The nucleolus is synthesized at which region?

Each chromosome has a disc-shaped structure called ...

Which of the following are used to define the karyotype of a species?
$1$. The number of chromosomes
$2$. The chromosome length
$3$. The positions of the centromeres

The part of the chromosome after the secondary constriction is called ......

Some chromosomes have non-staining secondary constrictions at a constant location. This gives the appearance of a small fragment called the ......

Centromere is part of