Giant chromosomes
Some cells at particular stages contain large nucleus with giant or large sized chromosomes. The giant chromosomes are Polytene and lamp brush chromosomes.
Polytene chromosome
It occurs in the tissues of salivary gland, gut, trachea, fat body cells and Malpighian tubules of many insects of the order Diptera. It was first reported by E G Balbiani in 1881. The name polytene was suggested by Kollar due the occurrence of many chromonemata.
The polytene chromosomes are larger in size. The polytene chromosome of Drosophila melanogaster has total length of 200 microns in comparison to the 7.5 microns of somatic chromosome. The enormous size of the polytene chromosome is achieved by the duplication of chromonema for nine or ten times and duplicated chromonemata are not separated.
The polytene chromosome consists of closely coiled or associated homologus pairs of chromosomes. This association is called somatic pairing.
The polytene chromosome contains about 1000 times more DNA material than the somatic chromosomes.
Dark and light bands
Polytene chromosome bears alternating dark and light bands. The dark bands take deep stain with basic chromosomal stain and contains large amount of DNA and small amount of RNA. These are mainly composed of euchromatin. The light bands take light chromosomal stain and contains small amount of DNA and large amount of RNA. They are mainly composed of heterochromatin.
The polytene chromosome is caused by the process of endomitosis. In endomitosis, chromonemata become duplicated and resulted chromonemata do not separate to form new chromosome but remain closely associated. Also in endomitosis, nuclear membrane does not rupture and no spindle fibre formation takes place.
During development stages, polytene chromosome of larva of Dipterous insect, develop swelling at particular point of dark and light band. Such swelling is called puffs or bulbs. The puffing process involves accumulation of acidic protein, synthesis of RNA and storage of synthesized mRNA.
The chromonema of polytene chromosome gives out series of loops laterally. These rings are known as Balbiani rings. They are rich in DNA and mRNA. The formation of Balbiani rings are similar to puffs.
Lampbrush chromosome
The yolk rich oocytes of many vertebrates such as fishes, amphibians, reptiles and birds, contain exceptionally large sized chromosomes known as lamp brush chromosomes. They were discovered by Ruckert.
The lamp brush chromosomes are much longer. The length of lamp brush chromosome in Urodele oocyte may reach up to 5900 microns.
The lamp bush chromosomes are composed of the main axis and lateral loops. The main axis is composed of 4 chromatids or two bivalent chromosomes. The chromonemata of these chromaitd give out fine loops at lateral sides giving appearance of lamp brush or test tube brush.
The loops of lamp brush chromosomes are rich in RNA and protein. The RNA appears to be mRNA. The synthesis of protein and yolk take place near the loop.
Cytogenetical functions of chromosomes
Chromosomes are the most significant component of cell. They control most of the biological and genetical activities of cell. They contain genetical material, DNA which ultimately influences the biological phenomena atg molecular, physiological and gross morphological level.
Crossing over
The linked genes may remain together during the process of inheritance showing complete linkage or many be separated or segregated during gametogenesis displaying incomplete linkage. The incomplete linkage takes place due to new combination or recombination of liked genes . this recombination in tis turn accomplished through a process of crossing over.
In crossing over, the non sister chromatids exchange chromosomal parts or segments. Crossing over is a process of formation of new combinations( recombinations) of genes by interchanging of corresponding segments between non sister chromatids of homologous chromosomes, the chromatins resulted from interchange of chromosomal parts are cross over.
Types
1. Somatic or mitotic crossing over
It occurs during mitotic cell division, it is rare and of no genetical significance. The somatic crossing over reported in Drosophila by Curt Stern.
2. Germinal or meiotic crossing over
It occurs during gamete formation in germinal cells. It is universal occurring and of great genetic significance.
Mechanism of crossing over
The process of crossing over includes four stages
1. Synapsis
2. Duplication of chromosome
3. Crossing over
4. Terminalization
Synapsis- during Zygotene stage of Prophase I of Meiosis, homologous chromosomes come close to each other and pairing of them takes place. The phenomenon of pairing is called synapsis. It is of prime importance because is provides mechanical basis of heredity and variation.
