Special Chromosomes

 • Chromosomes are the rod shaped, dark stained bodies located in the nucleus of a eukaryotic cell. 

• Play an important role in heredity i.e., transfer of genetic information from one generation to another generation.

• In some organisms  there are special tissues in which chromosomes undergoes structural specialisations. Such specialized chromosomes includes 1. Gaint chromosmes- like the Polytene chromosomes, Lampbrush chromosomes, B chromosomes

Polytene Chromosomes

Balbiani discovered the large or giant chromosomes in the salivary glands in Chironomous larvae (Diptera species) in 1881. These structues were long and sausage-shaped and marked by swellings and transverse bands. Unfortunately, he could not recognize them as chromosomes.  .

The cytogenetical importance, however, was not realized until many years later when Kosloff(1930) pointed out the similarities between their banded structure and the linearly arranged genes on the chromosomes. Panter, Heltz and Baner established the fact that each of the visible chromosomes actually consisted of a pair of homologous chromosomes intimately synapsed. Since, these chromosomes were discovered in the salivary glands cells, they were called salivary gland chromosomes.

Commonly occur in diplotene stage of meiosis. Have been observed in salivary glands cells, Malplhigian tubules, ovarian nurse cells and gut epithelial cells of the larvae of dipterean species like Drosophila, chironormous, sciara.

They have also been reported from the suspensor cells of developing embryos, antipodal cells and endosperm  cells of plants. 

Polytene or salivary gland chromosomes are the largest chromosomes known. In Drosophila melanogaster they are approximately 100 times the length of the somatic metaphase chromosome. Each salivary gland chromosome is 200 – 600 mm in length. Because of their larger size compared to metaphase chromosomes these chromosomes are called as giant chromosomes. They contain 1000-16000 times more DNA than normal cells. This is because of polyteny or multistranded nature of these chromosomes.

A common structural feature of all polytene chromosomes is that their many chromatin filaments (chromatids) are laying – more or less paralles – side by side. The reason for the higher number of chromatids is due to replication of the DNA several times without nuclear division. The duplication of DNA results in new chromatids which are not separated but remain within the same cell. This is called endo reduplication or endomitosis – chromosome duplication without cell division.

The resulting daughter chromatids do not separate but remain aligned side by side. For unknown reasons, the centromere regions of the chromosomes do not endoreplicate very well. As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter.

The number of replications rounds can lead to 256,512 or even 1024 strands existing side by side in a single chromosome.5. Because of presence of many strands or chromonemata  they were named as Polytene chromosomes by Kollar

These metabolically active tissues grow by an increase in the size of their constituent cells rather the increasing their cell numbers. This process generates giant cells whose volumes are thousands of times greater than normal cells. 

The development of giant cells is accompanied by successive rounds of replication

Another peculiar characteristic of the polytene chromosomes is that the maternal and paternal homologous chromosomes remain associated side by side. This phenomenon is called somatic pairing.

The polytene chromosomes shows transversal banding pattern (similar to a bar code) along the chromosome that can be easily observed in light microscopy. It consists of alternating darker and lighter chromosomal segments both in light and electron microscopy. It could be shown that the darker segments are represented by chromatin that is packaged relatively dense. While the lighter segments or interbands are made up by chromatin that is packed much more loosely. And this region shows intense gene transcription.

At particular developmental stage, the DNA at certain points uncoils to produce swellings called puffs or Balbiani rings. The DNA in the puff regions unfolds into open loops due to intense transcription and produces mRNA.

Some puffs are always found at the same chromosomal position. But, majority of puffs, however, are visible only at certain times of the development of an organism and they appear at different positions on the polytene chromosomes. This is explained by the fact that at different times during the development different genes are active. Furthermore, the activity of gene can be different from one tissue to another tissue.

Polytene chromosomes are well suited for studies of nucleic acid synthesis, gene action.  Polytene chromosomes provided the first evidence that eukaryotic gene activity is regulated at the level of RNA synthesis. The effects of deletion, addition, inversion, etc., of chromosomal segments or genes have been studied in detail due to occurrence of distinct bands and interbands in the polytene chromosomes. 

