Polytene chromosomes have a common point of attachment. What is this point called?

What do $a$ and $b$ represent in the given figure,respectively?

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

Two sister chromatids are held together at the chromosome by which of the following structures?

What is the site of Balbiani rings?

The movement of substances through the nuclear pore is facilitated by $..........$.