RNA and DNA strands have 5 prime and 3 prime ends, creating directionality that controls strand pairing, synthesis, and reading in IB Biology exam contexts.
Coverage 2012–2025 · Updated 14 Jul 2026
Nucleic-acid strands have direction because the sugar-phosphate backbone links the 5' carbon of one nucleotide toward the 3' carbon of the next.
During replication and transcription, a new nucleic-acid strand is synthesized 5' to 3' while its template is read 3' to 5'. During translation, the ribosome reads mRNA codons in the 5' to 3' direction.
For any strand, label both ends, keep complementary DNA strands antiparallel, and write the direction before applying base pairing.
A DNA template written 3'-A-G-C-5' directs a new strand written 5'-T-C-G-3'.
Reversing only the 5' and 3' labels changes the sequence interpretation; direction is part of the strand's identity.
This objective is assessed through structured response, commonly using Identify / State.
Identify / State
Build the answer around this relationship: Directionality of RNA and DNA should be described using precise molecular vocabulary.
Representative question
Identify the terminal indicated by Y on the diagram.
HL adds directionality, helix geometry, DNA packaging, and classic evidence. These ideas stay separate: 5′/3′ explains strand direction; purine-pyrimidine pairing explains width; nucleosomes explain packaging; Hershey-Chase and Chargaff explain evidence for DNA and base pairing.
