The membrane-bound nucleus is the most prominent feature of the eukaryotic cell.
Schleiden and Schwann, when setting forth the cell doctrine in the 1830s, considered that it had a central role in growth and development. Their belief has been fully supported even though they had only vague notions as to what that role might be, and how the role was to be expressed in some cellular action. The membraneless nuclear area of the prokaryotic cell, with its tangle of fine threads, is now known to play a similar role.
Some cells, like the sieve tubes of vascular plants and the red blood cells of mammals, do not possess nuclei during the greater part of their existence, although they had nuclei when in a less differentiated state. Such cells can no longer divide and their life span is limited. Other cells are regularly multinucleate. Some, like the cells of striated muscles or the latex vessels of higher plants, become so through cell fusion. Some, like the unicellular protozoan paramecium, are normally binucleate, one of the nuclei serving as a source of hereditary information for the next generation, the other governing the day-to-day metabolic activities of the cell. Still other organisms, such as some fungi, are multinucleate because cross walls, dividing the mycelium into specific cells, are absent or irregularly present. The uninucleate situation, however, is typical for the vast minority of cells, and it would appear that this is the most efficient and most economical manner of partitioning living substance into manageable units. This point of view is given credence not only by the prevalence of uninucleate cells, but because for each kind of cell there is a ratio maintained between the volume of the nucleus and that of the cytoplasm. If we think of the nucleus as the control centre of the cell, this would suggest that for a given kind of cell performing a given kind of work, one nucleus can ‘take care of’ a specific volume of cytoplasm and keep it in functioning order. In terms of material and energy, this must mean providing the kind of information needed to keep flow of materials and energy moving at the correct rate and in the proper channels. With the multitude of enzymes in the cell, materials and energy can of course be channelled in a multitude of ways; it is the function of some information molecules to make channels of use more preferred than others at any given time. How this regulatory control is exercised is not entirely clear.
The nucleus is generally a rounded body. In plant cells, however, where the centre of the cell is often occupied by a large vacuole, the nucleus may be pushed against the cell wall, causing it to assume a lens shape. In some white blood cells, such as polymorphonucleated leukocytes, and in cells of the spinning gland of some insects and spiders, the nucleus is very much lobed. The reason for this is not clear, but it may relate to the fact that for a given volume of nucleus, a lobate form provides a much greater surface area for nuclear-cytoplasmic exchanges, possibly affecting both the rate and the amount of metabolic reactions. The nucleus, whatever its shape, is segregated from the cytoplasm by a double membrane, the nuclear envelope, with the two membranes separated from each other by a perinuclear space of varying width. The envelope is absent only during the time of cell division, and then just for a brief period. The outer membrane is often continuous with the membranes of the endoplasmic reticulum, a possible retention of an earlier relationship, since the envelope, at least in part, is formed at the end cell division by coalescing fragments of the endoplasmic reticulum. The cytoplasmic side of the nucleus is frequently coated with ribosomes, another fact that stresses the similarity and relation of the nuclear envelope to the endoplasmic reticulum. The inner membrane seems to possess a crystalline layer where it abuts the nucleoplasm, but its function remains to be determined.
Everything that passes between the cytoplasm and the nucleus in the eukaryotic cell must transverse the nuclear envelope. This includes some fairly large molecules as well as bodies such as ribosomes, which measure about 25 nm in diameter. Some passageway is, therefore, obviously necessary since there is no indication of dissolution of the nuclear envelope in order to make such movement possible. The nuclear pores appear to be reasonable candidates for such passageways. In plant cells these are irregularly, rather sparsely distributed over the surface of the nucleus, but in the amphibian oocyte, for example, the pores are numerous, regularly arranged, and octagonal and are formed by the fusion of the outer and inner membrane
Which of the following kinds of cells never have a nuclei?
Show Hint
- Sieve Tubes
- Red blood cells of mammals
- Prokaryotic cells
- None of these
The Correct Option is C
Solution and Explanation
Sieve tubes and RBCs lose their nuclei during maturity, but once had them.
Thus, (c) is correct.
According to the first paragraph, the contention of Schleiden and Schwann that the nucleus is the most important part of the cell has
Show Hint
- been proved to be true.
- has been true so far but false in the case of the prokaryotic cell.
- is only partially true.
- has been proved to be completely false.
The Correct Option is A
Solution and Explanation
Thus, (a) is correct.
It may be inferred from the passage that the vast majority of cells are
Show Hint
- multinucleate.
- binucleate.
- uninucleate.
- anunucleate.
The Correct Option is C
Solution and Explanation
Hence, (c) is correct.
What is definitely a function of the nuclei of the normally binucleate cell?
Show Hint
- To arrange for the growth and nourishment of the cell.
- To hold hereditary information for the next generation.
- To make up the basic physical structure of the organism.
- To fight the various foreign diseases attacking the body.
The Correct Option is B
Solution and Explanation
Thus, (b) is correct.
The function of the crystalline layer of the inner membrane of the nucleus is
Show Hint
- generation of nourishment of the cell.
- holding together the disparate structures of the endoplasmic reticulum.
- helping in transversal of the nuclear envelope.
- Cannot be determined from the passage.
The Correct Option is D
Solution and Explanation
Thus, (d) is correct.
Why, according to the passage, is the polymorphonucleated leukocyte probably lobed?
Show Hint
- Because it is quite convoluted in its functions.
- Because it is the red blood cell which is the most important cell in the body.
- Because it provides a greater area for metabolism reactions.
- Because it provides greater strength to the spider web due to greater area.
The Correct Option is C
Solution and Explanation
Thus, (c) is correct.
Why, according to the passage, are fungi multinucleate?
Show Hint
- Because they need more food to survive.
- Because they frequently lack walls dividing the mycelium.
- Because the mycelium is area wise much bigger than other cells.
- Cannot be determined from the passage.
The Correct Option is B
Solution and Explanation
Thus, (b) is correct.
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