Special types of chromosomes exist as some tissues of certain organisms contain chromosomes which differ significantly from normal chromosomes in terms of either morphology or function. Such chromosomes are referred to as special chromosomes. The following are included under this category:
1. Giant chromosomes or polytene chromosomes
These were first discovered by E. G. Balbiani in 1882 in Dipteran salivary glands and hence commonly called salivary gland chromosomes.
These chromosomes replicate repeatedly but the daughter chromatids do not separate from one another and the cell also does not divide.
This phenomenon is known as endomitosis or endoreduplication. It results in the formation of many stranded giant chromosomes known as polytene chromosomes and the condition is known as polyteny.
Their size is 200 times or more than the normal somatic chromosomes (autosomes) and very thick. Hence they are known as giant chromosomes.
These chromosomes are somatically paired and their number in the salivary gland cells always appear to be half of that in the normal somatic cells.
Along the length of chromosomes, a series of dark bands are present alternate with clear bands known as interbands.
These bands have greatly helped in mapping of the chromosomes in cytogenetic studies. In the dark band region, the DNA is tightly coiled while in the interband region, DNA is less tightly coiled.
The morphological expression of such sites is represented by local enlargements of certain regions called puffs. These puffs are also known as balbiani rings. Puffs are the sites of active RNA synthesis.
2. Lamp brush chromosomes
These were first observed by W. Flemming in 1882 and were described in detail in oocytes of sharks by Rukert in 1892. They occur at diplotene stage of meiotic prophase in oocytes of all animal species.
Since they are found in meiotic prophase, they are present in the form of bivalents in which the maternal and paternal chromosomes are held together by chiasmata at those sites where crossing over has previously occurred. Each bivalent has four chromatids, two in each homologue.
The axis of each homologue consists of a row of granules or chromomeres, each of which have two loop like lateral extensions, one for each chromatid. Thus each loop represents one chromatid of a chromosome and is composed of one DNA double helix.
One end of each loop is thinner than other which is known as thickened. There is extensive RNA synthesis at thin ends of the loop while there is little or no RNA synthesis at the thick ends.
3. Accessory chromosomes
In many species some chromosomes are found in addition to normal somatic chromosomes. These extra chromosomes are called accessory chromosomes or B-chromosomes or supernumerary chromosomes.
These chromosomes are broadly similar to normal somatic chromosomes in their morphology, but have some peculiar functional aspects. For instance, presence of several such chromosomes often leads to reduction in vigour and fertility in males.
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These chromosomes are generally smaller in size than the normal somatic complement. They are believed to be generally inactive genetically. However they may not be completely devoid of genes. Origin of these chromosomes in most species is unknown.
4. Isochromosomes
An isochromosome is the one in which two arms are identical with each other in gene content and morphology. Such a chromosome is in essence a reverse duplication with centromeres separating the two arms.
Every isochromosome is metacentric. The attached ‘x’ chromosome of Drosophila is a classic example of an isochromosome. However its origin is uncertain. There is no evidence that isochromosomes had any evolutionary significance.
5. Allosomes / Sex chromosomes
Chromosomes differing in morphology and number in male and female are called allosomes.
They are responsible for determination of sex. Eg: X and Y chromosomes in human beings and Drosophila. Chromosomes which have no relation with determination of sex and contain genes which determine somatic characters of individuals are called autosomes and are represented by letter ‘A’.
In summary, plants are made of chromosomes. The number of chromosomes in a species is constant. Somatic chromosome number is the number of chromosomes found in somatic cells of a species and two copies of each chromosome except the sex chromosomes.
The size of the chromosome varies with stage of cell division being longest and thinnest during interphase (resting stage) but smallest and thickest during mitotic metaphase.
Plants have longer chromosomes than animals and species having lower chromosome number have longer chromosomes than those having a higher chromosome number. Dicots in general have shorter and higher number of chromosomes than monocots. Cytological principles of plant breeding are based on the chromosomal structure, composition and function of chromosomes of the plant.
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