What Are the important Functions of Organelles

Eukaryotic cells contain a number of cytoplasm organelles associated with different functions.

These organelles may be membrane-bound like nucleus, mitochondria, chloroplasts etc. or non-membranous like ribosome, centriole etc.

The membrane bound organelles compartmentalize the eukaryotic cells so that each compartment carries out a specific function. This compartmentalization is absent in prokaryotes.

NUCLEUS

Nucleus (pi. nuclei; L. nucleus or nucleus- kernel) is a double-membrane enclosed organelle found in most eukaryotic cells, containing most of the cell’s genetic material. It as nucleoproteins organized to chromatin, functioning to maintain the integrity and expression of the genetic materials in order to control the metabolic activities of cells and transmission of characters. Organized nucleus with a limiting membrane is absent in prokaryotic cell.

Nucleus was first described by Franz Bauer in 1802. But Scottish botanist Robert Brown in 1831 described the nucleus in detail. He observed an opaque area in the cells of outer layer of orchid flowers which he called areola or nucleus.

In 1838 Matthias Schleiden suggested a possible role of nucleus in generating new cells and introduced the name cytoblast- meaning cell builders. Between 1876 and 1878 Oscar Hertwig published several papers showing that an individual develops from a single nucleated cell which is formed by the fusion of sperm nucleus with that of ovum. Edward Strasburger in 1884 reported the similar results in plants.

The function of the nucleus in cell division and as a carrier of genetic information became clear only after the process of mitosis was discovered, and the Mendelian rules were rediscovered at the beginning of the 20th century. Presence of hereditary information inside the nucleus was demonstrated by Joachim Hammerling in 1953 in his experiment on single- celled alga Acetabularia.

Nucleus is usually central in position but in plant cells, due to the presence of a large central vacuole, it is pushed to the periphery. In green filamentous alga Spirogyra nucleus is suspended in the central vacuole by cytoplasm strands.

In adiposities nucleus is peripheral and in glandular cell’s nucleus is basal. In a typical mammalian cell the average diameter of nucleus is 11 to 22 mm (micrometer) and it occupies about 10% of the total volume of a cell.

There are cells which do not have nuclei and are called anucleated cells e.g. human RBC. The nucleated cells cannot divide to produce daughter cells. Mature human RBC in the young stage contains nucleus and loses the same during the process of maturation. R.B.C or erythrocytes undergo the process of erythropoietin in the bone marrow where they lose nucleus and other organelles.

The nucleus is expelled from an erythroblast to form reticulocytes, which is the immediate precursor of erythrocyte. A nucleated cell can also arise from defective cell division in which one daughter cell is a nucleate and the other is binueleate. Natural binucleate condition of the cells is also seen in the case of a protist Paramaecium where out of the two nuclei in the cell one controls the metabolic activities and the other possess the hereditary information. Poly nucleated cells contain multiple nuclei.

In humans, the skeletal muscle cells called myocytes, become poly nucleate during development, the resulting arrangement of nuclei near the periphery of the cell allows maximum intracellular space for myofibrils.

The multinucleate condition found in fungi and plants is a result of free nuclcar division without cytokinesis (division of cytoplasm) called coenocytic cells. Coenocyitic condition is due to the absence of septa or partition walls. Fungi like Rhizopus and algae like Vaucheria have coenocytic conditions.

The shape of the nucleus depends on cell type. Generally spherical in shape, nucleus can be oval or elliptical in plant cells due to the presence of large vacuole. It is disc shaped in squamous epithelial cells, irregularly branched in silk-spinning cells of insects.

The surface of the nucleus is usually smooth but as in leucocytes, the surface can have enfolding giving the nucleus a lobed appearance.

Ultra structure

The nucleus of a non-dividing yet metabolically active cell is known as inter phase nucleus. A typical inter phase nucleus has five parts: nuclear envelope, nuclear lamina, nuclear sap, chromatin and nucleolus.

