@ toshika
actually what happens when the dna replication starts , the dna molecule function as a template and form carbon copies .
thats the dna contains will double from 1C TO  2C in the case of haploid cells . 
and in the case of diploid cell , it will grow from 2C to 4C . 
as a result duplicate set of genes are formed. 
HOPE IT CLEARS YOUR DOUBT 
ALL THE BEST ..
						

							
HI DEAR
ACTUALLY C IS THE UNIT OF DNA MEASUREMENT . YOU CAN CALL IT AS PICOGRAM. AS WE MEASURE THINGS IN KILOGRAM , DNA IS MEASURED IN PICOGRAM {c}.
WHEN DNA CONTENT DOUBLES WE CALL IT AS DNA CONTENT HAS CHANGED FRON 2C TO 4C.
IT OCCURS AT TIME OF DIVSION. NOTE THAT CHROMOSOME NUMBER STILL REMAINS SAME . ONLY DNA CONTENT IS DOUBLEB
APPROVE MY ANSWER . POST IF STILL SOME DOUBT EXIST IN MIND
						

							C is nothing, just a unit for DNA amount . In diploid it is consider as 2C And during replication in the S phase.. it becomes 4C .
						

							
Unduplicated Chromosome before S phase of inter phase is a 2C chromosome. Duplicated chrome some in S Phase is a 4C Chromosome