The big idea: A strand of DNA or RNA has two different ends, called 5′ and 3′ after carbons of the sugar. That gives every strand a direction.
The points to remember
- The carbons of the pentose sugar are numbered 1′ to 5′.
- In the backbone, each phosphate links carbon 5′ of one sugar to carbon 3′ of the next.
- So every strand has a 5′ end (a free phosphate) and a 3′ end (a sugar with a free carbon 3′).
- In DNA the two strands are antiparallel: one runs 5′ to 3′, its partner 3′ to 5′.
Remember it as: Phosphate end 5′, sugar end 3′.
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The big idea: Because nucleotides can only be added to the 3′ end, every strand is built 5′ to 3′, and mRNA is read the same way.
The points to remember
- Enzymes add new nucleotides only to the 3′ end of a growing strand.
- So in replication every new DNA strand grows in the 5′ to 3′ direction.
- In transcription RNA is also built 5′ to 3′, alongside the template strand.
- In translation the ribosome reads the mRNA from its 5′ end to its 3′ end.
Real example: DNA copied in pieces: In 1968 Reiji and Tsuneko Okazaki found that one new DNA strand is made in short pieces. The reason is direction: DNA polymerase can only build 5′ to 3′, so on one template it must keep starting again.
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How it is asked: A drawn strand with a letter at one end: name the end. Look for the free phosphate (5′) or the free sugar (3′).
IB-style questionIdentify[1 mark]
The diagram shows part of a DNA strand. Identify the terminal indicated by Y.
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See the answer broken into clear points, with guidance on how each point earns marks.