The structure present on the chromosome is:

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

........ is present in a higher amount in the dark band of a polytene chromosome.

During anaphase,chromosomes appear in various shapes due to the presence of:

Identify the cell organelle based on the following characteristics:
- It is not a membrane-bound organelle.
- It is the site for active ribosomal $RNA$ $(rRNA)$ synthesis.
- It is a spherical structure.

The nucleus is separated from the surrounding cytoplasm by a nuclear membrane,which is