Duplication of chromosome- each homologous chromosome of bivalent splits longitudinally and form two identical sister chromatids. These are held together by unsplitted centomere. There are four chromatids and this stage is called tetrad.
Crossing over- crossing over occurs in the homologous chromosome during four stranded or tetrad stage. First, two non sister chromatids break at the corresponding points due to activity of endonuclease ( Stern and Hotta,1969). A segment of one side of break connects with a segment of opposite side of the break so that two non sister chromatids cross each other. The fusion of chromosomal segments with that of opposite one takes place due to action of ligase. The crossing of tow chromatids is called chiasma formation, the crossing overf includes breaking, their transposition and fusion.
Chiasma frequency
Crossing over may take place at several points making several chiasmata formation, the number of chiasmata depends upon length of chromosome. Longer the chromosome, greater the number of chiasmata formation. In a species, each chromosome has characteristic number of chiasmata. When two genes are located closer, there are lesser chances of chiasma formation between them and vice versa.
Terminalization
After crossing over, non sister chromatida start to repel cach other. The force of attraction between them decreases. The chromatids separate progressively form centromere towards chiasma. The movement of chiasma is known as terminalization . due to terminalization the homologous chromosomes are separated completely.
Types
1. Single crossing over
When chiasma is formed at one point only it is single crossing over. It produces two cross over chromatids and two non cross over chromatids.
2. Double crossing over
It is when chiasma occurs at two points in the same chromosome;. In this each chiasma is independent of other chiasma. There are 2 sub types
I. Reciprocal chiasma
The same two chromaids are involved in the second chiasma as in the first. The second chiasma restores the order which was changed by first. It produces two non cross over chromatids.
II. Complementary chiasma
When both chromatids in the second chiasma are different from those involved in the first chiasma. It is known as complementary chiasma. The complementary chiasm occurs when 3 or 4 chromatids of tetrad take part in the crossing over.
3. Multiple crossing over
It is when crossing over takes place at more than two places in the same chromosome pair. Such is known as multiple crossing over. It occurs rarely.
Significance
The phenomenon of crossing over is universal in its occurrence and found in Viruses, Bacteria, plants and animals. It is necessary for Natural Selection as the chances of variation increases.
Some cells at particular stages contain large nucleus with giant or large sized chromosomes. The giant chromosomes are Polytene and lamp brush chromosomes.
Polytene chromosome
It occurs in the tissues of salivary gland, gut, trachea, fat body cells and Malpighian tubules of many insects of the order Diptera. It was first reported by E G Balbiani in 1881. The name polytene was suggested by Kollar due the occurrence of many chromonemata.
The polytene chromosomes are larger in size. The polytene chromosome of Drosophila melanogaster has total length of 200 microns in comparison to the 7.5 microns of somatic chromosome. The enormous size of the polytene chromosome is achieved by the duplication of chromonema for nine or ten times and duplicated chromonemata are not separated.
The polytene chromosome consists of closely coiled or associated homologus pairs of chromosomes. This association is called somatic pairing.
The polytene chromosome contains about 1000 times more DNA material than the somatic chromosomes.
Dark and light bands
Polytene chromosome bears alternating dark and light bands. The dark bands take deep stain with basic chromosomal stain and contains large amount of DNA and small amount of RNA. These are mainly composed of euchromatin. The light bands take light chromosomal stain and contains small amount of DNA and large amount of RNA. They are mainly composed of heterochromatin.
The polytene chromosome is caused by the process of endomitosis. In endomitosis, chromonemata become duplicated and resulted chromonemata do not separate to form new chromosome but remain closely associated. Also in endomitosis, nuclear membrane does not rupture and no spindle fibre formation takes place.
During development stages, polytene chromosome of larva of Dipterous insect, develop swelling at particular point of dark and light band. Such swelling is called puffs or bulbs. The puffing process involves accumulation of acidic protein, synthesis of RNA and storage of synthesized mRNA.
The chromonema of polytene chromosome gives out series of loops laterally. These rings are known as Balbiani rings. They are rich in DNA and mRNA. The formation of Balbiani rings are similar to puffs.
Lampbrush chromosome
The yolk rich oocytes of many vertebrates such as fishes, amphibians, reptiles and birds, contain exceptionally large sized chromosomes known as lamp brush chromosomes. They were discovered by Ruckert.
