Chromosomal organisation in Prokaryotes and Eukaryotes

 The chromosomes are the nuclear components of special organisation, individuality and function.
 They are capable of self-reproduction and play a vital role in heredity, mutation, variation and evolutionary development of the species.
 Karl Nagli (1842) observed rod-like chromosomes in the nuclei of plant cells.
 E. Russow (1872) made the first serious attempt to describe chromosomes.
 E. Strasburger (1875) discovered thread-like structures which appeared during cell division.
 Walter Flemming (1878) introduced the term chromatin to describe the thread-like material of the nucleus that became intensely coloured after staining.
 W. Roux (1883) suspected the involvement of the chromosomes in the mechanism of inheritance.
 Benden and Bovery (1887) reported that number of chromosomes for each species was constant.
 The present name chromosome (Gr., chrom=colour, soma=body) was coined by W. Waldeyer (1888) to darkly stained bodies of nucleus.
 W. S. Sutton and T. Boveri in 1902 suggested that chromosomes were the physical structures which acted as messengers of
 In 1914, Robert Feulgen demonstrated a colour test known as Feulgen reaction for the DNA. In 1924, he showed that chromosomes contain DNA.

Prokaryotic genome
Bacterial DNA:
 In bacteria and blue-green algae, the hereditary material is organised into a single large circular chromosome composed of a circular molecule of double stranded DNA. It is known as bacterial chromosome or nucleoid.
 It lies free in the cytoplasm in the nuclear zone and has no protein around the DNA molecule, as is found in eukaryotic chromosome.
 Some RNA is found associated with it and forms backbone or core. DNA molecule is supercoiled.
 The circular chromosome of Escherichia coli has a contour length of about 1,360mµ and is about 20Ao broad.
 Its molecular weight is about 2.8×109. It has about 50 or more highly twisted or supercoiled loops and about four million nucleotide pairs.
 In addition to the single large circular chromosome, each bacterial cell contains from 1 to 20 much smaller circular duplex DNA molecules which are called plasmids.
 Some of the plasmids become incorporated into the bacterial chromosome and are called episomes.
 These are sometimes transferred from one bacterial cell to another during conjugation and give them new characteristics








EUKARYOTIC CHROMOSOMES
MORPHOLOGY:
Size:
 The size of chromosome is normally measured at mitotic metaphase and may be as short as 0.25 μm in fungi and birds, or as long as 30 μm in some plants such as Trillium.
 However, most metaphase chromosomes fall within a range of 3μm in fruitfly (Drosophila), to 5μm in man and 8μm to 12μm in maize.
 The organisms with less number of chromosome contain comparatively large-sized chromosomes than the chromosomes of the organisms having many chromosomes.
Number:
 The number of the chromosomes is constant for a particular species. Therefore, these are of great importance in the determination of the phylogeny and taxonomy of the species.
 The number or set of the chromosomes of the gametic cells such as sperms and ova is known as the gametic, reduced or haploid sets of chromosomes.
 The haploid set of the chromosomes is also known as the genome.
 The somatic or body cells of most organisms contain two haploid set or genomes and are knows as the diploid cells.
 The diploid cells achieve the diploid set of the chromosomes by the union of the haploid male and female gametes in the sexual reproduction.
 The number of chromosomes in each somatic cell is the same for all members of a given species.
 The organism with the lowest number of the chromosomes is the nematode, Ascaris megalocephalus univalens which has only two chromosomes in the somatic cells (i.e., 2n =2).
 In the radiolarian protozoan Aulacantha is found a diploid number of approximately 1600 chromosomes.
Autosomes and Sex chromosomes
 In a diploid cell, there are two of each kind of chromosome (these are termed homologous chromosomes), except for the sex chromosomes.
 One sex has two of the same kind of sex chromosome and the other has one of each kind.
 For example, in human, there are 23 pairs of homologous chromosomes (i.e., 2n = 46 ; a chromosome number which was established by Tijo and Levan in 1956).
 The human female has 44 non-sex chromosomes, termed autosomes and one pair of homomorphic (morphologically similar) sex chromosomes given the designation XX.
 The human male has 44 autosomes and one pair of heteromorphic or morphologically dissimilar sex chromosomes, i.e., one X chromosome and one Y chromosome.

