase (in transcription) can only add new nucleotides to the 3' end of a growing strand — they synthesise only 5' to 3'. …

Picture the two strands as running in opposite directions along the SAME axis, and connect this directly to the fact …

• Describing antiparallel as simply 'the two strands are different' without specifying the actual 5' …