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On the basis of structural organisation of their nucleus in 1957, scientist Dougherty classified types of cells into prokaryotic cells and eukaryotic cells. The cells those are possess a primitive type of nucleus devoid of nuclear membrane and chromosomes are the prokaryotic cells. This type of cells does not have a true organised nucleus. On the other hand, eukaryotic cells are those that possess a true, well organised nucleus having typical chromosomes and nuclear membrane. In the year 1966, scientist Dodge and some others proposed that there is a third types of cells which can be placed in between prokaryotic and eukaryotic cells. These types of cells are called Mesokaryotic cells.
Prokaryotic Cells: -Prokaryotic types of cells are primitive plant cells have being the simplest organisation. The cells of bacteria, blue green algae, Mycoplasma, rickettsiae and spirochae etc. are included in this type. Prokaryotic cells are generally smaller than eukaryotic cells. This type of cell is made up of a mass of protoplasm surrounded by a covering. All prokaryotic cell is invariably bounded by a complex cell wall. Beneath the cell wall lies and it sounds the protoplasm. The protoplasm of prokaryotic cells is not separated into nucleus and cytoplasm. The genetic material is localised within a distinct region called the nucleoid. The nucleoid is made up of DNA. The DNA is not complexes with histone proteins and does not form the chromosomes. The nucleoid is not separated from the surrounding cytoplasm by any delimiting membrane. For this reason the genetic material of prokaryotic types of cells is not considered as a true nucleus. This types of cells do not have membrane bounded organelles like mitochondria, plastids, endoplasmic reticulum, Golgi body, etc. Certain infolding of the plasma membrane which is called mesosomes may contain respiratory enzymes. Prokaryotic cells contain ribosomes and some inclusion bodies.
Eukaryotic Cells: -This types of cells are in general larger and more organised than prokaryotic cells. Most of the plants and animal cells are fall under this category. Eukaryotic cells are also made up by a mass of protoplasm and surrounded by a cover. In all this types of cells a plasma membrane bounds the protoplasm. The plant cells possess an additional, rigid outer covering, which is absent in animal cells and this covering is known as cell wall . As a result the plant cell has a fixed shape while the animal cell has a variation shape.All the eukaryotic types of cells' protoplasm is clearly divisible into cytoplasm and nucleus. The wall of nucleus contains nuclear material enclosed by double layered membrane called the nuclear membrane .
The genetic materiel i.e. DNA present in the nucleus remains combined with histone proteins and form the chromosomes. The eukaryotic cells' cytoplasm has a highly complex organisation. It contains several membrane bound organelles, such as mitochondria, plastids, endoplasmic reticulum, Golgi bodies, etc. and all have specific functions. Plastidsare presents in the plants cell only. In eukaryotic types of cells the ribosomes are mostly attached to endoplasmic reticulum and some of them may remain freely scattered in the cytoplasm. Various inclusion bodies in which different materials remain stored are also contained by the cytoplasm of eukaryotic cells.
Mesokaryotic Cells: -
Those types of cells that are intermediate between prokaryotic and eukaryotic type are called mesokaryotic cells. The cells of certain algae like Gymnodinium, Peridinium, etc., and protozoa like Noctiulea are the example of such cell. The mesokaryotic cells possess a membrane bound organised nucleus with chromosoms, which contains acidic no-histone protein. In these types of cells, the miotic spindle is not formed and mitosis does not happened.
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