Shape:
 The shape of the chromosomes is changeable from phase to phase in the continuous process of the cell growth and cell division.
 In the resting phase or interphase stage of the cell, the chromosomes occur in the form of thin, coiled, elastic and contractile, thread-like stainable structures, the chromatin threads.
 In the metaphase and the anaphase, the chromosomes become thick and filamentous. Each chromosome contains a clear zone known as centromere or kinetocore, along their length.
 The centromere divides the chromosomes into two parts, each part is called chromosome arm. The position of centromere varies form chromosome to chromosome and it provides different shapes.
 Telocentric. The rod-like chromosomes which have the centromere on the proximal end are known as the telocentric chromosomes.
 2. Acrocentric. The acrocentric chromosomes are also rod-like in shape but these have the centromere at one end and thus giving a very short arm and an exceptionally long arm. The locusts (Acrididae) have the acrocentirc chromosomes
 3. Submetacentric. The submetacentric chromosomes are J- or L-shaped. In these, the centromere occurs near the centre or at medium portion of the chromosome and thus forming two unequal arms.
 4. Metacentric. The metacentric chromosomes are V-shaped and in these chromosomes the centromere occurs in the centre and forming two equal arms. The amphibians have metacentric chromosomes.










Structure:
While describing the structure of the chromosomes during various phases of cell cycle, cell biologists have introduced many terms for their various components.
1. Chromatid. At mitotic metaphase each chromosome consists of two symmetrical structures, called chromatids. Each chromatid contains a single DNA molecule.
 Both chromatids are attached to each other only by the centromere and become separated at the beginning of anaphase, when the sister chromatids of a chromosome migrate to the opposite poles.
2. Chromonema (ta). During mitotic prophase the chromosomal material becomes visible as very thin filaments, called chromonemata (a term coined by Vejdovsky in 1912).
 A chromonema represents a chromatid in the early stages of condensation. Therefore, ‘chromatid’ and ‘chromonema’ are two names for the same structure : a single linear DNA molecule with its associated proteins. The chromonemata form the gene-bearing portions of the chromosomes.
3. Chromomeres. The chromomeres are bead-like accumulations of chromatin material that are sometimes visible along interphase chromosomes.
 The chromomere-bearing chromatin has an appearance of a necklace in which several beads occur on a string.
 Chromomeres become especially clear in the polytene chromosomes, where they become aligned side by side, constituting the chromosome beads.
 At metaphase the chromosomes are tightly coiled and the chromomeres are no longer visible.








4. Centromere and kinetochore. centromere is the region of the chromosome to which are attached the fibres of mitotic spindle.
 The centromere lies within a thinner segment of chromosome, the primary constriction. The regions of chromosome flanking the centromere contain highly repetitive DNA and may stain more intensely with the basic dyes.
 Centromeres are found to contain specific DNA sequences with special proteins bound to them, forming a disc-shaped structure, called kinetochore




