In DNA, why are some phosphate groups excluded from nucleotides while others aren’t?

Question

Labeled these in Imgur. My question stems from a Khan video.

[Here](https://imgur.com/a/W2XcQ0z) are the excluded phosphate groups in question.

[Here](https://imgur.com/a/ZduoMm3) is my drawing of the DNA with the nucleotide chains labeled with cyan dotted lines. I drew this refrencing Sal’s work.

In: Biology