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| Structure of Chromosomes |
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| The chromatin material just before cell division forms into chromosomes. Each chromosome is made up of two longitudinal strands called the chromatids. |
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| Structure of Chromosome |
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| Each chromatid has a double helical DNA molecule. The two chromatids are held together by a structure called the centromere. During cell division, the spindle fibres are attached to the centromeres. During anaphase, when the chromosomes or chromatids move apart, they form different shapes based on the position of the centromere. |
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| Types of Chromosomes |
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| The different types of chromosomes based on the position of the centromere are: |
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| Metacentric |
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| The centromere is at the centre and the chromosome is V-shaped during anaphase. |
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| Sub-metacentric |
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| The centromere is a little away from the centre and the chromosome gets an L shape. |
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| Acrocentric |
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| The centromere is nearer to one end and the chromosome is J-shaped. |
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| Telocentric |
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 The centromere is at the end and the chromosome is l-shaped. |
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The shape of the chromosome is specific and constant. |
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The sides of the chromosomes on either side of the centromere are called the arms. |
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The chromosomes are cylindrical structures with one or more constrictions. |
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| The centromere is associated with the primary constriction. In some chromosomes there occurs another constriction called the secondary constriction associated with the presence of nucleolus. |
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| The chromosomes are present in pairs. The pairs are called the homologous pairs. A species will always have the same number of chromosomes. This is called the chromosome number and it will always be an even number. This number is called the diploid number. During gamete formation, the homologous chromosomes separate and the gametes will have only half the number of chromosomes. This number is called the haploid number. Thus the somatic or the vegetative cells of all organisms are diploid and the gametes are haploid. |
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| The chromosome numbers of some of the plants and animals are given below: |
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| Whatever may be the number of chromosomes, it is possible to keep track of the behaviour of these chromosomes as they are all different in some respects. They are of a specific shape and carry specific genes at specific locations. The locations of genes on chromosomes are called the loci (sing. locus). The chromosomes can be numbered and studied. |
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| The pictorial representation of the entire set of chromosomes is called the karyogram. |
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| As can be seen from the human karyogram, the chromosomes are classified into two types, autosomes and sex chromosomes. The autosomes have homologous chromosomes as pairs whereas the sex chromosomes are of two different types - X and Y. A female has two X-chromosomes and a male has an X and a Y chromosome. |
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