The chromosomes are the nuclear components of a special organization, individuality, and function. They are capable of self-reproduction and play a vital role in the heredity, mutation, variation, and evolutionary development of the species.
Karl Nagli in 1842 observed rod-like chromosomes in the nuclei of plant cells. In 1872 E. Russow made the first serious attempt to describe chromosomes. A. Schneider (1873) published a most significant paper dealing with the relationship between chromosomes and stages of cell division. E. Strasburger (1875) discovered thread-like structures that appeared during cell division.
Walter Flemming (1878) introduced the term chromatin to describe the thread-like material of the nucleus that became intensely colored after staining. W. Roux (1883) suspected the involvement of the chromosomes in the mechanism of inheritance. Benden and Bovery (1887) reported that the number of chromosomes for each species was constant.
The present name chromosome (Gr., chrom= color, soma=body) was coined by W. Waldeyer (1888) for darkly stained bodies of the nucleus. W. S. Sutton and T. Boveri in 1902 suggested that chromosomes were the physical structures that acted as messengers of and one paternal member. He believed that chromosomes, acting in this way, maybe the physical basis for the Mendelian laws of heredity. He is credited as the originator of the theory of the chromosomal basis for heredity.
Thomas Morgan and Hermann Muller, in the early 1900s, established the cytological basis for the laws of heredity. Working with Drosophila chromosomes, they located 2000 genetic factors on the four chromosomes of the fruit fly in 1922.
In 1914, Robert Feulgen demonstrated a color test known as Feulgen reaction for the DNA. In 1924, he showed that chromosomes contain DNA. In 1942, using cytochemical procedures, Brachet demonstrated the presence of another nucleic acid, RNA, and not long thereafter, Mirsky and Pollister (1946) showed that there were proteins associated with chromosomal material.
Dupraw (1965) suggested ‘folded fibre model’ of the chromosome to suggest that it was made of a highly folded single molecule of DNA which is wrapped in chromosomal proteins. R. D. Kornberg (1974) proposed the ‘nucleosome model’ of the basic chromatin material. The term ‘nucleosome’ was coined by P. Outdet et al.,
Number of Chromosome
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 suffix “ploid” refers to chromosome “sets”. The prefix indicates the degree of the ploidy. 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. Among plants, chromosome number varies from 2n = 4 in Haplopappus gracilic (Compositae) to 2n = >1200 in some pteridophytes.
However, the diploid number of tobaccos is 48, cattle 60, the garden pea 14, the fruit fly 8, etc. The chromosome number of some animals and plants is tabulated in Table.
|Group||Common name||Scientific name||Chromosome number|
|Nematoda||Round worm||Ascaris lumbricoides||24|
|Arthropoda||House fly||Musca domestica||12|
|Algae||Chlamydomonas||10?; 12?; 16?; (Haploid sets)|
|Fungi||Bred mold||Mucor heimalis||2|
|Gymnosperm||Yellow pine||Pinus ponderosa||24|
|Sugar cane||Saccharum officinarum||80|
The diploid number of a species bears no direct relationship to the species position in the phylogenetic scheme of classification.
Lastly, while ‘n’ normally signifies the gametic or haploid chromosome number, ‘2n’ is then 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 example, in wheat Triticum aestivum 2n = 42; n = 21 and x = 7, showing that common wheat is a hexaploid (2n = 6x).
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). 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.
how many chromosomes do humans have?
In humans, there are 23 pairs of homologous chromosomes (i.e., 2n = 46; a chromosome number). 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.
What is the size of a Chromosome?
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 fruit fly (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. The monocotyledon plants contain large-sized chromosomes than the dicotyledon plants. The plants in general have large-sized chromosomes in comparison to the animals.
Further, the chromosomes in a cell are never alike in size, some may be exceptionally large and others may be too small. The largest chromosomes are lamp brush chromosomes of certain vertebrate oocytes and polytene chromosomes of certain dipteran insects.
What is the Shape of a Chromosome?
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 from chromosome to chromosome and it provides different shapes to the latter which are following.
1. 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 acrocentric 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 of Chromosome
While describing the structure of the chromosomes during various phases of the cell cycle, cell biologists have introduced many terms for their various components. Let us become familiar with the following terms to understand more clearly the structure of the chromosomes:
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 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. According to old view, a chromosome may have more than one chromonemata which are embedded in the achromatic and amorphous substance, called matrix. The matrix is enclosed in a sheath or pellicle. Both matrix and pellicle 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 chromosomal matrix disappears, the nucleolus appears and vice versa. Electron microscopic observations, however, have questioned the occurrence of pellicle and matrix in them.
