CHROMOSOME
• In the nucleus of each cell, the DNA molecule is packaged into thread-like structures
called chromosomes.
• Each chromosome is made up of DNA tightly coiled many times around proteins called
histones that support its structure.
• Chromosomes were first described by Strasburger (1815), and the term ‘chromosome’
was first used by Waldeyer in 1888.
• They appear as rod shaped dark stained bodies during the metaphase stage of mitosis
when cells are stained with a suitable basic dye and viewed under a light microscope
Size
The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25
µm in fungi and birds, or as long as 30 µm in some plants such as Trillium. However, most
metaphase chromosomes fall within a range of 3µm in fruitfly (Drosophila), to 5µm in man and
8µm to 12µm in maize. The organisms with less number of chromosome contain comparatively
large-sized chromosomes than the chromosomes of the organisms having many chromosomes.
The monocotyledon plants contain large-sized chromosomes than the dicotyledon plants. The
plants in general have large-sized chromosomes in comparison to the animals. Further, the
chromosomes in a cell are never alike in size, some may be exceptionally large and others may
be too small. The largest chromosomes are lampbrush chromosomes of certain vertebrate
oocytes and polytene chromosomes of certain dipteran insects
Shape
The shape of the chromosomes is changeable from phase to phase in the continuous process of
the cell growth and cell division. In the resting phase or interphase stage of the cell, the
chromosomes occur in the form of thin, coiled, elastic and contractile, thread-like stainable
structures, the chromatin threads. In the metaphase and the anaphase, the chromosomes become
thick and filamentous. Each chromosome contains a clear zone,known as centromere or
kinetocore, along their length. The centromere divides the chromosomes into two parts, each part
is called chromosome arm. The position of centromere varies form chromosome to chromosome
and it provides different shapes to the latter which are following
