Q1. What are polytene chromosomes and how they are formed?

Q1. Cell plate is formed by which organelle?

Polytene chromosomes are over-sized chromosomes which have developed from standard chromosomes and are commonly found in the salivary glands of Drosophila melanogaster.
Polytene chromosomes form when multiple rounds of replication produce many sister chromatids that remain synapsed together.

Cytokinesis in terrestrial plants occurs by cell plate formation. This process entails the delivery of Golgi-derived and endosomal vesicles carrying cell wall and cell membrane components to the plane of cell division and the subsequent fusion of these vesicles within this plate.