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Special chromosomes

Introduction:

  • Some cells at certain particular stages contain large nuclei with giant or large sized chromosomes.
  • The giant chromosomes are the polytene and lampbrush chromosomes.
  • Polytene chromosomes were discovered by Balbiani which are formed due to polyteny.
  • Lampbrush chromosomes are a special form of chromosome found in the growing oocytes (immature eggs) of most animals, except mammals.

Topics covered:

  • Polytene chromosomes
  • Lampbrush chromosome
  • Difference between the polytene and lampbrush chromosomes.

Polytene chromosomes:
 

Introduction:

  • This special type of chromosome is observed by Balbiani in salivary glands of the Chironomus larvae of Dipteran insects.
  • Since they were discovered in the salivary glands, they were also called salivary gland chromosomes.
  • The present name polytene chromosome was suggested by Kollar due to the occurrence of many chromonemata(DNA) in them.
  • Thus, some cells of Drosophila, Chironomus and mosquitoes become very large having high DNA content.
  • Polyteny of giant chromosomes is achieved by replication of the DNA several times without nuclear division and the resulting daughter chromatids do not separate but remain aligned side by side.

Detailed explanation:

  • Chromonema is required to form Polytene chromosomes which are formed by chromosomal replication without nuclear division.
  • The giant chromosomes are also formed by the process of somatic pairing between homologous chromosomes.
  • The resultant daughter chromosomes remain aligned with each other and not separated from each other.
  • This is very useful for the analysis of many facets of eukaryotic interphase chromosome organization and the genome as a whole.
  • The salivary gland cells do not undergo mitosis and die during metamorphosis.
  • This chromosome is symbolized by Balbiani rings or puffs which are swollen or puffy areas in polytene chromosomes.
  • This chromosome is the site of mRNA synthesis and is multi- stranded in nature.
  • The action of the basic dyes can result in the bearing of the number of dark bands in the polytene chromosome of varying size and intensity.
  • The dark bands are presumed to be formed by the juxtaposition of chromomeres of the different chromonemata of a polytene chromosome.
  • They are found in the permanent prophase stage of mitosis.
  • In puffs, DNA is uncoiled for rapid transcription of RNA.
  • Hence, the main function is the synthesis of RNA and proteins.


    balbiani-rings

    polytene-chromosomes
     

Lampbrush chromosome :
 

Introduction:

  • Lampbrush chromosomes were first observed in Salamander(amphibian) oocytes in 1882.
  • He coined the name because the chromosomes look like the brushes which were used for cleaning the glass chimneys of old fashioned paraffin or kerosene lamps.
  • This type of chromosome was observed by Flemming in 1882.
  • Lampbrush chromosomes occur in the diplotene chromosomes bivalents of most in animal oocytes.
  • It is also found in spermatocytes of several species, a giant cell of Acetabularia, and even in plants.
  • These chromosomes are even larger than the polytene chromosomes.

Detailed explanation:
 

lampbrush-chromosomes
 

  • Lampbrush Chromosomes are concerned with vitellogenesis (Yolk formation).
  • This is the special kind of synapsed mid prophase or the Diplotene bivalent stage
  • This chromosome occurs in pairs containing homologous chromosomes containing the point of contact known as chiasmata.
  • The chromatids forming the chromosomes bear a large number of chromomeres which are separated by interchromomeric stretches.
  • Many of the chromomeres give out lateral projections or loops which provide a lampbrush-like appearance to the chromosome pair.
  • These loops contain one to several transcriptional units which show transcription of mRNA required for the synthesis of the substances for growth and development of meiocytes.
  • Some mRNAs produced by lampbrush chromosomes may be stored as informosomes (mRNA + protein) for producing biochemicals during the early development of the embryo.
  • The lateral loops are withdrawn followed by the shortening of chromosomes, after the full development of meiocytes.
  • The main function is synthesis of RNA and proteins.
  • Lampbrush chromosome is a model useful for studying chromosome organization, genome function and gene expression during meiotic prophase.
  • This also allows the visualization of the individual transcription units.

