I’m trying to study but I’m struggling with simplifying this concept and with understanding it. I’ve never been that good at biology so I’m sorry if it’s a dumb question. It’s a semi-conservative process, because a new strand forms after a cell divides. And this, I have no issue with understanding. But if the 3’-5’ strand is the leading strand (because this is what our teacher has told us) and the DNA polymerase synthesises in the 5’-3’ direction, what about the 5’-3’ strand of the original cell? Our book shows a picture of a recently formed 3’-5’ strand. Does this mean the 5’-3’ strand can also be the leading strand? Sorry I’m just so confused and I think this is crucial to understanding the rest 😭
In: Biology
There is no “original cell” after division. When it splits, we wind up with two daughter cells.
Now, it’s been a while since I’ve had to remember the actual terminology, so I don’t understand this *5′-3’* you’re asking about but I at least know the process. With the replication of DNA strands, the entire molecule is split lengthwise and new bases are added to it to make (mostly) exact copies of the original. It’s semi-conservative because half of the original full strand goes to one daughter cell and the other half of the original goes to the other daughter cell.
Both strands get synthesized. You just start on the other end. Every strand of DNA has a 5′ and a 3′. They’re just on opposite sides of one another.
ETA: Does your book have a picture of the chemical structure of DNA? That might help you wrap your brain around it. 5′ and 3′ are just chemical sides of a DNA molecule. Maybe think of it like this: if you’re holding hands with a kid, one of you is holding right hand and the other is holding left hand. You can reverse that, right? Just move the kid to the other side. DNA molecules have 5′ AND 3′, it’s not an identity, it’s an orientation in space.
Hi I drew some crude images to try and explain. Let me know if this helped.
1- https://i.imgur.com/6GWYkZ7.jpeg
2- https://i.imgur.com/7I2fvfy.jpeg
3- https://i.imgur.com/GtCTmGi.jpeg
In these images the red would be the leading strand, green would be the lagging strand. In the final result only half of the original dna(blue) is preserved, hence it is semiconservative.
Edit: removed imgbb links and added imgur links to the images.
The 5′ and 3′ refer to the [numbered carbon atoms of the sugar backbone](https://qph.cf2.quoracdn.net/main-qimg-776b674ddbd0bc7b1b2c6902736906a2-pjlq) (deoxyribose in DNA, ribose in RNA).
The important part is that a new nucleotide can only be attached at the 3′ carbon of an existing one. For the 5′-3′ strand this is fine, it can be continuously built forward. The other strand cannot, so instead it is built backward with small steps at a time in its own 5′-3′ direction. These small steps are called [Okazaki fragments](https://en.wikipedia.org/wiki/Okazaki_fragments), and I think looking into these will help.
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