Special Chromosomes

  • Chromosomes are the rod shaped, dark stained bodies located in the nucleus of a eukaryotic cell. 

• Play an important role in heredity i.e., transfer of genetic information from one generation to another generation.

• In some organisms  there are special tissues in which chromosomes undergoes structural specialisations. Such specialized chromosomes includes 1. Gaint chromosmes- like the Polytene chromosomes, Lampbrush chromosomes, B chromosomes

Polytene Chromosomes

Balbiani discovered the large or giant chromosomes in the salivary glands in Chironomous larvae (Diptera species) in 1881. These structues were long and sausage-shaped and marked by swellings and transverse bands. Unfortunately, he could not recognize them as chromosomes.  .

The cytogenetical importance, however, was not realized until many years later when Kosloff(1930) pointed out the similarities between their banded structure and the linearly arranged genes on the chromosomes. Panter, Heltz and Baner established the fact that each of the visible chromosomes actually consisted of a pair of homologous chromosomes intimately synapsed. Since, these chromosomes were discovered in the salivary glands cells, they were called salivary gland chromosomes.

Commonly occur in diplotene stage of meiosis. Have been observed in salivary glands cells, Malplhigian tubules, ovarian nurse cells and gut epithelial cells of the larvae of dipterean species like Drosophila, chironormous, sciara.

They have also been reported from the suspensor cells of developing embryos, antipodal cells and endosperm  cells of plants. 

Polytene or salivary gland chromosomes are the largest chromosomes known. In Drosophila melanogaster they are approximately 100 times the length of the somatic metaphase chromosome. Each salivary gland chromosome is 200 – 600 mm in length. Because of their larger size compared to metaphase chromosomes these chromosomes are called as giant chromosomes. They contain 1000-16000 times more DNA than normal cells. This is because of polyteny or multistranded nature of these chromosomes.

A common structural feature of all polytene chromosomes is that their many chromatin filaments (chromatids) are laying – more or less paralles – side by side. The reason for the higher number of chromatids is due to replication of the DNA several times without nuclear division. The duplication of DNA results in new chromatids which are not separated but remain within the same cell. This is called endo reduplication or endomitosis – chromosome duplication without cell division.

The resulting daughter chromatids do not separate but remain aligned side by side. For unknown reasons, the centromere regions of the chromosomes do not endoreplicate very well. As a result, the centromeres of all the chromosomes bundle together in a mass called the chromocenter.

The number of replications rounds can lead to 256,512 or even 1024 strands existing side by side in a single chromosome.5. Because of presence of many strands or chromonemata  they were named as Polytene chromosomes by Kollar

These metabolically active tissues grow by an increase in the size of their constituent cells rather the increasing their cell numbers. This process generates giant cells whose volumes are thousands of times greater than normal cells. 

The development of giant cells is accompanied by successive rounds of replication

Another peculiar characteristic of the polytene chromosomes is that the maternal and paternal homologous chromosomes remain associated side by side. This phenomenon is called somatic pairing.

The polytene chromosomes shows transversal banding pattern (similar to a bar code) along the chromosome that can be easily observed in light microscopy. It consists of alternating darker and lighter chromosomal segments both in light and electron microscopy. It could be shown that the darker segments are represented by chromatin that is packaged relatively dense. While the lighter segments or interbands are made up by chromatin that is packed much more loosely. And this region shows intense gene transcription.

At particular developmental stage, the DNA at certain points uncoils to produce swellings called puffs or Balbiani rings. The DNA in the puff regions unfolds into open loops due to intense transcription and produces mRNA.

Some puffs are always found at the same chromosomal position. But, majority of puffs, however, are visible only at certain times of the development of an organism and they appear at different positions on the polytene chromosomes. This is explained by the fact that at different times during the development different genes are active. Furthermore, the activity of gene can be different from one tissue to another tissue.

Polytene chromosomes are well suited for studies of nucleic acid synthesis, gene action.  Polytene chromosomes provided the first evidence that eukaryotic gene activity is regulated at the level of RNA synthesis. The effects of deletion, addition, inversion, etc., of chromosomal segments or genes have been studied in detail due to occurrence of distinct bands and interbands in the polytene chromosomes. 

Lampbrush Chromosomes

In diplotene stage of the meiosis, the yolk rich ooctyes of many vebrates – fishes, amphibians, reptiles & birds contain exceptionally large-sized chromosomes

Which had an appearance of lampbrush or test tube brush. 

First observed by Flemming (1889) in amphibian oocyte

A detailed study of themws made  by J. Ruckert (1892) in oocyte of Shark. He coined the named lampbrush, because they appear as lamp cleaning brush.