Lampbrush Chromosomes

In diplotene stage of the meiosis, the yolk rich ooctyes of many vebrates – fishes, amphibians, reptiles & birds contain exceptionally large-sized chromosomes

Which had an appearance of lampbrush or test tube brush. 

First observed by Flemming (1889) in amphibian oocyte

A detailed study of themws made  by J. Ruckert (1892) in oocyte of Shark. He coined the named lampbrush, because they appear as lamp cleaning brush.

Size: in many animals lampbrush chromosomes may be more than 1000 microns in length and 20 microns in width. In some salamander oocytes the lampbrush chromosomes may reach a length of 5900 microns

Some lampbrush chromosomes are 3 times longer than the polytene chromosomes. 

Observed during prolonged diplotene stage, these are in form of bivalents

Structure: During diplotene, the homologous chromosomes begin to separate from each other and are held together at chiasmata. Under light microscope each chromosome is seen to consists of an axis along which is a row of dense granules or chromomeres. From each chromomere arises a pair of lateral loop.

Chromosome axis

Each chromosome of the pair of homologous chromosomes consists of two chromatids, which are represented by axial filaments. Thus, the pair of homologous chromosomes has four filaments in all. 

Chromomeres:

At certain points along their length the axis become tightly coiled. These points are the chromomeres. The chromomeres are found in pairs. They perhaps correspond to heterochromatin and transcriptionally inactive. 

The axial filaments and the chromomere consists of DNA.

 Loops:

From each chromomere arises a pair of lateral loop. The loops represent the lateral extension of the axial filaments

Each loop consists of an axial fibre which is covered by a matrix. When the loop is treated with Deoxyribonucleases ( an enzyme which breaks down DNA) it is broken down, indicating that it consists of DNA. When treated with ribonuclease ( an enzyme which breaks down RNA), trypsin and pepsin, the matrix of the loop is removed. From this it can be concluded that the matrix consists of RNA and Protein

They are of two main types- typical and special. Most of the loops are typical. Each typical loop consists of central axis from which are given off RNA fibrils of progressively increasing lengths. This makes the loop more     thicker on one side. The special loops have a marked asymmetry and have granules at the end of the fibrils.

Electron microscope studies by Miller andBeaty (1969) on the lampbrush chromosomes of the oocytes of the Salamander trityrus show the presence of dense granules on the DNA axial fibre. These granules are probably large molecules of the enzyme RNA polymerase, which synthesizes RNA. Arising from these RNA polymerase molecules are fine fibrils of RNA-protein (ribonucleoproteins).

Further, there is evidence that the chromosomal loop continuously spins out from the chromomere at one end, while it recoils into another end. Callan and Lyod have suggested that each pair of loop is associated with the activity of specific gene. Each loop is supposed to contain repeating DNA sequences – gene arranged in series. At each chromomere is supposed to be a master ‘ gene’ copy which transfers information to several ‘slave gene’ copies on the same loop (master and slave hypothesis). Only the slave gene take part in RNA synthesis, but not the master gene. 

The intense period of transcription lasts for many day in some animals; then the RNA is given off and loop collapses into the main body of the chromosome. Most of the RNA formed in the oocyte is used to form ribosomes i.e., it is rRNA. In addition,some mRNA is synthesized this mRNA is stored in the cytoplasm and is used later in embryogenesis.

LC assist in fulfilling the high demand for transcripts during oogenesis. Studies on oogenesis in vertebrates reveal that extensive transcription occur during prophase I diplotene of meiosis. The loops of lampbrush chromosomes have been shown to be regions of active transcription, whose products are required during early stages of embryogenesis.

B Chromosomes:

The growth, development and reproduction of an organism relies on genetic material that is organsized into chromosomes. The chromosome complement carried by all members of a species is refereed to as the A chromosomes.

A subset of individuals within a species may also possess extra chromosomes that are non-essesntial and not member of the standard A chromosome set. These supernumerary chromosomes are commonly refereed to as B Chromosomes. First described in 1907, B chromosmes have now been identified in hundreds of species across many different taxa and it is estimated that B chromosomes may be present  in 15% of all eukaryotic species.

B chromosomes shows following characteristics:

Different morphology from that of the chromosomes belongining to the normal complement, usually of smaller size

Genetic constitution which does not strongly influence the individual – usually having an almost non-existent gentic action.

