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| Summary |
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 Chromosomes are tiny thread-like structures found in the nucleus of a cell. |
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 Chromosomes store and transmit the coded information which is responsible for all the life processes of an organism. Hence, chromosomes are commonly described as carriers of heredity. |
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 In bacteria, the entire hereditary material is packed into a single, irregularly packed, compact mass called nucleioid or bacterial chromosome. |
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 Bacterial chromosomes lack a nuclear membrane and are not associated with histones. |
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 Eukaryotic chromosomes are visible only during metaphase stage of mitosis. |
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 The chromosome number of a given species is generally described as the diploid number (2n) since chromosomes occur in pairs. |
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 Haploid number of chromosomes occur in gametes and spores. Some adult organisms (chlamydomonos, male honey bee) have haploid number of chromosomes. |
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 Chromosomes range in size from 0.1 to 30 mm in length and 0.2 to 2.0 mm in thickness. When the number is less, the chromosomes are larger in size. |
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 A metaphase chromosome shows two identical components called chromatids, joined by a centromere (primary constriction). Each chromatid has two arms. |
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 A secondary constriction may occur sometimes. Such a chromosome is called sat-chromosome. |
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 Chemically the chromosome is composed of nucleoproteins present in the form of a highly coiled chromonema. |
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 Chromonema is composed of a chain of bead-like structures called nucleosomes (nu bodies). |
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 A nucleosome has a core particle formed by proteins called histones surrounded by a DNA strand. |
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 The part of DNA not associated with histones is called Linker DNA. |
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 Normally a chromosome has only one centromere (monocentric). It can be sometimes dicentric or polycentric. Very rarely it may lack a centromere (acentric). |
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 Monocentric chromosome can be distinguished into metacentric, submetacentric, acrocentric and telocentric, based on the position of centromere. |
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 Chromosomes, which are extremely larger than the normal ones are called giant chromosomes. They occur in some animal cells. |
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 Lampbrush chromosomes of amphibian oocytes and polytene chromosomes of Drosophile one common examples of giant chromosomes. |
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 Genome is the term used to describe the sum total of all the genes present in a haploid set of chromosomes. |
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 In unisexual organisms the chromosomes can be distinguished into autosomes (somatic chromosomes) and allosomes (sex chromosomes). |
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 Allosomes have a role in sex-determination and in the expression of sex-linked characters. |
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