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.
Chromomeres are regions of tightly folded DNA and have a great interest in the cell biologists. They are believed to correspond to the units of genetic function in the chromosomes. In fact, for a long time, most geneticists considered these chromomeres as genes, i.e., the units of heredity.
4. Centromere and kinetochore.
Originally it was considered that the centromere consists of small granules or spherules. The centromere of the chromosome of the Trillium has a diameter of 3μm and the spherules have a diameter of 0.2 μm.
The chromonema remains connected with the spherules of the centromere. Currently, it is held that centromere is the region of the chromosome to which are attached the fibers of the mitotic spindle. The centromere (a term much preferred by the geneticists) lies within a thinner segment of a chromosome, the primary constriction.
The regions of chromosome flanking the centromere contain highly repetitive DNA and may stain more intensely with the basic dyes. (i.e., it is constitutive heterochromatin). Centromeres are found to contain specific DNA sequences with special proteins bound to them, forming a disc-shaped structure, called kinetochore (a term that is much preferred by the cytologists).
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 the nucleosome. Further, emanating from the convex surface of the outer layer of the kinetochore, in addition to the microtubules, a “corona” or “collar” of fine filaments has been observed.
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 center of assembly for microtubules, i.e., it serves as a nucleation center 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.
The chromosomes of the Ascaris megalocephala and hemipterans have diffused type of the centromere. 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.
(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 telomeres. If a chromosome breaks, the broken ends can fuse with each other due to a 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.
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. Chromosome satellites are a morphological entity and should not be confused with satellite DNAs which are highly repeated DNA sequence.
Karyotype and Idiogram
The term karyotype has been given to the group of characteristics that identifies a particular set of chromosomes. 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.
A diagrammatic representation of a karyotype (or morphological characteristics of the chromosomes) of a species is called idiogram (Gr., idios = distinctive; gramma = something written). 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 homologs are ordered in a series of decreasing size. A karyotype of human metaphase chromosomes is obtained from their microphotographs.
The individual chromosomes are cut out of the microphotographs and lined up by size with their respective partners. The technique can be improved by determining the so-called centromeric index, which is the ratio of the lengths of the long and short arms of the chromosome.
Some species may have special characteristics in their karyotypes; for example, the mouse has acrocentric chromosomes, many amphibians have only metacentric chromosomes and plants frequently have heterochromatic regions at the telomeres.
Uses of karyotypes.
The karyotypes of different species are sometimes compared and similarities in karyotypes are presumed to represent evolutionary relationship. A karyotype also suggests primitive or advanced features of an organism.
It may be symmetric or asymmetric. A karyotype exhibiting large differences in smallest and largest chromosomes of the set and containing fewer metacentric chromosomes, is called an asymmetric karyotype.
In comparison to a symmetric karyotype (e.g., Pinus), an asymmetric karyotype (e.g., Ginkgo biloba,) is considered to be a relatively advanced feature. Levitzky (1931) suggested that in flowering plants there is a prominent trend towards asymmetric karyotypes.
This trend has been well studied in the genus Crepis of the family Compositae. In many cases it was shown that increased karyotype asymmetry was associated with specialized zygomorphic flowers.
Material of Chromosome
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:
Portions of chromosomes that stain lightly are only partially condensed; this chromatin is termed euchromatin. It represents most of the chromatin that disperses after mitosis has completed. Euchromatin contains structural genes that replicate and transcribe during G1 and S phase of interphase. The euchromatin is considered genetically active chromatin, since it has a role in the phenotype expression of the genes. In euchromatin, DNA is found packed in 3 to 8 nm fiber.
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). It is late replicating (i.e., it is replicated when the bulk of DNA has already been replicated) and is not transcribed.
It is thought that in heterochromatin the DNA is tightly packed in the 30 nm fiber.
Types of heterochromatin.
In an interphase nucleus, usually, there is some condensed chromatin around the nucleolus, called perinucleolar chromatin, and some inside the nucleolus called intranucleolar chromatin. Both types of heterochromatin appear to be connected and together, they are referred to as nucleolar chromatin.