Numberical variability between different cells or different tissues within individuals or populations.

Abnormal behavior  at meiosis, - lack homology with the normal complement

Abnormal behavior at mitosis- chromatid non-disjunction at anaphase

Morphology

Generally, B chromosomes are noticeably smaller in dimension than the A chromosomes, and often have  more or less globular appearance. Particularly interesting thing is that the centromere is frequently in a terminal position –telocentric 

In majority  of cases the B chromosomes are distinctly heterochromatic and non-coding. But some B chromosomes of  maize contain sizeable euchromatin segments.

They are not essential to normal cellular function, and donot follow normal patterns of replication and segregation during cell division.

They are not homologous with any of the basic A-chromosomes. Their inheritance is non-mendelian. Some times due to non-disjunction, their number is increased in the progeny

Chromosomes

 Chromosomes

Karl Nageli in 1842 observed darkly stained thread like structures in the nucleus. In 1876, Balbiani described rod-like structures that were formed in the nucleus before cell division. In 1879 Walter Flemming used the word “Chromatin” (Gr. Chroma-colour) to describe the substance that stained intensely with basic dyes in interphase nuclei.

He suggested that the affinity of chromatin for basic dyes was due to their content of “nuclein”, a phosphorous –containing compound isolated from pus cells by Meischer in 1871, a substance that is now called DNA.

In 1888 Waldeyer used the word Chromosome of interphase nuclei and rod-like objects observed during mitosis.

Chromosomes are concerned with the transfer of genetic traits from one generation to next and hence referred to as ‘Physical basis of heredity’. Sutton and Boveri proposed the chromosome theory which states that chromosomes are the vehicles of hereditary.

Chromosome number

The number of chromosomes varies from species to species, but is constant for a given species. Therefore, they are of great importance in the determination of phylogeny and taxonomy of the species. 

The somatic cells contain two sets of chromosomes. This number is called diploid number which is represented by 2n. The gametes contain only one set of chromosomes. This number is called haploid number and it is represented by n. 

The lowest number of chromosomes is 2 (2n=2) found in roundworm Ascaris megacephala. The maximum number of chromosomes is 1700 found in Radiolarian (Aulacantha, protozoan).

Among plants, chromosome number varies from 2n=4 in Haplopappus gracilis (compositae) to 2n = 1200 in some pteridophytes. 

Size:

The size of mitotic metaphase chromosomes generally varies from 0.5µ to 30 µ in length and between 0.2 µ to 30 µ in diameter. Trillium contains the largest chromosomes that may attain 30-32 µ in length.

In general plants have longer chromosomes than animals. Species having lower chromosome number possess longer chromosomes than those having higher chromosome number.

Structure

Mitotic metaphase chromosomes are the most suitable for studies on chromosome morphology. Chromosomes shows 1. Chromatids, 2. Centromere, 3. Secondary constriction, 4. Satellite,   5. Telomere.

Chromatids:

The chromosome contains two identical, spirally coiled filaments or arms called chromatids. Each chromatid contains a single DNA molecule.

The two chromatids of a chromosome are held together at a point called centromere. The DNA of each telophase chromosome (composed of a single chromatid) replicates during the synthesis (S) phase of interphase. This produces an identical copy of the chromatid so that during prophase and metaphase the chromosome is made up of two chromatids.

Since the two chromatids of a prophase chromosome are produced through replication, they are referred to as sister chromatids. 

The two chromatids separate from each other during anaphase and move to opposite poles. As a consequence, each chromosome is represented by a single chromatid during telophase.

Centromere

The region where two sister chromatids of a chromosome are held together during mitotic metaphase is known as centromere. Under light microscope, the centromere generally appears as a constriction in the chromosome. Therefore, it is also termed as primary constriction.

Centromere divides the chromosome into two parts called arms. In most cases, one are of a chromosome is longer than the other, hence they are termed as long arm (q) and short arm (p), respectively.

There are two cup like discs in the centromere which are called kinetochores. They contain highly repetitive DNA with special proteins attached. 

Under electron microscope, the kinetochore shows a trilaminar structure. It shows a 10nm thick dense proteinaceous layer, a middle layer of low density and a dense inner layer tightly bound to the centromere. 

