Structure of Chromosome

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Structure of Chromosome

Strasburger in 1875 discovered thread-like structures which appeared during cell division. These thread-like structures were called chromosomes (chroma = colour) due to their affinity for basic dyes. In all types of higher organisms (eukaryote), the well organized nucleus contains a number of chromosomes of definite size and shape. These chromosomes are invisible in the nucleus but can be easily seen during cell division, whether mitosis or meiosis. At leptotene stage of meiotic prophase, chromosomes appear as beaded structures, bead-like nodules being known as chromomeres. Size of chromomeres and that of interchromomeric regions are not constant, so that every leptotene has its own particular pattern. The DNA is though known to concentrate in the chromomeres, but is believed to be present in the inter-chromomeric regions also. A diploid nucleus has two chromosomes of each type. Two similar chromosomes, are known as homologous chromosomes, which come in contact at zygotene and pair lengthwise throughout their length.

Number, Size and Shape of Chromosomes

Chromosome number varies from 2n = 4 (n=2) in haplopappus gracilis (Compositae) to 2n=>1200 in some pteridophytes. In Aulacantha a radiolarian, a diploid number of approximately 1600 chromosomes is found (consult Swanson, Cytology and Cytogenetics). While ‘n’ normally significs the gametic or haploid chromosome number, 2n’ is the somatic or diploid chromosome number in an individual. In polyploid individuals, however, it becomes necessary to establish an ancestral primitive number, which is represented as ‘x’ and is called the base number. For instance, in common wheat (Triticum aestivum), 2n = 42; n = 21 and x = 7, showing that common wheat is a hexaploid (2n = 6x).

A chromosome is normally measured at mitotic metaphase and may be as short as 0.25μ (u .001 mm) in fungi and birds, to as long as 30μ in some plants like Trillium. As a rule, most metaphase chromosomes fall within a range of 3μ in fruitfly (Drosophila), 5u in man and 8-12μ in maize.

Chromosome shape is usually observed at anaphase, when the position of primary constriction or what is also called centromere determines chromosome shape. This constriction or centromere can be terminal, sub-terminal or median in position. A terminal centromere will give rod shaped chromosome, a sub-terminal one would give J-shaped and a median centromere would give a V-shaped chromosome.

Morphology of Chromosomes

Corresponding to different positions of centromere, chromosomes would be called:

acrocentric or telocentric, having terminal centromere,
submetacentric having sub-terminal centromere and
metacentric having median centromere.

Besides centromere, which produces a primary constriction in chromosomes, secondary constrictions can also be observed in some chromosomes. Such a secondary constriction if present in the distal region of an arm would pinch off a small fragment called trabant or satellite. The satellite remains attached to rest of the body of chromosome by a thread of chromatin. Secondary constrictions may be found in other regions also and are constant in their position, so that these constrictions can be used as useful markers. Secondary constrictions can be distinguished from primary constriction or centromere, because chromosome bends or shows angular deviation only at the position of centromere. Chromosomes, each having a satellite, are marker chromosomes and are called SAT-chromosomes. The chromosome extremities or terminal regions on either side are called telomeres. If a chromosome breaks, the broken ends can fuse due to lack of telomeres. A chromosome, however, cannot fuse at the telomeric ends, suggesting that a telomere has a polarity which prevents other segments from joining with it. Telomeres have been studied in great detail at the molecular level in recent years.

Detailed study of chromosome morphology reveals a coiled filament throughout the length of chromosome. This filament is called chromonema (Vejdovsky, 1912). The chromonemata from the gene-bearing portions of the chromosomes. The chromonemata are embedded in the achromatic substance known as matrix. Matrix is enclosed in a sheath or pellicle. Both matrix and sheath are non-genetic materials and appear only at metaphase when the nucleolus disappears. It is believed that nucleolar material and matrix are interchangeable i.e., when matrix disappears, nucleolus appears and vice versa. A distinction also needs to be made between chromonema and chromatid. While a chromatid is a half chromosome (two chromatids remain connected at the centromere), the chromonema is a structure which is of a sub-chromatid nature and there can be more than one chromonemata in a chromatid.

