Introduction:

E. Strasburger in the year 1875 first discovered thread-like structures that appeared throughout the cell division. Such thread like structures was termed as chromosomes due to their affinity for basic dyes. The word chromosome is derived from the two Greek words: chrom = color, soma = body. This word was first employed by Waldeyer in the year 1888.

Chromosomes:

Of all the components of cell, the chromosomes have been studied most widely and possibly more is recognized about them than any other cell organelle. The chromosome has bigger constancy than any other cell component and it maintains it special qualities from one cell generation to the other. The Chromosomes contributed to the division of cells and they are of main significance as they carry the genes that are the hereditary material.

Chromosome number:

The number of chromosomes in a specific species is usually constant. All the members of the species ordinarily encompass definite and usually a constant somatic and gametic chromosome number. Somatic chromosome number is the number of chromosomes found in the somatic cells of a species and is represented via 2n.

Generally somatic cells have two copies of each chromosome apart from the sex chromosomes. Both the copies are ordinarily similar in morphology, gene content and gene order is therefore termed as homologous chromosomes. Gametic chromosome number is precisely half of somatic chromosome number and is represented by n. It signifies the number of chromosomes found in gametes of a species. The number of chromosomes differs greatly from 2n = 4 (n = 2) in Haplopappusgracilis (Compositae) to 2n = > 1200 in some Pteridophytes.

Chromosome Size: 

The size of the chromosome exhibits a remarkable variation based on the phase of cell division. The chromosomes are the thinnest and longest throughout Interphase (resting phase) and therefore not visible beneath light microscope. Chromosomes are the thickest and smallest throughout mitotic metaphase. In general, plants have longer chromosomes than animals and species having lower chromosome number encompass longer chromosomes than those containing a higher chromosome number. Among plants, dicots in general encompass shorter and higher number of chromosomes than monocots. Among the higher plants, the longest mitotic chromosomes are such of Trillium spp., which might reach 32 μ in size. In most of the fungi all chromosomes are very minute. Chromosome size is not proportional to the number of genes present on the chromosome.

Chromosome Morphology:  

The outer covering or casing of a chromosome is termed as pellicle that encloses the matrix. In the matrix lies the chromatin. Fleming proposed the word chromatin in the year 1879. The word chromatin signifies to the Feulgen positive materials viewed in Interphase nucleus and later throughout nuclear division. Chromatin readily stains by means of basic dyes particularly Basic Fuchsin that is specific for DNA which in turn is a main constituent of chromosomes. The chromosome morphology changes throughout cell division and mitotic metaphase is the most suitable phase for studies on chromosome morphology. In mitotic metaphase chromosomes, the following structural characteristics can be seen below the light microscope.

1) Chromatid:  Each and every metaphase chromosome appears to be longitudinally splitted into two similar parts each of which is termed as chromatid. Both chromatids of a chromosome appear to be joined altogether at a point termed as Centromere. The two chromatids of chromosome separate from one other throughout mitotic anaphase (and throughout anaphase II of meiosis) and move towards the opposite poles. As the two chromatids making up a chromosome are produced via replication of a single chromatid throughout synthesis (S) stage of Interphase, they are termed to as sister chromatids. In contrary, the chromatids of homologous chromosomes are termed as non-sister chromatids.

2) Centromere:  Centromere and telomere are the main stable portions of chromosomes. The area where two sister chromatids come out to be joined throughout mitotic metaphase is termed as Centromere. It usually appears as constriction and therefore termed as primary constriction. Centromere is localized and simply detectable morphological area of the chromosomes that assists in the movement of the chromosomes to opposite poles throughout anaphase of the cell division. The Centromere splits the chromosomes into two transverse parts termed as arms. The Centromere comprises of two disk shaped bodies termed as kinetochores. The kinetochores don't form part of the chromatid however lies one on each side of the chromosome in such a way that each and every chromatid is having its own kinetochore. One kinetochore is joined to the spindle fibres towards one pole and the other likewise towards the other pole. Beads on position of the Centromere, chromosomes can be assembled as:

a) Metacentric: Centromere is positioned precisely at the centre of chromosome, that is, both arms are equivalent in size. These chromosomes suppose 'V' shape at anaphase.

b) Submetacentric: The Centromere is positioned on one side of the centre point in such a way that one arm is longer than the other. Such chromosomes become 'J' or 'L' shaped at anaphase.

c) Acrocentric: Centromere is positioned close to one end of the chromosome and therefore giving a much short arm and a very long arm. Such chromosomes get 'J' shape or rod shape throughout anaphase.

d) Telocentric: Centromere is positioned at one end of the chromosome so that the chromosome consists of only one arm. Such chromosomes are 'I' shaped or rod shaped. In general chromosomes are monocentric having one Centromere each.

