How RNA primer on leading strand is removed during DNA replication and how the gap is filled?
The group of cellular enzymes that remove RNA primers include the proteins FEN1 (flap endonuclease 1) and RNase H. The enzymes FEN1 and RNase H remove RNA primers at the start of each leading strand and at the start of each Okazaki fragment, leaving gaps of unreplicated template DNA. Once the primers are removed, a free-floating DNA polymerase lands at the 3' end of the preceding DNA fragment and extends the DNA over the gap. However, this creates new nicks (unconnected sugar-phosphate backbone).
In the final stage of DNA replication, the enyzme ligase joins the sugar-phosphate backbones at each nick site. After ligase has connected all nicks, the new strand is one long continuous DNA strand, and the daughter DNA molecule is complete.

Source: Boundless. “DNA Replication in Eukaryotes.” Boundless Biology. Boundless, 08 Jan. 2016. Retrieved 14 Mar. 2016 from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/dna-structure-and-function-14/dna-replication-101/dna-replication-in-eukaryotes-437-12941/