The nuclear envelope consists of two membranes; each 70-90A⁰ thick arranged parallel to one another and separated by a perinuclear space of about 10 to 5011m. It completely encloses the nucleus separating its contents from the cytoplasm.

The outer nuclear membrane is continuous with the membrane of endoplasmic reticulum and similarly may be studded with ribosome at some places giving it a rough appearance. The inner nuclear membrane is smooth.

A large number of nuclear pores or perforations present on the nuclear envelope provides for aqueous channels. These pores are composed of proteins collectively known as nucleoporins. The pores are about 100nm in total diameter enclosing a passage of about 9nm wide through which the molecules freely diffuse.

The central passage through the pore is reduced in width because of regulatory systems present within the centre of the pore. The pores are about 125 million Daltons in molecular weight and consist of around 50 (in yeast) to 100 proteins (in vertebrates).

The nucleus of a typical mammalian cell may have about 3000 to 4000 pores on its envelope, each of which has a eightfold- symmetric ring shaped structure at a position where the inner and outer membranes fuse.

Attached to the ring is a structure called nuclear basket that extends into the nucleoplasm, and a series of filamentous extensions that reach into the cytoplasm. Both these structures mediate binding to the nuclear transport proteins called Karyopherins. Most proteins, ribosomal subunits and some RNAs are transported through the pores in a process mediated by karyopherins.

The karyopherins mediating movement out of nucleus is called exportins and those into the nucleus are called importins. In some cases (like mammals) the nuclear pores may have blebs or annuli. In such pores there are one central ring surrounded by eight fold peripheral symmetric ring shaped structures.

Below the envelope, the nuclear matrix forms a dense fibrous network called nuclear lamina, to provide structural support for nuclear envelope and anchoring sites for chromosomes and nuclear pores. It is mostly composed of lamin proteins.

Lamin monomers form two types of intermediate filaments called lamin and lamin which forms the mesh work of nuclear lamina. The lamins are also found inside the nucleoplasm where they form another structure called nucleoplasm veil. Here the lamin binds to chromatin.

The inter phase nucleus contains an intact nuclear envelope, which breaks down during cell division by the depolymerization of lamin and then reappears after nuclear division due to repolymerisation of lamins.

The nuclear sap or nucleoplasm is also known as karyolymph or karyoplasms. The inter phase nucleus is filled with a homogenous, transparent, semi fluid granular, acidophilic ground substance known as nuclear sap.

It contains enzymes required for synthesis and maintenance of DNA, RNA and nucleoproteins, and also some proteins essential for spindle formation during cell division. The chromatin materials are found scattered in the nucleoplasm. One or more nucleolus is also found in the nucleoplasm.

The nucleus also contains a number of other non-membranous bodies. These include:

Cajal bodies:- A nucleus contains 1 to 10 cajal bodies or coiled bodies (CB) of 0.2 to 2nm diameter responsible for processing of small nucleolar RNA(Sno RNA) and small nuclear RNA(sn RNA )

Gems: – Gems or Gemini of coiled bodies assist cajal bodies in processing Sn RNA. Gems do not contain sn RN Ps.

PIKA domains; – Polymorphic interphase karyosomal associations were first described in 1991. They are thought to be associated with active DNA replication, transcription and RNA processing.

PML bodies: – promyelocytic leukemia bodies (PML) are spherical bodies found scattered throughout the nucleoplasm, measuring around 0.2 – 1.0 mm. They are also known as nuclear domain Io (NDio), KREMER BODIES and PML oncogenic domains. They play a role in regulating transcription.

Some other sub nuclear structures appear as part of abnormal disease process. Presence of small intranuclear rods has been reported in nemaline myopathy. This condition arises due to mutation in actin genes.

The nucleus contains the hereditary materials, DNA in the form of DNA- protein and RNA in thread like net work called chromatin network or chromatin reticulum. In 1882 Walter Fleming used the term ‘chromatin network’ for the first time as it gets stained with some basic dyes (chroma-colour).

