Again here with an important article on nucleosome,Polytene Chromosome and lampbrush chromosome. Here this article includes the following topic:
- Structure of nucleosome
- Discoverer of polytene chromosome
- Occurrence of polytene chromosome
- Structure of polytene chromosome
- Function of polytene chromosome
- Discoverer of lampbrush chromosome
- Characteristics of lampbrush chromosome
- Importance of lampbrush chromosome
- B chromosome
Nucleosome is the basis unit of DNA packaging in eukaryotes consisting of a segment of DNA wound in sequence around eight histone protein core. This structure is often compared to thread wrapped around a spool.
Each nucleosome contains a core particle consisting of histone octamer,a single molecule of linker histone H1 and super coiled DNA of about 147 base pairs. This forms a chromatin subunit known as chromatosome.
In the histone octamer ,two molecules of each four core histones,H2A, H2B, H3 and H4 are present. The core histones are capable of self assembling into the histone octamer which is a flat ended cylindrical particle, about 11 nm in diameter and 5.7 nm in thickness. The DNA appears to wrap(left handed) the outside of the nucleosome corea about one and three quarter times. The core particles are connected to one another by a stretch of linker DNA or spacer DNA.
Together the DNA of nucleosome core and linker consist of about 200 base pairs. One molecule of H3 histone is normally located just outside the core particle. It is associated with both ends of the DNA as it enters and exits from the core particles. Electron microscopy reveals this DNA-protein complex as 10 nm chromatin fibre with an appearance of ‘beads on a string’. The string is the naked linker DNA while each bead in the nucleosome is a core particle.
Polytene chromosome (also called salivary gland chromosome) is a type of giant chromosome that is found in the salivary gland of drosophila larva.
Poly means many and tene means thread.
Polytene chromosome was first discovered by F.G. Balbiani in 1881.
Where polytene chromosome is found ?
This giant chromosome is found in various tissues like salivary gland, mid gut and malphighian tubules, in the larvae of some flies (drosophila) and in several species of protozoans and plants.
Structure of polytene chromosome
Polytene chromosomes are large sized chromosome with a linear series of alternating bands, interbands and puffs. The length of the chromosome reach 200 to 600 µm. It has characteristic dark and light banding pattern. Dark bands are highly coiled, heterochromatic, least transcriptionally active region where as light bands are least coiled, euchromatic and high transcriptionally active regions. Light band is present between two dark bands. Some times many interband regions are joined together to form a long loop like structure that gives a swelling appearance. These very structures are called chromosome puffs. Chromosome puffs are diffused uncoiled region of polytene chromosome that are the site of RNA transcription due to presence of m-RNA and RNA pol II.
Characteristics of polytene chromosome
- Present in the salivary gland sale of drosophila larva.
- Much bigger in size than normal chromosome.
- Here two types of regions (dark bands and light bands) are alternatively present.
- Presence of chromosome puffs.
Functions of polytene chromosome:
- In addition to increasing the volume of cell’s nucexpansion polytene cell’s nuclei and causing cell expansion, polytene cell’s have a metabolic advantage as multiple copies of genes permits a high level of gene expression
- The banding pattern of polytene chromosomes are especially helpful in research as they provide an excellent visualisation of transcriptionally active chromatin.
- Polytene chromosomes are also used to identify the species of chironomid larvae that are notoriously difficult to identify.
In the year 1882 scientist Flemming first observed lampbrush chromosome and in 1892 scientist Ruckert observed these chromosomes in animal ovum as it looked like a brush to clean chimney,he named tt he chromosome as lampbrush chromosome.
Discovery: In 1892,Ruckert discovered the lampbrush chromosome.
Where lampbrush chromosome is found?
Lampbrush chromosome is found in the ovum of vertebrate species.
Characteristics of lampbrush chromosome:
- It is an excessively large type of chromosome found in the oocytes of many animals.
- It has long, thread like projecting loops, giving it a hairy brush like appearance.
- Lampbrush chromosome occur during the diplotene stage of meiosis I.
- They are meiotic bivalent each consisting of two sister chromatids.
- Each chromatid axis is composed of many chromere.
- Each chromere has a loop like structure on both side and in total look like a thick dark cotton like structure.
- In each set of lampbrush chromosome there are as many as 10,000 loops present.
- Each loop has one Axis which is composed of a DNA double helix.
- Due to synthesis of massive amount of RNA, the DNA form a loop like structure.
- The RNA are mainly heterogeneous RNA(hn RNA).
- Lampbrush chromosomes are clearly visible even in the light microscope, where they are seen to be organised into a series of large chromatin loops emanting from a linear chromosomal axis.
- A given loop always contains the same DNA sequence and it remains extended in the same manner as the oocytes grows.
Importance of lampbrush chromosome:
Giant chromosome in the lampbrush form are useful model for studying chromosome organisation and gene expression during meiotic prophase, since they allow the individual transcription units to be visualised.
In addition to normal chromosomes some wild populations of certain animal, plant and fungi have B chromosomess. It is also called supernumerary or accessory chromosome.
These chromosomes are not essential for the life of a species and are lacking in most of the individuals. Thus a population would have individuals with 0, 1, 2, 3 etc number of chromosome.
Characteristics of B chromosomes:
- Most of the b chromosomes are mainly heterochromatic (non coding).
- In some cases ( like in maize) they contain some euchromatic (coding )regions.
- It has been seen that the role and characters of B chromosome are changed to give the species some selective evolutionary or adaptive advantages.
- B chromosomes do not follow normal mendelian mode of inheritance.
- It is assumed that these chromosomes have originated from the heterochromatic segments of normal chromosomes by a special process called genetic polymorphism.
Significance of B chromosomes:
- Give evolutionary advantages to the species to remain alive in many unfavourable environment.
- Because I’m also have some effects over normal chromosome ( A chromosomes) as below……
- Increase chaisma formation.
- Increase crossing over and recombination frequencies.
- Sometime causes increased unpaired chromosome that results into infertility.