The big idea: A strand of DNA has two different ends. DNA polymerase can only add nucleotides to one of them: the 3' end.
The points to remember
- The 5' end of a strand finishes with a phosphate (on carbon 5 of the deoxyribose).
- The 3' end finishes with the sugar: its carbon 3 carries a free -OH group.
- DNA polymerase joins the 5' phosphate of a new nucleotide to the 3' carbon of the strand.
- So a new strand always grows in the 5' to 3' direction.
- The two strands of a helix are antiparallel: they run in opposite directions.
Remember it as: Five to three, never the other way.
Real example: E. coli. Every DNA polymerase known, in bacteria, plants and humans, works only 5' to 3'. That one rule shapes how every cell copies its DNA.
Get the ends right: The 5' end has the phosphate; the 3' end has the sugar. Polymerase adds to the 3' end, never the 5' end.
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One rule, two templates: The templates are antiparallel, but each new strand must grow 5' to 3'. So the two new strands cannot be made the same way.
Helicase
- Opens the helix at the fork.
- Gives two templates running in opposite directions.
One template
- Its new strand grows 5' to 3' towards the fork.
- It can be made in one piece.
The other template
- Its new strand grows 5' to 3' away from the fork.
- It has to be made in short pieces.
Why this matters
- Antiparallel templates + a 5' to 3' only polymerase = two different ways of copying.
- This is why there is a leading strand and a lagging strand.
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How it is asked: A long Describe on the directionality of DNA and what it means for replication, or a 1A item on which end a nucleotide is added to.
IB-style questionDescribe[5 marks]
Describe the implications of the directionality of DNA for its replication.
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See the answer broken into clear points, with guidance on how each point earns marks.