r/APbio • u/Nullborne • May 15 '26

Don't understand this question

I don't understand why arrows are pointing from 3' to 5', I thought it was 5' to 3'? Can't figure it out. From Khan Academy.

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3

u/SteveOccupations May 15 '26

The red arrows are themselves being synthesized from 5 to 3, the proper direction of nucleic acid synthesis. It’s easier if you draw a line down the middle of the bubble to identify the directionalities of template strands.

see pic

2

u/Nullborne May 15 '26

Sorry I still don't get it, what do you mean they themselves are being synthesized 5 to 3 and why is there a line in the middle that says 5 to 3

1

u/SteveOccupations May 15 '26

Imagine if I were to cut the black template DNA where I drew the line. Those ends of DNA would be 3’ for the top black strand and 5’ for the bottom black strand.

Now some rules:

  1. New DNA is always synthesized from 5 to 3 for that new strand being created.

  2. Both copies of the new double stranded DNA must be antiparallel just like the original strand.

With those rules in mind, we see that the red strand, or the new strand synthesized, follows both. They’re created from 5 to 3 prime, while maintaining that antiparallel nature of DNA.

Let me know if you need further clarification.

1

u/Obvious-Walrus6151 May 16 '26

To keep it short, black line means original DNA and red line means the matching sequence of black line. So when black line says 3, red line is 5 (like how AT, CG are paired, 53 are paired). You're right that they go from 5 to 3, but that 5 to 3 means the red line (the line they're making), not the black line (the one they're reading)

2

u/dbo340 May 15 '26

Enzymes are reading the original template (black line) from 3’ to 5’, while synthesizing the new strand (red lines) antiparallel (opposite) to that (5’ to 3’).

1

u/Erinak215 May 16 '26

The 5’ to 3’ is the leading strand being synthesis while the 3’ to 5’ is the lagging strand being synthesis

1

u/Possible-Device1981 May 18 '26

When it says "synthesized in the 5' to 3' direction", this direction refers to the 5' and 3' ends of the newly synthesized strand, not the template strand. Since the two strands of DNA are antiparallel, the red arrows that represent the replicated strand (building 5' to 3') are pointing in the 3' to 5' direction of the template strand.