How many layers are present in the nuclear envelope?

What is a centromere? How does the position of the centromere form the basis of the classification of chromosomes? Support your answer with a diagram showing the position of the centromere on different types of chromosomes.

Chromosomes having equal or almost equal arms are called

$A$ : Telocentric chromosome has two unequal arms.
$R$ : The centromere is situated close to its end forming one extremely short arm.

The common point of attachment of all the arms of polytene chromosome is known as

Distinguish between: Euchromatin and Heterochromatin.