Size: in many animals lampbrush chromosomes may be more than 1000 microns in length and 20 microns in width. In some salamander oocytes the lampbrush chromosomes may reach a length of 5900 microns

Some lampbrush chromosomes are 3 times longer than the polytene chromosomes. 

Observed during prolonged diplotene stage, these are in form of bivalents

Structure: During diplotene, the homologous chromosomes begin to separate from each other and are held together at chiasmata. Under light microscope each chromosome is seen to consists of an axis along which is a row of dense granules or chromomeres. From each chromomere arises a pair of lateral loop.

Chromosome axis

Each chromosome of the pair of homologous chromosomes consists of two chromatids, which are represented by axial filaments. Thus, the pair of homologous chromosomes has four filaments in all. 

Chromomeres:

At certain points along their length the axis become tightly coiled. These points are the chromomeres. The chromomeres are found in pairs. They perhaps correspond to heterochromatin and transcriptionally inactive. 

The axial filaments and the chromomere consists of DNA.

 Loops:

From each chromomere arises a pair of lateral loop. The loops represent the lateral extension of the axial filaments

Each loop consists of an axial fibre which is covered by a matrix. When the loop is treated with Deoxyribonucleases ( an enzyme which breaks down DNA) it is broken down, indicating that it consists of DNA. When treated with ribonuclease ( an enzyme which breaks down RNA), trypsin and pepsin, the matrix of the loop is removed. From this it can be concluded that the matrix consists of RNA and Protein

They are of two main types- typical and special. Most of the loops are typical. Each typical loop consists of central axis from which are given off RNA fibrils of progressively increasing lengths. This makes the loop more     thicker on one side. The special loops have a marked asymmetry and have granules at the end of the fibrils.

Electron microscope studies by Miller andBeaty (1969) on the lampbrush chromosomes of the oocytes of the Salamander trityrus show the presence of dense granules on the DNA axial fibre. These granules are probably large molecules of the enzyme RNA polymerase, which synthesizes RNA. Arising from these RNA polymerase molecules are fine fibrils of RNA-protein (ribonucleoproteins).

Further, there is evidence that the chromosomal loop continuously spins out from the chromomere at one end, while it recoils into another end. Callan and Lyod have suggested that each pair of loop is associated with the activity of specific gene. Each loop is supposed to contain repeating DNA sequences – gene arranged in series. At each chromomere is supposed to be a master ‘ gene’ copy which transfers information to several ‘slave gene’ copies on the same loop (master and slave hypothesis). Only the slave gene take part in RNA synthesis, but not the master gene. 

The intense period of transcription lasts for many day in some animals; then the RNA is given off and loop collapses into the main body of the chromosome. Most of the RNA formed in the oocyte is used to form ribosomes i.e., it is rRNA. In addition,some mRNA is synthesized this mRNA is stored in the cytoplasm and is used later in embryogenesis.

LC assist in fulfilling the high demand for transcripts during oogenesis. Studies on oogenesis in vertebrates reveal that extensive transcription occur during prophase I diplotene of meiosis. The loops of lampbrush chromosomes have been shown to be regions of active transcription, whose products are required during early stages of embryogenesis.

B Chromosomes:

The growth, development and reproduction of an organism relies on genetic material that is organsized into chromosomes. The chromosome complement carried by all members of a species is refereed to as the A chromosomes.

A subset of individuals within a species may also possess extra chromosomes that are non-essesntial and not member of the standard A chromosome set. These supernumerary chromosomes are commonly refereed to as B Chromosomes. First described in 1907, B chromosmes have now been identified in hundreds of species across many different taxa and it is estimated that B chromosomes may be present  in 15% of all eukaryotic species.

B chromosomes shows following characteristics:

Different morphology from that of the chromosomes belongining to the normal complement, usually of smaller size

Genetic constitution which does not strongly influence the individual – usually having an almost non-existent gentic action.

Numberical variability between different cells or different tissues within individuals or populations.

Abnormal behavior  at meiosis, - lack homology with the normal complement

Abnormal behavior at mitosis- chromatid non-disjunction at anaphase

Morphology

Generally, B chromosomes are noticeably smaller in dimension than the A chromosomes, and often have  more or less globular appearance. Particularly interesting thing is that the centromere is frequently in a terminal position –telocentric 

In majority  of cases the B chromosomes are distinctly heterochromatic and non-coding. But some B chromosomes of  maize contain sizeable euchromatin segments.

They are not essential to normal cellular function, and donot follow normal patterns of replication and segregation during cell division.

They are not homologous with any of the basic A-chromosomes. Their inheritance is non-mendelian. Some times due to non-disjunction, their number is increased in the progeny