Summarized Bic Concepts in Cytology

Basic Concepts in Cytology

1. Every organism consists of cells. These possess, normally, a cell wall, a colourless semi-liquid material cytoplasm, and a nucleus, both the latter constituting the protoplasm or living substance. In addition. the cell may include plastids, which often bear pigments, e.g., chloroplasts. These are embedded in the cytoplasm. The cytoplasm contains immense numbers of minute granules of substances formed by it and also larger units, rod-like or granular, called mitochondria. In the cytoplasm are usually spaces, the vacuoles, empty in appearance but filled with cell-sap. They may coalesce into a single large vacuole filling the central part of the cell. 
2. The nucleus consists of a nuclear membrane, enclosing a limpid, semi-solid material, the karyolymph. In this is suspended a hollow reticulum composed of chromatin. In this network are numerous knots or lumps of heterochromatin. Attached to the reticulum are one, or some­times two, nucleoli which differ from the chromatin in composition. 
3. Cell division is preceded by nuclear division or mitosis. The reticulum is resolved into a number of separate threads, whose number is fixed and characteristic for each species. These threads thicken and are called chromosomes. Each of them has a distinctive shape or size. They split lengthways, and the portions or chromatids separate and move to opposite ends of the cell, where they reconstitute two daughter nuclei. The chromosomes are drawn apart by two sets of cytoplasmic fibrils called the spindle fibres, which appear only during nuclear division. The nucleolus and the nuclear membrane disappear in the early stages of division and are reformed at its conclusion. Between the daughter nuclei a cell plate is formed, which becomes a new cell wall. 
4. Sexual reproduction consists essentially of the union of two nuclei, one from each of the parents. Thus after fusion the nucleus contains two sets of chromosomes, each pair of the same type forming what is called an homologous pair. As this would inevitably lead to a progressive doubling in the number of chromosomes, from generation to generation, there is a periodic halving or meiosis of the chromosome number. The sexual nuclei have always the half number (monoploid), and their union re-establishes the double number (diploid). 
5. The special reduction division or meiosis does not always occur at the same point in the life-cycles of different organisms, but it is always associated with the formation of some type of reproductive cell, either sexual or non-sexual. Even in the latter case, which is common in plants, it may be regarded as a preparation for eventual conjugation. The process of reduction division involves two immediately successive nuclear divisions, resulting in the formation of a quartet of cells. In the first division (heterotypic) the chromosomes of homologous pairs unite together and then disjoin, each chromosome being then separated to form part of a daughter nucleus. In other words, at this stage whole chromosomes are segregated instead of halves as in mitosis. The two daughter nuclei thus contain half the number of whole chromosomes. In the second division (homotypic) each of these chromosomes splits, as in mitosis, followed by separation as before. The final result is four nuclei each with the reduced number of chromosomes. 

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