Difference between Lampbrush and polytene chromosomes:
 

S.no. Lampbrush chromosomes Polytene chromosomes
1. They were firstly observed by Flemming. They discovered by Balbiani
2. These are found in yolk-rich primary oocytes of Amphibians like Newt (Triturus), spermatocytes of many animals, and the giant nucleus of Acetabularia. They were observed in the cell of salivary glands of Chironomus larvae of Dipterian Insect. These are also found in malpighian tubules, endosperm, antipodal cells and salivary glands of Drosophila.
3. They are found In permanent diplotene stage of meiosis. They are found in the permanent prophase stage.
4. The size up to 5.9 mm (5900pm). The size of polytene chromosomes is 2000pm.
5. Special Characteristic: The axis of the lamp-brush is composed of DNA and a matrix of RNA. Proteins in its lateral loops help In synthesis of RNA and yolk. Special Characteristic: They become giant due to endomitosis or endoduplication. Large swellings are found on some places of each strand that are called puffs (Balbiani rings). In puffs DNA is uncoiled for rapid transcription of RNA.
6.


Frequently Asked Questions: FAQs

Question 1. Why do lampbrush chromosomes are observed during oogenesis and what roles do they play in egg formation?
Answer:

  • The Loops are clearly seen in lampbrush chromosomes during meiotic prophase in oocytes of many species.
  • Lampbrush Chromosomes are concerned with vitellogenesis (Yolk formation).
  • This is the special kind of synapsed mid prophase or the Diplotene bivalent stage
  • This chromosome occurs in pairs containing homologous chromosomes containing the point of contact known as chiasmata.
  • It stores components needed for rapid cell divisions during the early development of the fertilized egg.

Question 2. In which type of stage are lampbrush chromosomes observed?
Answer:

  • Lampbrush chromosomes occur in the Diplotene stage of the meiosis I division cycle.

Question 3. How are lampbrush chromosomes helpful?
Answer:

  • Lampbrush Chromosomes are concerned with vitellogenesis (Yolk formation).
  • The loops present in the lampbrush chromosomes contain one to several transcriptional units which show transcription of mRNA required for the synthesis of the substances for growth and development of meiocytes.
  • Some mRNAs produced by lampbrush chromosomes may be stored as informosomes (mRNA + protein) for producing biochemicals during the early development of the embryo.
  • The lateral loops are withdrawn followed by the shortening of chromosomes, after the full development of meiocytes.
  • The main function is synthesis of RNA and proteins.
  • Lampbrush chromosome is a model useful for studying chromosome organization, genome function and gene expression during meiotic prophase.
  • This also allows the visualization of the individual transcription units.

Question 4. Why are polytene chromosomes useful in genetic studies?
Answer:

  • Chromonema is required to form Polytene chromosomes which are formed by chromosomal replication without nuclear division.
  • The giant chromosomes are also formed by the process of somatic pairing between homologous chromosomes.
  • The resultant daughter chromosomes remain aligned with each other and not separated from each other.
  • This is very useful for the analysis of many facets of eukaryotic interphase chromosome organization and the genome as a whole

Question 5. How are polytene chromosomes produced?
Answer:

  • Chromonema is required to form Polytene chromosomes which are formed by chromosomal replication without nuclear division.
  • The giant chromosomes are also formed by the process of somatic pairing between homologous chromosomes.
  • The resultant daughter chromosomes remain aligned with each other and not separated from each other.
  • The salivary gland cells do not undergo mitosis and die during metamorphosis.
  • This chromosome is symbolized by Balbiani rings or puffs which are swollen or puffy areas in polytene chromosomes.
  • This chromosome is the site of mRNA synthesis and is multi- stranded in nature.

Question 6. Differentiate between lampbrush chromosomes and polytene chromosomes.
Answer:

 

S.no. Lampbrush chromosomes Polytene chromosomes
1. They were firstly observed by Flemming. They discovered by Balbiani
2. These are found in yolk-rich primary oocytes of Amphibians like Newt (Triturus), spermatocytes of many animals, and the giant nucleus of Acetabularia. They were observed in the cell of salivary glands of Chironomus larvae of Dipterian Insect. These are also found in malpighian tubules, endosperm, antipodal cells and salivary glands of Drosophila.
3. They are found In permanent diplotene stage of meiosis. They are found in the permanent prophase stage.
4. The size up to 5.9 mm (5900pm). The size of polytene chromosomes is 2000pm.
5. Special Characteristic: The axis of the lamp-brush is composed of DNA and a matrix of RNA. Proteins in its lateral loops help In synthesis of RNA and yolk. Special Characteristic: They become giant due to endomitosis or endoduplication. Large swellings are found on some places of each strand that are called puffs (Balbiani rings). In puffs DNA is uncoiled for rapid transcription of RNA.
6.

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