They belong to a class of specialized chromosomes discovered in the oocytes of animals except for mammals. Walter Flemming discovered these specialized structures. They show the presence of highly extended regions. The tailed and the tailless amphibians, birds, and insects show these specialized chromosomes. These chromosomes do not occur in plants. The geneticists visualize the lampbrush chromosomes even under the light microscope. Hence, they involve an easy staining procedure. The lampbrush chromosomes exist as bivalents, each having two sister chromatids. These chromosomes get visualized mainly in the diplotene stage of prophase I of meiosis-l. During this stage, the chromosomes get converted into lampbrush shaped. Meaning, they resemble a brush for cleaning the chimneys. They consist of loops occurring in pairs. They occur as one on either side of the axis. These chromosomes involve active transcription. Mainly, the loops involve vigorous RNA synthesis. The basic units of lampbrush chromosomes include a pair of deoxyribonucleoprotein (DNP) fibers. They run along the axis of the chromosomes. However, the centromere of the lampbrush chromosome lacks the lateral loops. It is known as a chromomere. It gets flanked by the condensed regions showing the axial bars.
The axial bars include the pericentromeric heterochromatin. These regions show the presence of highly repetitive satellite DNA. Thus, it does not involve any looped regions. The lampbrush chromosomes do not occur in the males. However, exceptions include Drosophila Y chromosomes showing lampbrush loops. These regions involve active transcription process. The lampbrush chromosomes show a greater thickness as compared to the 30nm chromatin fiber. It is due to the high amount of ribonucleoprotein arising due to active transcription. The amphibian lampbrush chromosomes show the presence of telomeres. These regions look like granules. However, the telomeres of the bird lampbrush chromosomes consist of additional loops attached to the telomeric granules at one end. The other end carries a free telomeric sequence involving transcription from only the C- rich strand.
The axial bars include the pericentromeric heterochromatin. These regions show the presence of highly repetitive satellite DNA. Thus, it does not involve any looped regions. The lampbrush chromosomes do not occur in the males. However, exceptions include Drosophila Y chromosomes showing lampbrush loops. These regions involve active transcription process. The lampbrush chromosomes show a greater thickness as compared to the 30nm chromatin fiber. It is due to the high amount of ribonucleoprotein arising due to active transcription. The amphibian lampbrush chromosomes show the presence of telomeres. These regions look like granules. However, the telomeres of the bird lampbrush chromosomes consist of additional loops attached to the telomeric granules at one end. The other end carries a free telomeric sequence involving transcription from only the C- rich strand.
The homologous loops differ in their size and structure. The lampbrush chromosomes involve uninemy of the chromosomes. Meaning, they consist of one DNA duplex.
Image: Lampbrush Chromosomes
Transcription process in the loop:
Attachment of the RNA polymerase enzyme to the principal axis of the loop gets revealed through the electron microscopy. The RNA fibrils start extending upon the action of RNA polymerase enzyme. These fibrils go on increasing their length. The hnRNA or the heterogeneous nuclear RNA covers different kinds of RNA molecules possessing different lengths. These molecules are known as the precursors for mRNAs in the eukaryotic cells. The loops mainly perform the transcription of the hnRNAs. The structure of the lateral loop consists of the asymmetrical matrix consisting of RNA transcripts. The hnRNA transcripts also consist of their respective binding proteins. The synthesis of RNA starts at the thinner end. It gets completed at the thicker end. Ultimately, the ribonucleoprotein gets released. Important components of the cell involved in the structure of the lampbrush chromosomes are known as actin filaments. Actin is a kind of protein possessing filaments.
