Chromosome is a specific form of DNA in mitosis or meiosis. In the nucleus, DNA is tightly coiled around proteins called histones and packaged into a linear structure. The number, size and morphology of chromosomes vary with species, cell types and developmental stages.
When a cell does not divide, chromosomes are not visible in the nucleus – and so are they under the microscope. However, the DNA that makes up the chromosomes becomes more compact during cell division, and the chromosomes are visible under the microscope.
Each chromosome has a contraction point called centromere, which divides the chromosome into two parts, the “arm”. The short arm is “p arm”; The long arm is “q arm”. The position of the centromere on each chromosome provides a unique shape for the chromosome, which can be used to help describe the position of a specific gene.