Question

how chromatin network is changed into a thread like structure so called chromosome

Answer 1

Chromatin is a complex of DNA and proteins (histones). Histones are basic protein that condenses the DNA.
Chromatin structure depends on the present stage of the cell cycle. During Interphase, chromatin is loose to allow the access of DNA polymerase and RNA polymerase to initiate replication and transcription respectively.
When the cell prepares to divide during mitosis or meiosis, the organisation of chromatin into chromosome takes place.

There are three levels of chromatin organisation:
1. Histone proteins wrap DNA forming nucleosome ,"The beads on a string "structure. Eight histone proteins are present, consisting of 4 pairs of H2A, H2B, H3 and H4 organised into histone core. The length of DNA around 147 base pairs makes two turns around the histone core.

2. Multiple histones wrap around DNA into a 30nm fibre. This is secondary coiling of the beads on a string structure involving histone H1.

3. Next higher level of DNA packaging of 30nm fibre results into formation of chromosome (during mitosis and meiosis).

Chromosome now contains single continuos piece of DNA, which contain many genes.