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| Structure of Chromosomes |
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| The most ideal stage to study the structure of chromosomes is the metaphase of mitosis. A metaphase chromosome consist of two identical components called chromatids, which are held together at a specific region called primary constriction. It is usually found in the centre and hence it is commonly described as centromere. It shows a plate-like proteinaceous structure called kinetochore where the microtubules of the spindle become attached during cell division. The portions of a chromatid found on either side of a centromere, are called arms which may be equal (isobrachial) or unequal (heterobrachial), based on the position of centromere. |
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| Sometimes, a chromosome may have an additional constriction apart from the centromere, called secondary constriction. Portion of the chromatid situated beyond the secondary constriction, is called as a satellite body. Such a chromosome with a satellite body is known as a sat-chromosome. Secondary constrictions most often take part in the formation of nucleolus. Hence, such secondary constrictions are called as nucleolar organizers. |
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| fig. 19.3 Structure of Chromosomes |
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| The terminal end of a chromosome is called as telomere. It is functionally different from the rest of the chromosome. A chromatid may breakup into pieces and the pieces may rejoin, but no segment becomes connected to the telomere. Thus, the telomere shows a sort of polarity. |
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| The chromatid, when observed under the electron microscope appears to be composed of a very fine filamentous structure called chromonema. It represents a molecule of double stranded DNA extending from one end of the chromosome to the other. The chromonema occurs in a highly coiled state. The chromonema is in turn composed of a chain of tiny, bead-like structures called nucleosomes. A nucleosome or nubody consists of a core particle surrounded by a DNA strand. The core particle is formed by a group of proteins called histones. In a nucleosome there is an octomer of 8 histone molecules, two each of H2A, H2B, H3 and H4. In between the nucleosomes is the DNA portion not associated with histones and is called Linker DNA. A linker DNA with the nucleosome is described as a chromatosome. |
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| Chemically, the chromosomes are described as nucleo-proteins, since they contain DNA and proteins. The chromosomal proteins have nothing to do with the genetic potency of the organism. These proteins, called histones, regulate the gene action. |
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