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- The nucleus is a specialize membrane-bound structure or organelle.
- It is the controlling center of eukaryotic cells.
- It is a characteristic feature of eukaryotic cells and is absent in prokaryotes like bacteria and viruses.
- It contains most of the genetic material of an organism.
- Most of the eukaryotic cells contain one nucleus but some cells may have more than one nuclei.
- These cells are multinucleated cells. For instance, Paramecium contains two nuclei viz. a larger macronucleus and a smaller micronucleus.
- Macronucleus governs all the general cellular activities such as water balance etc while micronucleus is responsible for sexual reproduction.
- Some mammalian cells even lack a nucleus. For instance mature red blood cells or erythrocytes.
- In addition to this nucleus also differs in size depending upon the cell type.
- Mostly nuclei are spherical but multilobed nuclei are also evident in leukocytes.
- Thus leukocytes are also known as polymorphonuclear leukocytes.
Image Source: Helmut Schiessel, Article: The Biology and Polymer Physics Underlying Large Scale Chromosome Organization
A nucleus has four components:
- Nuclear envelope
- Nuclear matrix
1. Nuclear envelope
- The nuclear membrane consists of two concentric membranes:
- Inner nuclear membrane and Outer nuclear membrane
- There lies a perinuclear space between these two membranes.
- The outer nuclear membrane is continuous with endoplasmic reticulum and the perinuclear space is continuous with the lumen of the endoplasmic reticulum.
- The outer nuclear membrane also has bound ribosomes to its cytoplasmic side.
- In this manner, it is very similar to the endoplasmic reticulum’s membrane. Conversely, the inner nuclear membrane has proteins specific to the nucleus.
- The nuclear envelope contains nuclear pores for the transport of macromolecules between the cytoplasm and nucleus.
- Each nuclear pore comprises of nuclear pore complex.
- It is a cylindrical structure comprising of eight spokes surrounding a central channel.
- The inner and outer membranes fuse at the nuclear pore complex.
2. Nuclear matrix
- It is also known as a nuclear scaffold.
- It is a network of protein filaments in the nucleus which provides a structural framework for organizing chromatin.
- It also facilitates transcription and replication.
- The nuclear matrix is the biochemical entity.
- Isolation of the nuclear matrix from a cell’s nuclei is possible after sequential extraction with non-ionic detergents, nucleases, and high salt buffers.
- Ultrastructure shows the presence of nuclear lamina and a network like an internal nuclear matrix.
- The structural characteristics of the internal nuclear matrix are less known.
- The nuclear lamina is a network of intermediate filaments.
- It comprises of lamin proteins.
- It is a non-membrane-bound dynamic entity present inside the nucleus.
- It disappears in late prophase and reappears in the Telophase stage of cell division.
- The number of nucleoli per nucleus differs.
- For instance, the yeast cell contains one relatively large nucleolus with respect to its nuclear volume.
- On the other hand, Xenopus oocytes contain over 1000 nucleoli per nucleus.
Structure of nucleolus
It consists of three major regions:
Fibrillar centers: It contains rRNA i.e. ribosomal RiboNucleic Acid genes in the form of partly condensed chromatin.
Fibrillar component: These surround the fibrillar centers which contain RNA molecules in the process of transcription.
Granular regions: These are the outer most regions having mature ribosomal precursor particles.
4. Chromatin and chromosomes
Chromatin is an organized structure of DNA and proteins found in the nucleus of eukaryotic cells. It contains double-stranded Deoxy RiboNucleic Acid in coiled and condensed form.
Are chromatin and chromosome the same?
- Chromatin and chromosome are basically the same things.
- The only difference is that the condensation is more in chromosome than that found in chromatin.
- The extent of chromatin condensation depends and varies with phases of the cell cycle.
- In interphase, most of the chromatin remains decondensed.
- There exist two types of chromatin depending on condensation:
- Euchromatin: Lesser condensation is evident. As a result, it is light staining. This is transcriptionally active because there is space available for enzymes to work upon.
- On the other hand, Heterochromatin: Much more condensation is evident. As a result, it is the dark staining form. Therefore it is generally transcriptionally inactive.
Regions in chromosome
- The constricted region of linear chromosomes is known as the centromere.
- But despite its name, it is usually not exactly located in the center of the chromosome.
- The regions on either side of the centromere are called chromosome arms.
- The site off the association of sister chromatids.
- Attachment sites for microtubules of a meiotic spindle which is essential for movements in the cell cycle.
They are specialized structures capping the ends of eukaryotic chromosomes and consist of a long array of short tandem repeat sequences.
- These are important for the protection of DNA.
- Prevention of shortening of genetic material in eukaryotes each time the DNA is replication occurs.
Origin of replication
This is the site where replication initiates. One chromosome contains multiple origins of replication.
Chromosome numbers in the eukaryotic nucleus
- Chromosome numbers in eukaryotes are species-specific.
- As a result, every species maintain a characteristic chromosome number.
- It may vary from two to several hundred.
- Eukaryotes can be:
- Monoploid: having one set of chromosomes
- Diploid: having two sets of chromosomes
- Polyploid: having more than two sets of chromosome
Chromosome number in some organisms is as follows:
Species Haploid number of chromosome
Saccharomyces cerevisiae (budding yeast) ———————- 16
Schizosaccharomyces pombe (fission yeast) ——————– 03
Caenorhabditis elegans ——————————————– 06
Drosophila melanogaster —————————————— 04
Homo sapiens ——————————————————- 23
- Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, and Peter Walter. 2014. Molecular Biology of the Cell. 6th Edition. Garland Science.
- Harvey Lodish; Arnold Berk; Chris A. Kaiser; Monty Krieger; Anthony Bretscher; Hidde Ploegh; Angelika Amon; Kelsey C. Martin. 2016. Molecular Cell Biology. 8th Edition.
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