Dense clumps of deeply staining chromatin often occur in close contact with the inner membrane of the nuclear envelope (i.e., with the nuclear lamina) and are called condensed peripheral chromatin. Between the peripheral heterochromatin and the nucleolar heterochromatin are regions of lightly staining chromatin, called dispersed chromatin.
In the condensed chromosomes, the heterochromatic regions can be visualized as regions that stain more strongly or more weakly than the euchromatic regions, showing the so-called positive or negative heteropyknosis of the chromosomes.
Heterochromatin has been further classified into the following types:
1. Constitutive heterochromatin.
In such a heterochromatin the DNA is permanently inactive and remains in the condensed state throughout the cell cycle. This most common type of heterochromatin occurs around the centromere, in the telomeres and in the C-bands of the chromosomes.
In Drosophila virilis, constitutive heterochromatin exists around the centromeres and such pericentromeric heterochromatin occupies 40 % of the chromosomes. In many species, entire chromosomes become heterochromatic and are called B chromosome, satellite chromosomes or accessory chromosomes and contain very minor biological roles.
Such chromosomes comprising wholly constitutive heterochromatin occur in corn, many phytoparasitic insects and salamanders. In the fly Sciara, large metacentric heterochromatic chromosomes are found in the gonadal cells, but are absent in somatic cells.
Entire Y chromosome of male Drosophila is heterochromatic, even though containing six gene loci which are necessary for male fertility. Constitutive heterochromatin contains short repeated sequences of DNA, called satellite DNA.
This DNA is called satellite DNA because upon ultracentrifugation, it separates from the main component of DNA. Satellite DNA may have a higher or lower G + C content than the main fraction.
For example, the mouse satellite DNA is a 240 base pair sequence that is repeated about 1000,000 (106) times in the mouse genome, constituting 10 % of the total mouse DNA. The exact significance of constitutive heterochromatin is still unexplained.
2. Facultative heterochromatin.
Such type of heterochromatin is not permanently maintained in the condensed state; instead it undergoes periodic dispersal and during these times is transcriptionally active. Frequently, in facultative heterochromatin one chromosome of the pair becomes either totally or partially heterochromatic.
The best-known case is that of the X-chromosomes in the mammalian female, one of which is active and remains euchromatic, whereas the other is inactive and forms at interphase, the sex chromatin or Barr body (Named after its discoverer, Canadian cytologist Murray L. Barr).
Barr body contains DNA which is not transcribed and is not found in males. Indeed, the number of Barr bodies is always one less than the number of X chromosomes (i.e., in humans, XXX female has two Barr bodies and XXXX female has three Barr bodies).
Dosage compensation and lyonization. In mammals all female cells contain two X chromosomes, while male cells contain one X and one Y chromosome. Presumably because a double dose of X chromosome products would be lethal, the female cells have evolved a mechanism for permanently inactivating one of the two X chromosomes in each cell (this process is called dosage compensation).
Process of X chromosome inactivation is often termed lyonization after the name of British cytogeneticists Mary Lyons. In mice, this occurs between the third and the sixth day of development, when one or the other of the two X chromosomes in each cell is chosen at random and condensed into heterochromatin (or Barr body).
Because the inactive chromosome is faithfully inherited, every female is a mosaic composed of clonal groups of cells in which only the paternally inherited X chromosome (Xp) is active and a roughly equal number of groups of cells in which only the maternally inherited X chromosome (Xm) is active.
Function and Significance of Chromosomes
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.
Genetic Code Storage:
The chromosome contains the genetic material that is required by the organism to develop and grow. DNA molecules are made of a chain of units called genes. Genes are those sections of the DNA that code for specific proteins required by the cell for its proper functioning.
Humans have 23 pairs of chromosomes out of which one pair is the sex chromosome. Females have two X chromosomes and males have one X and one Y chromosome. The sex of the child is determined by the chromosome passed down by the male. If the X chromosome is passed out of the XY chromosome, the child will be a female and if a Y chromosome is passed, a male child develops.
Control of Cell Division:
Chromosomes check the successful division of cells during the process of mitosis. The chromosomes of the parent cells ensure that the correct information is passed on to the daughter cells required by the cell to grow and develop correctly.
Formation of Proteins and Storage:
The chromosomes direct the sequences of proteins formed in our body and also maintain the order of DNA. The proteins are also stored in the coiled structure of the chromosomes. These proteins bound to the DNA help in proper packaging of the DNA.