Which of the following statement(s) is/are correct about S-phase (synthetic phase)? 
i) It occurs between  $G_1$ and$G_2$   phase.
ii) It marks the period during which centrosome duplicates.
iii) At the end of this phase, DNA is doubled but the number of chromosomes remains unchanged

 The interphase is divided into three further phases: 

l) G1 phase (Gap 1) ,II) S phase (Synthesis) lII) G2 phase (Gap 2)

S or synthesis phase marks the period during which DNA synthesis or replication takes place. During this time the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C then it increases to 4C. However, there is no increase in the chromosome number; if the cell had diploid or 2n number of chromosomes at G1 , even after S phase the number of chromosomes remains the same, i.e., 2n.