Karyotypes

A group of plants or animals comprising a species is characterised by a set of chromosomes, which have certain constant features. These features include chromosome number, size and shape of individual chromosomes and other attributes listed above. The term karyotype is given to the group of characteristics that identifies a particular chromosome set and is usually represented by a diagram called idiogram, where chromosomes of haploid set of an organism are ordered in a series of decreasing size. The karyotypes of different groups are sometimes compared and similarities in karyotypes are presumed to represent evolutionary relationships.

Karyotype also suggests primitive or advanced feature of an organism. A karyotype showing large differences between smallest and largest chromosome of the set and having fewer metacentric chromosomes, is called asymmetric karyotype, which is considered to be a relatively advanced feature when compared with symmetric karyotypes. In 1931 G.A. Levitzky, a Russian scientist suggested that in flowering plants there is a predominant trend towards karyotype asymmetry. This trend has been carefully studied in the genus Crepis of the family Compositae. In several cases it was shown that increased karyotype asymmetry was associated with specialised zygomorphic flowers.

The above technique of karyotype preparation does not allow identification of chromosomes with similar morphological attributes (e.g. size of chromosome, position of centromere, etc.). The development of chromosome banding techniques during late 1960s and early 1970s proved very useful for the preparation of karyotypes in which linear differentiation due to distinct banding patterns allow identification of chromosomes that are similar in morphology. The banding technique allow detection of GC or AT rich regions or the regions with repetitive DNA. A variety of different kinds of bands (e.g., Q. C, G or R bands) have been used in animal materials. Similarly, Giemsa C bands and N bands have been utilized in plant systems.

In situ hybridization (ISH) using DNA probes, labelled with radioactive or non-radioactive molecules, can also be used to locate the position of specific DNA sequences on chromosomes. DNA may also be labelled with fluorochromes to allow the use of one or more colours to locate the positions of one or more DNA sequences simultaneously on the same chromosome. These techniques are described as fluorescence in situ hybridization (FISH) and multicolour fluorescence in situ hybridization (McFISH). They have been extensively utilized in animals (including humans) and plants in recent years.

Karyotypes can also be prepared by a relatively recent technique called flow cytometry. In this technique a suspension of many thousands of chromosomes is made and the suspended chromosomes are stained with a DNA binding fluorochrome. As these chromosomes pass through cytometer, the fluorescence in measured for individual chromosomes giving a histogram, each peak in this histogram representing one chromosome or a group of chromosomes of same size. The technique allows detection of differences as small as 1.5 to 4.0 Mb (megabase = 106 bases), so that aneuploidy or duplications/ deletions can be easily detected. Karyotypes of many plant species like tomato, faba been wheat have actually been prepared using the technique of flow cytometry.

Euchromatin and Heterochromatin

When chromosomes are stained with stains like acetocarmine or feulgen (basic fuchsin) at prophase, a linear differentiation into regions having dark stain and those having light stain becomes conspicuous. In 1930’s and 1940’s Emil Heitz and other cytologists studied this aspect. The darkly stained regions were called heterochromatic and light regions were called euchromatic. Heterochromatic regions are constituted into three structures namely chromemeres, chromocentres and knobs.

Chromomeres are regular features of all prophase chromosomes, large enough to reveal them, but their number, size, distribution and arrangement are specific for a particular species at a particular stage of development.
Chromocenters are heterochromatic regions of varying size, which occur near the centromeres in proximal regions of chromosome arms. At mid-prophase, many chromocenters can be resolved into strings of chromomeres, which are larger than chromomeres found in distal regions. In some dipteran salivary glands, the chromocenters of different chromosomes fuse to form a large chromocenter. The relative distribution of chromocenters are sometimes considered to be of considerable evolutionary value.
Knobs are spherical heterochromatin bodies which may have a diameter equal to the chromosome width but may reach a size having a diameter which is several times the width of the chromosome. Very distinct chromosome knobs can be observed in maize at pachytene stage. Knobs are valuable chromosome markers for distinguishing chromosomes of related species and races.

Constitutive and Facultative Heterochromatin

Certain heterochromatic regions of chromosomes, particularly those proximal to centromeres are constant, and are called constitutive heterochromatic regions serving as chromosome markers. There are other heterochromatic regions called facultative hererochromatic and represented by whole sex chromosomes which become heterochromatic only at certain stage. For instance in female humans, one X-chromosome is inactivated or becomes heterochromatic. Only facultatively. Similarly in plants accessory chromosomes are heterochromatic. In plants also, among dioecious genera like Melandrium and Rumex, one or both sex chromosomes may undergo partial or complete heterochromatinization.