The composition and properties c chromatin vary from one cell type to the other, during development of a specific cell type and also at different stages of the cell cycle. Besides DNA and proteins associated’ with packaging the DNA within the nucleus, many enzymes are also associated with chromatin reticulum.

They are the enzymes involved in DNA transcription, replication! And in post translational modifications of his tone proteins. Scaffold proteins encompass! Chromatin proteins such as insulators, domain boundary factor and cellular memory! Module (MMs). Inter chromatin granule clusters also known as spicing speckles! Associated with chromatin are rich in Sn RNPs (Small nuclear Ribonucleo protein) and other splicing proteins necessary for pre-m RNA processing.

The chromatin fibers in the interphone nucleus are distributed throughout the nucleoplasm and are differentiated into two regions: euchromatin and hetero chromatin. Euchromatin is less compact DNA for which it stains lightly and is present in the form of diffused fibers. Heterochromatin is more compact form of DNA for which it is darkly stained and is present in condensed, granular form.

The functional part of chromatin is found in the region of euchromatin to take part in transcription. The heterochromatin is further categorized into facultative heterochromatin consisting of genes that are organized as heterochromatin only in certain cell types or at certain stages of cell development while constitutive heterochromatin is found in all cell types at all stages of development.

Constitutive heterochromatin forms the structural components of chromosomes like telomeres and centromeres. Interphone nucleus has chromatin organized into discrete individual patches called chromosome territories and the active genes found in euchromatin regions tend to be located towards the boundary of chromosome territory.

The nucleolus (plural- nucleoli) is a no membranous, densely stained sub organelle found in the nucleus. It was first discovered by Fontana in I78tand named by Bowman in 1940. Generally 1-4 nucleoli are found in a nucleus but the number can be as high as 1600 as in the Oocytes of Xenopus.

They are found attached to the nucleolar organizer region (NOR) of the chromatin. The nucleolar organizer regions on DNA are constituted by tandem repeats for transcription of r RNA. The main role of nucleolus is to process ribosomal RNAs and assembly ribosomal components.

The transcription, post-trai. Scriptional processing and assembly of rRNA occurs in the nucleolus aided by small nucleolar RXA (SnO RNA) molecules. According to Wilson there are two kinds of nucleolus: Plasmosome – these are positive to acidic stains, have transparent exterior and dense interior and karyosome: these are positive to basic dyes. In some instances plasmosomes and karyosomal combine to form amphinuleoli (e.g. Molluscans eggs.). Fine structure of nucleolus includes three components – Granular portion, Fibrillar portion and Amorophus Matrix.

A. Granular portion: This is the peripheral region of the nucleolus composed of granules of size 150 – 250 A°. The granules are composed of ribonucleic acid and protein. The nucleolus is non membranous. The presence of granular portion makes the border of nucleolus distinct from surrounding chromatin and nucleoplasm.

B. Firbrillar portion: This is also known as nucleolonema. Fibrils of rib nucleoproteins of size 40 to 80 A⁰ form a fine network. The number and size of the febrile components varies with cellular activity and ribosome production.

C. Amorphous matrix: This is the electron dense, amorphous ground substance of nucleolus. The matrix is first to disappear at the time of cell division. This region is also known as pars amorpha.

Recent studies have indicated additional functions of nucleolus like Trafficking of various small RNAs Regulation of cell cycle Interaction with viral components Regulation of tumor suppressor and oncogene activities Assembly of signal recognition particles. Control of ageing and modulating telomerase function. Chromosome

Chromosomes (Gk. Chroma – color, soma – body) are the deeply staine condensed from of chromatin fibers formed during nuclear division. They are the bearers of hereditary materials or genes.

The name was given by German anatomy Heinrich Von Waldeyer in 1888. The chromosomal behavior in animal cell (Salamander) was described by Walter Fleming in 1882. The chromosomes id their true sense (i.e. DNA complexed with histone proteins) is found only in eukaryotic cells. Naked DNA (devoid of histone) found in prokaryotes and RNA / DNA in virus are also sometimes referred to as prokaryotic and viral chromosomes respectively because of functional similarity.