During mitosis, 4 to 40 microtubules of mitotic spindle become attached to the kinetochore.  Thus, they function to provide a centre of assembly for microtubules.i.e., a nucleation centre for polymerization of tubulin protein into microtubules. Thus,centromere is the region of attachment of microtubules of spindle fibres during cell division. As a result, centromeres are the first parts of chromosomes to be seen moving towards the opposite during anaphase.

The position of centromere varies from chromosome to chromosome and it provides different shapes to the chromosome. Based on the position of the centromere, the chromosomes are classified as follows:

1. Metacentric:

The centromere is located in the middle of the chromosome, forming two arms of equal length. During anaphase they appear in the shape of ‘V’.

2. Sub-metacentric:

The centromere is located slighted away from the middle of the chromosomes, forming two unequal arms. During anaphase they appear as J or L shaped.

3. Acrocentric:

The centromere is located close to one end of chromosome, forming two unequal arms in which one arm is very long and one arm is very short. 

4. Telocentric:

The centromere is located at the end of the arm. Only one arm is present. During anaphase they appear I shaped. 

The chromosomes of most organisms contain only one centromere and are known as monocentric chromosomes. In some chromosomal abnormalities, chromosomes break and fuse with other, producing chromosomes without centromere called acentric chromosomes or with two centromeres called dicentric chromosomes or with more than two centromeres called polycentric chromosomes. 

Sometimes a chromosome may undergo a break into two, so that only one part has the centromere while the other is without the centromere. The part lacking the centromere is called acentric chromosome. It doesnot take part in mitosis, as spindle fibre cannot be attached to it. It loss usually results in lethality.

Dicentric chromosomes with two chromosomes are sometimes produces, as for example during pairing of structurally different homologous chromosomes (produced as a result of translocation). If there two centromeres move to opposite poles during anaphase the chromosomes breaks. 

Some species have diffuse centromeres i.e  the centromere is not located in one position but lies in a diffused condition along the length of the chromosome, so that the  microtubules attached along the length of the 

Telomere:

The two ends of a chromosome are known as telomeres (Gr telo- far, meros- part). The individuality of each chromosome is related to the fact that it is terminated at either end by a telomere, a term coined by H.J. Muller to indicate the uniqueness and stability of this portion of the chromosome.

The telomeres show polarity which prevents the other segments of chromosomes joining with them. Thus, they provide stability to chromosomes and protect their individuality. If a chromosome breaks, the broken ends can fuse with each other due to lack of telomeres. 

The telomere is rich in G repeats. The replication of telomere is brought about by telomerase. The repeated division of chromosome during cell division results in shortening of telomere. The aging of man is said to be 

Secondary Constriction:

In addition to primary constriction, the arms of chromosome may show one or more additional constrictions called the secondary constrictions. They can be distinguished from the primary constriction by the absence of bending of chromosomes during the anaphase.

Satellite:

The chromosome region lying between the secondary constriction and telomere is known as satellite. Chromosomes bearing secondary constriction and satellite are called as SAT chromosomes. The prefix SAT – stands for Sine Acid Thymonucleinico – (without thymonucleic acid or DNA). 

Nucleolus is always associated with the secondary constriction of sat-chromosomes. Therefore, secondary constrictions are also called Nucleolus Organizer Regions (NOR). NOR contains several hundred copies of the gene coding for ribosomal RNA (rRNA).

Heterochromatin and Euchromatin

When a chromosome in a higher organism is observed during the mitotic cycle under light microscope, with conventional fixation and staining procedures, different parts of it show different condensation and therefore different staining cycles.

In 1928, Heitz defined heterochromatin as those regions of chromosomes that remain condensed during interphase and early prophase and form the so-called chromocentres. The rest of the chromosome, which is in a non-condensed state, was called euchromatin.

The heterochromatin regions can be visualized in condensed chromosomes as regions that stain more strongly or more weakly than the euchromatin regions, showing what is called a positive or a negative heteropyknosis of the chromosomes (hetero – different, pyknosis – staining)

It is thought that in heterochromatin DNA remains tightly packed in the 30nm fiber, which probably represents the configuration of transcriptionally inactive chromatin.

Two types of heterochromatin are generally recognized: constitutive heterochromatin, which is permanently condensed in all types of cells, it is present in the same position on two homologous chromosomes and is inherited.