It is established that DNA replicates in heterochromatic region at a time different than that in euchromatic regions. It is also established that genes in heterochromatic region are inactive, but the earlier belief that no genes are found in heterochromatic regions is not correct because, genes could be located in heterochromatic regions in several cases like Drosophila and tomato. The genes in heterochromatic region perhaps become active for a short period. Y chromosome is another example of heterochromatic chromosomes having inactive genes in several dioecious plants and animals.

Single-stranded and Multi-stranded hypotheses for Chromosomes

When chromosomes are compared in related species which differ widely in DNA content, such differences may be attributed to one of the two causes: (i) lateral multiplication of chromonemata leading to multiple strandedness or (ii) tandem duplication, where length-wise duplication instead of lateral multiplication is responsible for difference in DNA content. This later condition will retain the single stranded feature of chromosomes. Although multiple strandedness has been demonstrated in several cases of plants like Vicia faba and animals like dipteran salivary gland chromosomes, there are evidences against such a hypothesis to become a generalization. In vicia faba chromosomes. Sub-chromonemata were actually observed, while in related V Sativa they could not be observed. In other genera like Allium and Lolium, also it has been shown that increase in DNA content is mainly brought about by tandem duplication rather than by lateral multiplication leading to multi-stranded feature. Therefore, the two hypotheses assuming single stranded and multi- stranded nature of chromosomes are complementary and not exclusive to each other.

Chemical composition of chromosomes

The major chemical components of chromosomes are DNA, RNA, histone proteins and non-histone proteins. Calcium is also present in addition to these constituents.

DNA-

As we know, DNA is the most important of chemical components of chromatin, since it plays the central role of controlling heredity. Quantitative measurements of DNA have been made in a large number of cases which are reviewed by M.D. Bennett and I.J, Leitch in 1995 (Annals of Botany 76: 113-176). The most convenient measurement of DNA is picoram (10-12 g) which is equivalent to 31 cm of double helical DNA (for details of the double helical structure of DNA, consult Chapter 31). It has been found that quantity of DNA varies greatly in cells from different kinds of organisms. The haploid genome of mammals usually contains 1000 times DNA content of bacteria. Other eukaryotes may similarly have 10 go 100 times the bacterial DNA content. It is interesting to note that a human diploid cell has 174 cm (5-6 picograms) of DNA, so that all cells in a human being may have DNA equal to 2.5 x 10¹0 km (100g), a length which is equal to 100 times the distance from earth to sun. Similarly a diploid cell of Trillium has 37 meters (120 picograms) and that Drosophila salivary glands has 91 meters (293 picograms) of DNA. In comparison of these enormous lengths, the DNA of bacteria measures only 1.1 mm-1.4 mm.
Histones-

There are five fractions of histones, which have been differently designated. H1 histone is most easily removed and so is least tightly bound. This may thus be concerned with holding together a chromosome fibre.H3 and H4 are extremely conserved, having same structure in different species and should thus have a common structural role.

Histones, isolated from diverse materials showed considerable similarity. It is also assumed that general similarities in histones have been conserved during evolution. This feature alone suggested that these proteins should play a structural role rather than a regulatory role. However, some important experiments involving chromatin reconstitution and other experiments conducted in recent years have established that histones do play a regulatory role. This regulatory role of histones is more of general nature rather than specific and is exercised by repressing the activity of genes.

Non-histones-

The non-histone proteins display more but still limited diversity. In a variety of organisms, number of non-histones can vary from 12 to little more than 20. Heterogeneity of these proteins suggested that these proteins are not as conserved in evolution as histones. These non-histone proteins differ even between different tissues of the same organism suggesting that they regulate the activity of specific genes. Chromatin reconstitution experiments described in 1973 by R.S. Gilmour and J. Paul of Institute for Cancer Research at Glasgow (U.K.), established conclusively for the first time that specific non-histone proteins switch on specific genes. The results of these experiments were later confirmed in a number of cases (Barrett et al., 1974; Groner et al, 1975). During 1980s and 1990s, many of the transcription factors have been shown to be non-histone proteins, providing further evidence that non-histone proteins exercise positive control on the activity of specific genes.

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Basic Structure of Chromosome