The number of chromosomes per cell in eukaiyotes varies from 2 (Ascaris) to more than thousand (1600 in Aulacantha – a haploid protozoa). But the chromosome number is generally fixed or constant for a species.

The chromosomes are found in single set in haploid forms and two sets in diploid forms. Diploid organisms having two sets of chromosomes produce gametes (sperm and ovum) with only one set of chromosomes.

The haploid set of chromosome number is referred to as ‘n’ and the diploid set of chromosome number is referred to as ‘211’. In diploid cells each of the the chromosomes in the haploid set finds a partner or homologue in other set. The homologous pair of chromosomes are identical in size and carry identical or similar genes in the corresponding positions or loci.

The general morphology of a set of chromosomes is known as Karyotype and its diagrammatic presentation is known as idiogram.

During interphase the chromosomes are present in the nucleus as fine threads and their shape is not clearly visible under light microscope. However, during cell divisions the chromosomes are distinctly visible as the fine thread like chromatins get condensed to shorter and thicker structures.

Gross Morphology: The metaphasic chromosome consists of two halves or chromatids. The two chromatids are actually two daughter chromosomes yet to be separated. During cell division each chromosome replicates into two and form two chromatids held together at a point called centromore or primary constriction.

The two chromatids separate into two poles during anaphase (next to metaphase). The centromere or the primary constriction has a darkly staining granule surrounded by a relatively clear region. Spindle fieres formed during cell division attach to the chromosomes at a specialized structure in the centromere called kinetochore.

Usually there is one centromere in a chromosome (chromosome is monocentric). But di-or polycentric chromosomes are also there in some species. Basing on the position of centromere, four types of chromosomes are found.

They are met centric with centrally located centromore, submetacentric with centromere being one sided resulting in two unequal chromosome arms ; areocentric when the centromere is close to one end of the chromosome; telocentric with terminal centromere.

Chromosomes may also be acentric with complete absence of centromere. When the two arms of chromosomes are equal as in metacentric chromosome it is said to be telocentric chromosomes are rarely found in the plants. Mark (1957) considered them as unstable chromosomes causing genetic imbalance.

Isobrachial and otherwise it is hetero brachial. Some chromosomes may possess an additional constriction called secondary constriction. Often, the chromosomes bear a small fragment or rounded body separated by secondary constriction called satellite or trabant.

These satellite regions are without thymonucleic acid (Sine Acedo Thymonucleirico=SAT). The secondary constrictions are associated with ribosomal RNA synthesis that induces the formation of nucleolus. Hence this region is also called nucleolar organizing region.

The tips of the chromosome are called telomeres. Telomeres stabilize the chromosome. Yeast telomere is about 100 base pair long and is constituted by repeated sequences of DNA. According to classical cytogeneticists each chromatid is constituted by very thin and highly coiled subunits called chromonemata.

Cytologists observe the presence of certain bead-like structures formed due to accumulation of chromatin material which are visible along the entire length of the chromonema.

These are called chromomeric and are believed to be the regions representing genes. Contrary to the observations made under light microscope by classical workers a chromosome is actually formed by direct condensation of a single chromatin.

Chemical composition

Chemically eukaryotic chromosomes are DNA associated with some amount of RNA and proteins. The DNA-protein ratio is almost 1:2.

Out of the proteins 70% are the basic his tone proteins and rest is acidic non-his tone proteins. The tone proteins interact specifically with each other and .with DNA to give it a compact
packaging. The non-his tone proteins that are. Found associated with chromatin fall into several functional categories like:

Chromatin-bound enzymes

High mobility group (HMG) proteins

Transcription factors Scaffold proteins.

Transition proteins (testis specific proteins)

Protamines (present in mature sperm).

Unlike most other proteins which have an overall negative charge, the tones have positive charge, owing to an abundance of basic amino acids viz… arginine and lysine.