The loop gets extended from the chromomeric axis with the help of actin filaments. In the end, the loops disappear. The transcriptional regions in the lampbrush chromosomes include lateral loops. An important structure of the lampbrush chromosome known as chromomere plays a crucial role here. The chromomere is the axial condensate of the lampbrush chromosome. It appears like that of a bead. The paired loops extend from the chromomere. The composition of the chromomere includes tight packaging of the DNA with histones forming arrays of deoxyribonucleoprotein. The newt chromosomes consist of 104 lateral loops in one haploid complement. Thus, lateral loops are easy to identify. They possess distinct units with different morphologies. They also act as the units of inheritance. Since they get involved in the transcription process, they are known as the units of transcription.
The loop gets extended from the chromomeric axis with the help of actin filaments. In the end, the loops disappear. The transcriptional regions in the lampbrush chromosomes include lateral loops. An important structure of the lampbrush chromosome known as chromomere plays a crucial role here. The chromomere is the axial condensate of the lampbrush chromosome. It appears like that of a bead. The paired loops extend from the chromomere. The composition of the chromomere includes tight packaging of the DNA with histones forming arrays of deoxyribonucleoprotein. The newt chromosomes consist of 104 lateral loops in one haploid complement. Thus, lateral loops are easy to identify. They possess distinct units with different morphologies. They also act as the units of inheritance. Since they get involved in the transcription process, they are known as the units of transcription.
Staining lampbrush chromosomes:
Silver staining technique involves staining the objects of study using silver. It is possible to stain the lampbrush chromosomes with silver staining. Alternatively, they get stained with antibodies against the RNA processing proteins. Actively expressing regions become clearly visible under the fluorescence microscope. The RNA splicing machinery appears like round-shaped granules under the microscope.
The procedure for preparing lampbrush chromosomes:
The procedure worked well firstly in the urodele amphibian oocytes. The first step involves culturing the actively dividing tissues or cells extracted from the organism. Next, the cultures get processed to obtain only the oocytes. The nucleus of the oocytes gets extracted removing the membrane. Next, the lampbrush chromosomes get freed up from the nucleus of the oocyte. These chromosomes get fixed on the coverslip for microscopic examination.
Consequences of stretching the lampbrush chromosomes:
The lampbrush chromosomes consist of highly extended regions. Upon stretching these chromosomes, the breaks occur in them. These breaks occur transversely. The chromosomes thus held together with the help of fibers, thereby forming loops. The looped regions consist of one double-stranded DNA molecule. The axis regions consist of two double-stranded DNA molecules. Digestion of the lampbrush chromosomes with DNase-I creates two adjacent cuts. The rate of production of breaks shows a direct relationship with the square of the length of the time of digestion. Two double-stranded DNA molecules present in the axis have a different equation. The rates of production of breaks in them show a direct relationship with the digestion time raised to the power of four.
Drosophila lampbrush chromosomes:
The lampbrush chromosomes in Drosophila also show the presence of loops. However, they undergo different mechanisms. The loops consist of simple sequences. They show highly repetitive DNA, retrotransposons, and middle repetitive DNA sequences. All of them get transcribed. Out of many, an at least one loop thread out and shows the genes encoding Dynein. It forms the outer arms of the microtubules in the sperm tails. The loops show relatively larger sizes. The reason includes the presence of high amounts if intronic sequences. Male Drosophila lampbrush chromosomes do not exhibit crossing over during meiosis. Thus, they do not have any mechanisms to prevent rapid growth of clusters of the satellite DNA.
The significance of lampbrush chromosomes:
These chromosomes mainly play a role in the transcription process. Apart from this, lampbrush chromosomes get involved in the maternal gene expression. During oogenesis, the lampbrush chromosomes provide important clues regarding the sequence expression. The maternal mRNA synthesis mainly occurs in the oocytes. It helps the early embryos to get the mRNA from the maternal part. Thus, they serve in the embryonic development.
References:
[1] Lampbrush Chromosomes, Harold G. Callan
[2] Chromosomes: Organization and Function, Chapter 14, Adrian T. Sumner
[3] Principles of genetics, 8th edition, Page 134, Gardner, M. J. Simmons, D. P. Snustad
Copyright, 2019 All Rights Reserved
.png)