The lamp brush chromosomes are much longer. The length of lamp brush chromosome in Urodele oocyte may reach up to 5900 microns.
The lamp bush chromosomes are composed of the main axis and lateral loops. The main axis is composed of 4 chromatids or two bivalent chromosomes. The chromonemata of these chromaitd give out fine loops at lateral sides giving appearance of lamp brush or test tube brush.
The loops of lamp brush chromosomes are rich in RNA and protein. The RNA appears to be mRNA. The synthesis of protein and yolk take place near the loop.
Cytogenetical functions of chromosomes
Chromosomes are the most significant component of cell. They control most of the biological and genetical activities of cell. They contain genetical material, DNA which ultimately influences the biological phenomena atg molecular, physiological and gross morphological level.
Crossing over
The linked genes may remain together during the process of inheritance showing complete linkage or many be separated or segregated during gametogenesis displaying incomplete linkage. The incomplete linkage takes place due to new combination or recombination of liked genes . this recombination in tis turn accomplished through a process of crossing over.
In crossing over, the non sister chromatids exchange chromosomal parts or segments. Crossing over is a process of formation of new combinations( recombinations) of genes by interchanging of corresponding segments between non sister chromatids of homologous chromosomes, the chromatins resulted from interchange of chromosomal parts are cross over.
Types
1. Somatic or mitotic crossing over
It occurs during mitotic cell division, it is rare and of no genetical significance. The somatic crossing over reported in Drosophila by Curt Stern.
2. Germinal or meiotic crossing over
It occurs during gamete formation in germinal cells. It is universal occurring and of great genetic significance.
Mechanism of crossing over
The process of crossing over includes four stages
1. Synapsis
2. Duplication of chromosome
3. Crossing over
4. Terminalization
Synapsis- during Zygotene stage of Prophase I of Meiosis, homologous chromosomes come close to each other and pairing of them takes place. The phenomenon of pairing is called synapsis. It is of prime importance because is provides mechanical basis of heredity and variation.
Duplication of chromosome- each homologous chromosome of bivalent splits longitudinally and form two identical sister chromatids. These are held together by unsplitted centomere. There are four chromatids and this stage is called tetrad.
Crossing over- crossing over occurs in the homologous chromosome during four stranded or tetrad stage. First, two non sister chromatids break at the corresponding points due to activity of endonuclease ( Stern and Hotta,1969). A segment of one side of break connects with a segment of opposite side of the break so that two non sister chromatids cross each other. The fusion of chromosomal segments with that of opposite one takes place due to action of ligase. The crossing of tow chromatids is called chiasma formation, the crossing overf includes breaking, their transposition and fusion.
Chiasma frequency
Crossing over may take place at several points making several chiasmata formation, the number of chiasmata depends upon length of chromosome. Longer the chromosome, greater the number of chiasmata formation. In a species, each chromosome has characteristic number of chiasmata. When two genes are located closer, there are lesser chances of chiasma formation between them and vice versa.
Terminalization
After crossing over, non sister chromatida start to repel cach other. The force of attraction between them decreases. The chromatids separate progressively form centromere towards chiasma. The movement of chiasma is known as terminalization . due to terminalization the homologous chromosomes are separated completely.
Types
1. Single crossing over
When chiasma is formed at one point only it is single crossing over. It produces two cross over chromatids and two non cross over chromatids.
2. Double crossing over
It is when chiasma occurs at two points in the same chromosome;. In this each chiasma is independent of other chiasma. There are 2 sub types
I. Reciprocal chiasma
The same two chromaids are involved in the second chiasma as in the first. The second chiasma restores the order which was changed by first. It produces two non cross over chromatids.
II. Complementary chiasma
When both chromatids in the second chiasma are different from those involved in the first chiasma. It is known as complementary chiasma. The complementary chiasm occurs when 3 or 4 chromatids of tetrad take part in the crossing over.
3. Multiple crossing over
It is when crossing over takes place at more than two places in the same chromosome pair. Such is known as multiple crossing over. It occurs rarely.
Significance
The phenomenon of crossing over is universal in its occurrence and found in Viruses, Bacteria, plants and animals. It is necessary for Natural Selection as the chances of variation increases.
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