 Under the EM, the kinetochore appears as a plate - or cup-like disc, 0.20 to 0.25 nm, in diameter situated upon the primary constriction or centromere.
 In thin electron microscopic sections, the kinetochore shows a trilaminar structure, i.e., a 10 nm thick dense outer proteinaceous layer, a middle layer of low density and a dense inner layer tightly bound to the centromere
 The DNA of centromere does not exist in the form of nucleosome
 During mitosis, 4 to 40 microtubules of mitotic spindle become attached to the kinetochore and provide the force for chromosomal movement during anaphase.
 The main function of the kinetochore is to provide a centre of assembly for microtubules, i.e., it serves as a nucleation centre for the polymerization of tubulin protein into microtubules
 The chromosomes of most organisms contain only one centromere and are known as monocentric chromosomes.
 Some species have diffuse centromeres, with microtubules attached along the length of the chromosome, which are called holocentric chromosomes.
 In some chromosomal abnormality (induced for example by X-rays), chromosomes may break and fuse with other, producing chromosomes without centromere (acentric chromosomes) or with two centromeres (dicentric chromosomes).
 Both types of these chromosomal aberrations are unstable. The acentric chromosomes cannot attach to the mitotic spindle and remain in the cytoplasm. The dicentric chromosomes lead to fragmentation, since, two centromeres tend to migrate to opposite poles.
 5. Telomere. (Gr., telo=for; meros=part). Each extremity of the chromosome has a polarity and therefore, it prevents other chromosomal segments to be fused with it. The chromosomal ends are known as the telomeres. If a chromosome breaks, the broken ends can fuse with each other due to lack of telomeres.
6. Secondary constriction. The chromosomes besides having the primary constriction or the centromere possess secondary constriction at any point of the chromosome.
 Constant in their position and extent, these constrictions are useful in identifying particular chromosomes in a set.
 Secondary constrictions can be distinguished from primary constriction or centromere, because chromosome bends (or exhibits angular deviation) only at the position of centromere during anaphase.
 7. Nucleolar organizers. These areas are certain secondary constrictions that contain the genes coding for 5.8S, 18S and 28S ribosomal RNA and that induce the formation of nucleoli.
 The secondary constriction may arise because the rRNA genes are transcribed very actively and, thus, interfering with chromosomal condensation.
 In human beings, the nucleolar organizers are located in the secondary constrictions of chromosomes 13, 14, 15, 21 and 22, all of which are acrocentric and have satellites.
 8. Satellite. Sometimes the chromosomes bear round elongated or knob-like appendages known as satellites.
 The satellite remains connected with the rest of the chromosome by a thin chromatin filament.
 The chromosomes with the satellite are designated as the sat chromosomes. The shape and size of the satellite remain constant.
KARYOTYPE AND IDIOGRAM
 All the members of a species of a plant or the animal are characterized by a set of chromosomes which have certain constant characteristics.
 These characteristics include the number of chromosomes, their relative size, position of the centromere, length of the arms, secondary constrictions and satellites.
 The term karyotype has been given to the group of characteristics that identifies a particular set of chromosomes.
 A diagrammatic representation of a karyotype (or morphological characteristics of the chromosomes) of a species is called idiogram.
 Generally, in an idiogram, the chromosomes of a haploid set of an organism are ordered in a series of decreasing size.
 Sometimes an idiogram is prepared for the diploid set of chromosomes, in which the pairs of homologues are ordered in a series of decreasing size.







MATERIAL OF THE CHROMOSOMES
 The material of the chromosomes is the chromatin. Depending on their staining properties, the following two types of chromatin may be distinguished in the interphase nucleus :
1. Euchromatin. Portions of chromosomes that stain lightly are only partially condensed; this chromatin is termed euchromatin.
 It represents most of the chromatin that disperse after mitosis has completed. Euchromatin contains structural genes which replicate and transcribe during G1 and Sphase of interphase.
 The euchromatin is considered genetically active chromatin, since it has a role in the phenotype expression of the genes.
2. Heterochromatin. In the dark-staining regions, the chromatin remains in the condensed state and is called heterochromatin.
 In 1928, Heitz defined heterochromatin as those regions of the chromosome that remain condensed during interphase and early prophase and form the so-called chromocentre.
 Heterochromatin is characterized by its especially high content of repetitive DNA sequences and contains very few, if any, structural genes (i.e., genes that encode proteins).
CHEMICAL COMPOSITION
 Chromatin which has been isolated from rat liver contains DNA, RNA and protein. The proteinof chromatin is of two types : the histones and the non-histones. Rat liver chromatin has been used as a model for chromatin.
1. DNA
 DNA is the most important chemical component of chromatin, since it plays the central role of controlling heredity. The most convenient measurement of DNA is picogram (10-12 gm).
2. Histones
 Histones are very basic proteins, basic because they are enriched in the amino acids arginine and lysine to a level of about 24 mole present. There are five types of histones in the eukaryotic chromosomes, namely H1, H2A, H2B, H3 and H4.
 Histones determining the structure of chromatin play a regulatory role in the repression activity of genes.