Facultative heterochromatin develops later during the course of development of the organism and is condensed only in certain cell types or at special stages of development. Constitutive heterochromatin in both paternal and maternal chromosomes responds directly and similarly to development since it is inherited.

Facultative heterochromatin is related to development but it occurs in one of two genetically alike complements so that two chromosomes behave differently. Its significance is due to its capacity for shutting off normal gene function for relatively longer or shorter periods during the lifetime of an organism.

Euchromatin: portion of chromosomes that stain lightly and are only partially condensed

Nucleosome- Solenoid Model

Proposed by Korneber and Thomas in 1974

The length of the DNA is far greater than the size of the nucleus in which it is contained. To fit into the nucleus, the DNA has to be condensed in some manner. 

When eukaryotic nuclei were observed on EM, it was found that chromatin has a repeating structure of beads about 10 nm in diameter connected by a string of DNA. 

The existence of a repeating unit of chromatin – called nucleosome- was predicted from the biochemical studies. 

It was found that the DNA molecule in a eukaryotic chromosome is not free but is bound to basic proteins called the histones, in a structure called chromatin. Histones are small proteins that are basic because they have a high content ( 10 to 20%) of the basic amino acids arginine and lysine There are five major classes of histones – H1, H2A, H2B, H3 and H4. 

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The DNA-histone complex is the basic unit of structure in the chromosome. The two components are present roughly in equal amounts by weight, and they generally account for  60-90% of the chromatin mass. Both are formed at the same stage of cell division – S (synthesis).

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DNA is a polyanion by virtue of the continuous sequence of acidic phosphate (PO4--) groups, and these are neutralized and stabilized by the histones, which because of their basic nature act as polycations. Being basic, histones bind tightly to DNA, which is acid

By electron microscopy chromatin looks like a regularly beaded thread. The bead units in chromatin are called nucleosomes. It is an oblate particle about 50b-55 A0 high and 110 A0 in diameter. 

Each unit has a definite composition – namely, one molecule of H1, two molecules each of H2A, H2B, H3 and H4, and one segment of DNA containing about 200 nucleotides pairs. Prolonged treatment of the nucleosomes with the nuclease removes some of the DNA and causes loss of H1.

The resulting structure, called the core particle, consists of an octomer of pairs of H2A, H2B, H3 and H4

, around which the remaining 145 base pair length of DNA is wound in about 13/4 turns. Thus, histones are located internally in the beads, with the DNA wrapped around the outside of the clustered proteins.

Thus, a nucleosome consists of a core particle and linker DNA to which H1 is attached. Possibly, H1 also binds to adjacent core particles to make a more compact structure. The H1 is not located internally within beads; it is external in the inter bead stretches of DNA, and it may be added after the beaded structure is formed.

DNA lining the H1 particle is termed as linker DNA. Its length is 15 – 100 bp, depending on the cell type. The linker DNA is coiled or folded in the normal state of chromatin.

Assembly of DNA and histones is first stage -A sevenfold reduction in the length of DNA.

The beaded flexible fibre of 11nm wide is roughly five times the width of free DNA

The second level folding is the shortening of 11 nm fibre to form a solenoidal supercoil with six nucleosome per turn, called the 30 nm fibre – the supercoiled nucleosome is called solenoid.

The further folding of the 30 nm fibre is less well understood.

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Chromatin can be isolated from nuclei in various stages of cellular activity, either in an extended state or visibly compacted as chromosome.

In extended state, the chromatin exists in the form of long strands having a diameter ranging from 10nm to 100nm

As a cell passes through its growth cycle, the structure of its chromatin changes. In a resting cell the chromatin is dispersed and fills the entire nucleus. Later, after DNA replication, the chromatin condenses about 100-fold and chromosomes form. 

Functions

The main function of chromosomes is to carry the genetic information from one cell generation to another.

Another important function is to protect the genetic material i.e., DNA from being damaged during cell division.

Gene action in eukaryotes is regulated through the histones and non-histone proteins associated with chromosomes.

The specialized structures of telomeres prevents end to end interactions between chromosomes. This ensures stability of the different chromosomes.