3) Telomere:  The two ends of chromosomes are termed as telomeres. They are very stable and don't fuse or unite by telomeres of other chromosomes due to the polarity effect. Any broken end of a chromosome is not stable and can attach by a piece of any other chromosome. However the telomeres impart stability to the chromosome that retains its identity and individuality via cell cycle and for numerous cell generations.

4) Secondary constriction: The constricted or narrow area other than that of Centromere is termed as secondary constriction and the chromosomes having secondary constriction are termed as satellite chromosomes or sat chromosomes. Chromosome might have secondary constriction in one or both arms of it. Chromosomal end distal to the secondary constriction is termed as satellite. Production of nucleolus is related by secondary constriction and thus it is as well termed as nucleolus organizer area and satellite chromosomes are often termed to as nucleolus organizer chromosomes.

5) Chromomere:  In certain species such as maize, rye and so on. Chromosomes in pachytene phase of meiosis exhibit small bead like structures termed as chromomeres. Chromomeres are visible throughout meiotic prophase (or pachytene) and invisible in the mitotic metaphase chromosomes. The distribution of chromomeres in chromosomes is extremely characteristic and constant. The pattern of distribution being dissimilar for different chromosomes. They are evidently visible as dark staining bands in the huge salivary gland chromosomes. Chromomeres are areas of tightly folded DNA. Chromomeres of single chromosome exhibit considerable variation in size. They might differ in size as in the case of maize or they might be of uniform size as in the case of rye. 

6) Chromonema: A chromosome comprises of two chromatids and each chromatid comprises of thread like coiled structures termed as chromonema (plural chromonemata). The word chromonema was introduced through Vejdovsky in the year 1912. The chromonemata form the gene bearing part of the chromosomes.

7) Matrix:  The mass of acromatic material that surrounds the chromonemata is termed as matrix. The matrix is surrounded in a sheath that is termed as pellicle. Both the matrix and pellicle are non genetic materials and appear only at metaphase, when the nucleolus disappears.

Composition of chromosomes: 

The material of which chromosomes are composed is termed as chromatin. N. Fleming proposed the word chromatin in the year 1879. Chromatin was categorized into two groups by means of cytologists on the basis of its affinity to basic dyes such as acetocarmine or feulgen (that is, basic fuchsin) reagent at prophase. The darkly stained areas were termed as heterochromatin, whereas lightly stained areas were termed as euchromatin. This differential staining capacity of dissimilar parts of chromosomes is termed as 'heteropycnosis'. Generally heterochromatin is found in the centromeric and telomeric areas and these areas of chromosome usually replicate later than the euchromatic areas of chromosomes. The genes in the heterochromatic areas are generally inactive. Mainly, the genome of an active cell is euchromatic and the genes within this euchromatic area are expressed. Heterochromatin is further categorized into two groups: a) Constitutive and b) Facultative heterochromatin.

Special types of Chromosomes:

A few tissues of certain organisms have chromosomes that differ significantly from normal chromosomes in terms of either morphology or function. Such chromosomes are termed to as special chromosomes. The following are comprised beneath this group:

1) Giant chromosomes or polytene chromosomes:

These were primary discovered by E. G. Balbiani in the year 1882 in Dipteran salivary glands and therefore generally termed as salivary gland chromosomes. Such chromosomes replicate repeatedly however the daughter chromatids don't separate from one other and the cell as well doesn't divide. This phenomenon is termed as endomitosis or endoreduplication. It outcomes in the formation of numerous stranded giant chromosomes termed as polytene chromosomes and the condition is termed as polyteny. Their size is 200 times or more than the normal somatic chromosomes (or autosomes) and much thick. Therefore they are termed as giant chromosomes. Such chromosomes are somatically paired and their number in the salivary gland cells for all time appears to be half of that in the normal somatic cells.

2) Lamp brush chromosomes:

These were first noticed by W. Fleming in the year 1882 and were explained in detail in oocyte of sharks by Rukert in the year 1892. They take place at diplotene phase of meiotic prophase in oocytes of all animal species. As they are found in meiotic prophase, they are present in the form of bivalents in which the maternal and paternal chromosomes are held altogether by chiasmata at such sites where crossing over has earlier occurred.

3) Accessory chromosomes:  

In most of the species some chromosomes are found in the addition to normal somatic chromosomes. Such extra chromosomes are termed as accessory chromosomes or B-chromosomes or supernumerary chromosomes. Such chromosomes are mostly similar to normal somatic chromosomes in their morphology; however encompass some peculiar functional aspects.

4) Isochromosomes:

An isochromosome is the one in which two arms are similar by one other in gene content and morphology. Such a chromosome is in essence a reverse duplication with Centromere separating the two arms. Each and every isochromosome is Metacentric.

5. Allosomes or sex chromosomes: 

Chromosomes varying in morphology and number in male and female are termed as allosomes. They are responsible for finding out of sex. Example: X and Y chromosomes in human beings and Drosophila.

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