Chromosome || The Genetic material

E. Strasburger in 1875, discovered thread like structure in the cell which appeared during cell division. These thread like structure were called chromosome - a term coined by W. Waldeyer in 1888. In all eukaryotic cells, the well organised nucleus contains a definite number of chromosome of distinct size and shape.

The chromosome become distinct only during cell division. At the interphase stage of the cell cycle, the chromosome with in the nucleus remain in the form of long, loosely coiled, irregular strands called chromatin reticulum. Chromatin contains DNA and some base proteins called histones, some non histone proteins and also RNA. At the S-phase they replicate and the two chromatids remain attached. With the onset of mitosis or meiosis I, the long replicated chromosome condense.

Number | Shape | Size

The number of chromosome in a cell of an organism varies considerably from species to species, but is constant for a species. In animals, the lowest number of chromosome occur in Ascaris megalocephala where the haploid number is one, and perhaps the highest in Aulacntha  with haploid number 800.
In human beings the deploid number is 46. Of these one set of 23 chromosome is derived from the mother egg cell and the other of 23 chromosome from the father sperm cell.

The size of chromosome also varies from species to species. A chromosome, measured at mitotic metaphase, may be as short as 0.25 um in fungai and bird or as long as 30 um in plants like Trillium.

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