Karyotype

The sequential arrangement of all chromosomes of the individual is called karyotype. A karyotype describes the chromosomes of a cell in terms of Number, their relative size, position of centromere, length of the arms, secondary constrictions and satellites.

A diagrammatic representation of a karyotype of a species – idiogram (Gr. Idios = distinctive, gramma –something written)

Genrally, in an ideogram, the homologous chromosomes are arranged in a series of decreasing size. That is, largest chromosome is placed in the first position and the smallest one is placed at last. The sex chromosomes are usually placed in their appropriate positions according to their size and are marked as X and Y. 

Karyotypes suggest primitive or advanced nature of an organism.

Heterochromatin

The darkly stained condensed region of the chromatin

Characterized by high content of repetitive DNA sequences and very few structural genes

DNA is packed in 30 nm fibre

Constitutive heterochromatin :  DNA is permanently inactive and remains condensed state throughout the cell cycle. – around centromere, in telomere

Facultative: it is essentially euchromatin that has undergone heterochromatization

Euchromatin:

Portion of chromosomes that stain lightly are only partially condensed

Nucleus

 Nucleus ( Latin nuculeus – kernel or nut)

Nucleus is a membrane bound organelle containing chromosomes and nucleolus. The nucleus is the brain of eukaryotic cells.

Robert Brown in 1833 discovered a prominent body within the cell and termed it nucleus. Van Hammerting’s experiment (1934) showed the role of the nucleus in controlling the shape and features of the cell.

Usually, the nucleus is round and is the largest organelle in the cell. The nucleus varies in diameter from 11 to 22 micrometers. The nucleus typically occupies about 10% of a eukaryotic cell’s volume, making it one of the cell’s most prominent features.

Generally, there is only one nucleus per cell, but nucleus is absent in RBC’s of Human beings and sieve cells of plants.

Some organisms like paramecium  and tapetal cells has two nucleus and this condition is called as binucleate. Some organisms like slime moulds (Mucor), siphonales (Vaucheria) have more than two nucleus, this condition is called multinucleate. The multinucleate animal cells are called syncytial cells (e.g., epidermal cells of Ascaris) and the multinucleate plant cells are called coenocytic.

The shape of the nucleus varies considerably. In most of the cells, it is spherical in shape. It may be cylindrical or elliptical.

Structure:

The nucleus is comprised of the following structures:

1. The nuclear membrane or karyotheca              2. The nuclear sap or nucleoplasm, 

3. Chromatin fibres   and                                     4. The nucleolus

Nuclear Envelope

The nucleus is enclosed by a double membrane called the nuclear envelope or nuclear membrane or karyotheca. The nuclear envelope seperates the nuceloplasm from the cytoplasm. Each membrane is about 75 to 90 Ao thick and lipoproteinous in nature. The two membranes remain separated by a space of 100 - 150 Ao.  The space between two membranes is called as perinuclear space.

The outer membrane is continuous with rough endoplasmic reticulum and is studded with ribosomes. The inner membrane is linked by firbrous filament protein called lamins, which forms a scaffold of filaments called the nuclear lamina.

Nuclear Pore

The nuclear envelope is perforated with thousands of pores called the nuclear pore. The nuclear pore were first demonstrated by Callan and Tomlin (1950) in amphibian oocytes.

The number of pores per unit area of nuclear envelope varies with the cell type and with the physiological state of the cell. The nuclear pores are circular in surface view and have an diameter between 10 – 100 nm.

The nuclear pore has a complex organisation. So the entire structure of nuclear pore is called nuclear pore complex. The pore complex consists of two rings or annuli with an inside diameter of 80nm

. A large particle is present in the nuclear pore forming the central plug.

Eight radial subunits or spokes extend from the inner and outer membrane where they join. Actually, they form a ring of subunits 15-20 nm in diameter. Each sub-units project a spoke-like unit into the centre, so that the pore looks like a wheel with 8 spokes from the top.

The nuclear pore forms an aqueous channel connecting the cystol with the interior of the nucleus. When materials are to be transported through the pore, it opens up to form a channel some 25nm wide, that is large enough to get such large assemblies as ribosomal subunits through. The nuclear pore complexes are active; that its, it requires energy.

