DNA and RNA grow only at the 3' end
- The idea
- Predict
- See it
- Why
- Check
The idea
“A new nucleotide can join a DNA or RNA strand at its 5' end, or at either end.”
A strand written as a row of letters looks like it could grow on either side. The two ends are chemically different, though.
The short version
The mistake: A new nucleotide can join a DNA or RNA strand at its 5' end, or at either end.
What's actually true: A polymerase adds each new nucleotide to the free 3' OH at the end of the strand, so strands grow 5' to 3'.
- A nucleotide's phosphate sits on the sugar's 5' carbon, and an OH sits on its 3' carbon. So a strand's two ends are different.
- A polymerase bonds the new nucleotide's inner phosphate to the strand's 3' OH. Releasing its other two phosphates powers this, so the strand must supply the OH.
- Each new nucleotide becomes the 3' end. So DNA and RNA are built 5' to 3', and the template is read 3' to 5'.
On the exam: New nucleotides join only the free 3' OH. Every DNA or RNA strand is built 5' to 3'.