On which part of the DNA will the new DNA nucleotide connect? My question is: > Where will the new DNA nucleotide connect on the (part of the) DNA shown on the picture? Will it connect to the phosphate group? What does the 5' --> 3' mean? !enter image description here Thank you in advance.

The numbers come from the numbering of the carbon atom of the ribose in the backbone of the DNA. The ribose is numbered as shown in the figure below:

!enter image description here

In the nucleotides the phosphate groups are attached to the 5' carbon atom and new nucleotide will be added at the 3' carbon atom. See the image below:

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First you have a OH-group at the 3' position which is then "replaced" by the new nucleotide.