All proteins synthesized in cytoplasm and those needed by the nucleus must be imported into it through the NCPs, these includes histones, ribosomal proteins, transcription factors, splicing factors etc.

Molecules and macromolecular assemblies like ribosomal subunits, mRNA, tRNA are exported from the nucleus through the NCPs.

Function of  Nuclear Envelope

The Nuclear envelope regulates and facilitates transport between the nucleus and the cytoplasm.

It serves to separate the genetic component (chromosomes) from the protein synthesising machinery (ribosomal and ER). It provides protection to DNA against mutagenic effects of cytoplasmic enzymes.

The nuclear envelope provides a surface for the attachment of structural elements of the cytoplasm i.e., microtubules and microfilaments.

The attachment of chromosomes to nuclear envelope assists in their condensation and separation during cell division.

Envelope is involved in the formation of ER, Golgi complex.

Nucleoplasm:

The nucleus is filled with a homogenous, transparent, acidophilic fluid called the nucleoplasm or nuclear sap or karyolymph.

The nuclear components such as chromatin threads and the nucleolus remain suspended in the nucleoplasm.

The nucleoplasm is composed of nucleic acids, proteins, enzymes and minerals. The most important nuclear enzymes are the DNA polymerase, RNA polymerase, NAD synthetase, alkaline phosphatise.

In nucleoplasm events like – i. Replication of DNA, ii. Transcription of RNA, iii. Transport of materials etc occurs.

Nucleolus:

The nucleolus (- a small nucleus) is a large, spherical, ball like body found in nucleus. It was discovered by Fontana. In cell biology, nucleolus is a ‘suborganelle’of the cell nucleus, which is an organelle.

The size of the nucleolus is found to be related with the synthetic activity of the cell. Therefore, the cells which synthesize the proteins contain comparatively large-sized nucleoli, eg., Oocytes, neurons and secretory cells.

No membrane seperates the nucleolus from the nucleoplasm. Calcium ions are supposed to maintain its intact organisation. Nucleoli are made of protein and ribosomal DNA (rDNA) sequences of chromosomes. The rDNA is a fundamental component since it serves as the template for transcription of the ribosomal RNA (rRNA) for inclusion into new ribosomes.

Nucleoli disappear during metaphase of cell division. After daughter cells complete separation, nucleoli reform around the nucleolus organizer regions (NORs) of the chromosomes.

During the period between cell division, when the chromosomes are in their extended state, 1 or more chromosomes have loops extending into a spherical mass called the nucleolus. The nucleolus is organized from the ‘nucleolar organising regions’on different chromosomes. Nucleoli are formed around the DNA loop that extends from the nucleolar organizer.  A number of chromosomes get together and transcribe ribosomal RNA at this site. Here three kinds of ribosomal RNA molecules (28S, 18S, 5.8S) used in the assembly of the large and small subunits of ribosomes are synthesized. 

28S, 18S and 5.8S ribosomal RNA is transcribed by RNA polymerase I from hundred of tandemly-arranged rDNA genes distributed on different chromosomes. The rDNA-containing regions of these chromosomes cluster together in the nucleolus.

The nucleolus that is active in the synthesis of ribosomes typically exhibits three regions under electron microscope.

1. Fibrillar Centre (FC):

This pale-staining part represents the innermost region of the nucleolus. It is made up by a network of fine (4-5 nm thick) fibrils. These fibrils are formed of rDNA, rRNA and ribonucleoprotein particles(RNP).

2. Granular component (GC):

This is the outermost region of the nucleolus where processing and maturation of pre-ribosomal particles occur.

. 3. Amorphous matrix: this is the proteinous ground substance in which granules and fibrils remain suspended.

Chromatin fibres

the nucleoplasm  contains  thread like, coiled and much elongated structures called the chromatin fibres. Chromatin fibres are observed only in the interphase nucleus. During cell division chromatin fibres become thick and ribbon-like structures known as the chromosomes.

 

Chemically chromatin is a nucleoprotein. It is composed of nucleic acid and protein.

Function of Nucleus

Nucleus is the dynamic centre which controls and regulates various metabolic activities like growth, protein synthesis, metabolism, reproduction.

Vehicle for transmission of hereditary characters through chromosomes during cell division.

The synthesis of ribosomal RNA takes place inside the nucleolus