Which is the most important asset of the cell that governs all its characteristic features? It is the genetic material that regulates all the functions and characteristics of the cell. So you can guess that protecting the genetic material is one of the top priorities of the cell. Primitive cells such as the prokaryotic bacteria and archaebacteria were not advanced enough to develop a special enclosure for protecting their genetic material but as the eukaryotic cells evolved, a well defined membrane developed to enclose the genetic material. This membranous structure is called the nuclear envelope which is formed of two nuclear membranes and with its emergence, a well-defined nucleus was formed in eukaryotic cells.
But what triggered the formation of the nuclear envelope? What is it made up of? What other functions does it serve? Let us get the answers to these questions through this article.
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The nucleus is the most essential part of the cell which directs and controls all activities. It is compared to the control room of a factory. A well defined nucleus surrounded by a nuclear membrane or nuclear envelope is present only in eukaryotic cells. Nucleus in eukaryotes is composed of 9-12% DNA, 15% basic proteins or histones, 5% RNA, about 65% acid proteins and neutral proteins and traces of mineral ions (Ca2+, K+, Na+).
An interphase nucleus is distinguished into the following parts:
Let us now get into the details of the nuclear envelope.
The presence of a nuclear envelope separating the nuclear material from cytoplasm was demonstrated by O. Hertwig in 1893. It was described by him as the nuclear membrane. Anderson named it the nuclear envelope.
There are several ideas circling the evolutionary origin of the nuclear envelope in eukaryotic cells. Studies have suggested that the emergence of the nucleus in the primitive eukaryotic ancestor was triggered by the symbiosis between archaeal hosts and the aerobic bacterial cells that they had engulfed which are considered to be the precursors to eukaryotic mitochondria. Some ideas propose that the nuclear envelope emerged due to invagination of the cell membrane in some prokaryotic ancestor while others suggest the synthesis of a new membrane after the establishment of proto-mitochondria in the host archaeal cells. This must have been an adaptation to protect the genetic material from the reactive oxygen species (ROS) produced during aerobic respiration by the proto-mitochondria.
The nuclear envelope is formed of two phospholipid bilayer membranes, an outer nuclear membrane and an inner nuclear membrane, which are separated by a fluid-filled perinuclear space that is around 100-500 Å wide. Each nuclear membrane is about 90 Å thick and has a unit membrane structure similar to that of the plasma membrane.
The nuclear envelope also has some nuclear pores to facilitate the exchange of materials between the cytoplasm and the nucleoplasm. These pores help to connect the outer and inner nuclear membranes.
The nuclear envelope is visible only in interphase nuclei. During prophase of nuclear division of a cell undergoing cell division, the nuclear membrane fragments and is reformed during telophase, around the two sets of chromatin fibres at the poles, by the accumulation of vesicles from the endoplasmic reticulum.
This membrane bears ribosomes and is often connected with the membrane of the Endoplasmic reticulum. The perinuclear space is continuous with the lumen of the ER. It contains higher concentrations of proteins compared to the endoplasmic reticulum.
It surrounds the nucleoplasm and is smooth in nature due to absence of ribosomes. The inner side of the inner nuclear membrane is supported by the nuclear lamina which is a mesh like structure formed by intermediate filaments formed of protein lamins. The nuclear lamina helps to stabilise the nuclear membrane, provide anchor and support to the chromatin fibres, help in replication and transcription of the genetic material and helps the nucleus to maintain its shape and size.
The inner nuclear membrane fuses with the outer nuclear membrane around the edges of the nuclear pores. Although the two membranes of the nuclear envelope are linked to the endoplasmic reticulum, the proteins embedded in these membranes do not move across the continuum and remain in their respective positions.
The nuclear envelope is perforated by numerous pores, ranging from 600-1000 Å in diameter. Nuclear pores were described by Callan and Tomlin (1950). The pores are circular or polygonal, more commonly octagonal in outline. The nuclear pores are not simple open channels but often possess extra materials such as blebs, diaphragms, septum, plug nucleoplasmin, granules and annuli or microcylinders. These additional structures together form the nuclear pore complex or NPC. The complexes are constructed of proteins called nucleoporins. Each nuclear pore is plugged with a hollow cylindrical annulus and is covered with a fine diaphragm.
These pores are highly selective and regulate the nucleo-cytoplasmic exchange of materials. Ribosomal subunits synthesised by the nucleolus, come out of the nucleus into the cytoplasm through the nuclear pore but active ribosomes are not allowed to enter the nucleoplasm. Similarly, proteins synthesised in the cytoplasm can enter the nucleus but ions like K+, Na+ or Cl- are not allowed to enter.
1. Which of the following structures helps to support the inner membrane of the nuclear envelope?
Solution: The inner side of the inner nuclear membrane is supported by the nuclear lamina which is a mesh like structure formed by intermediate filaments formed of protein lamins. The nuclear lamina helps to stabilise the nuclear membrane, provide anchor and support to the chromatin fibres, help in replication and transcription of the genetic material and helps the nucleus to maintain its shape and size. Thus, the correct option is c.
2. Which of these molecules is transported from the cytoplasm to the nucleoplasm across the nuclear pores?
Solution: The nuclear pores are highly selective and allow newly synthesised ribosomal subunits to move out from the nucleoplasm into the cytoplasm but do not allow the entry of active ribosomal subunits from the cytoplasm into the nucleoplasm. They allow rRNA, mRNA, tRNA to leave the nucleoplasm and enter the cytoplasm and also allow proteins newly synthesised in the cytoplasm to enter the nucleoplasm. Thus, the correct option is d.
3. Which of the following statements is true?
Solution: The outer nuclear membrane is rough and bears ribosomes. It is often connected with the membrane of the endoplasmic reticulum. The perinuclear space is continuous with the lumen of the ER. It contains higher concentrations of proteins compared to the endoplasmic reticulum.
The inner nuclear membrane is smooth and does not contain any ribosomes and is hence smooth in nature.
4. Which of the following is not a function of the nuclear membrane?
Solution: rRNA is synthesised by the nucleolus. The functions of the nuclear envelope include -
Answer: The part of the endoplasmic reticulum which lies close to the nucleus is called the perinuclear endoplasmic reticulum. Its lumen is continuous with the lumen of the nuclear envelope lumen, that is the perinuclear space.
Answer: Mutations in the genes that code for lamin proteins of the nuclear lamina result in some rare genetic disorders known as laminopathies. These disorders can lead to muscular dystrophy, diabetes, neuropathy, dysplasia, premature ageing, leukodystrophy, etc. Most of these symptoms are exhibited during childhood or adolescence. Some can even result in death at a young age.
Answer: Nuclear envelope spectrin repeat proteins or nesprin proteins are present in the outer nuclear membrane of mammalian cells and help in connecting the filaments of the cytoskeleton to the nucleoskeleton. These connections mediated by the nesprin proteins help in positioning the nucleus in the cytoplasm and also help the cell in its mechanosensory functions.
Answer: Generally in eukaryotic cells, the nuclear membrane disintegrates during early prophase of cell division but in yeast cells, the nuclear membrane remains intact and is not disintegrated during cell division. Such a cell division is called closed cell division. The spindle fibres are either formed within the nuclear membrane or penetrate it without causing any damage or tear.