Lampbrush Chromosome

Lampbrush Lampbrush ChromosomeChromosome

Lampbrush ChromosomeLampbrush Chromosome First seen by Flemming in 1882 It is the largest chromosome known and is

found in the amphibian oocytes (immature eggs).

Also seen in diplotene in oocytes of fish, birds, reptiles, and some invertebrates.

Lampbrush chromosomes occur during the diplotene stage of meiosis I.

They are meiotic bivalents, each consisting of each consisting of two pairs of sister chromatids.

Lampbrush chromosomes are so called because the chromosomes seem to have a chromomeric pattern with loops projecting in pair from majority of chromosomes and they appear like the brushes, that were used for cleaning glass chimneys of Kerosene lamps.

First observed by Flemming in 1882 and were decribed in detail in shark oocyte by J.Ruckert in 1892 who later coined the name.

In vertebrates during the diplotene stage of meiosis, lampbrush chromosomes (discovered by Ruckert in 1892) are formed in the nuclei of yolk rich  oocytes during the active synthesis of mRNA molecules for the future use by the egg during cleavage when no synthesis of mRNA molecules is possible due to active involvement of chromosomes in the mitotic cell division.  

400–800 μm long as opposed to at the most 15–20 μm during later stages of meiosis.

Thus the lampbrush chromosomes are ~30 times less tightly packed.

The total length of the entire lampbrush chromosome set is 5 to 6 mm and is organized into ~5000 chromomeres.

It is larger in size. Hence it is called a giant chromosome.

Lampbrush chromosomes are clearly visible even in the light microscope, where they are seen to be organized into a series of large chromatin loops emanating from a linear chromosomal axis giving a furry, brush-like appearance.

Each lamp brush chromosome consists of a main axis and many lateral loops.

The main axis of each chromosome is formed of 4 chromatids.

The main axis contains a series of thickenings called chromomere.

From each chromomere a pair of lateral loops arise one on each side.

Each loop has an axial fibre, which is a continuation of the chromonema of the main axis.

Loops have transcriptional activity.

The axial fibre of the loop is surrounded by a matrix. The matrix is formed gives a fuzzy appearance.

The synthesis of proteins and yolk  takes in the lateral loops

These chromosomes are producing large amounts of RNA for the oocyte, and most of the genes present in the DNA loops are being actively expressed.

The majority of the DNA, however, is not in loops but remains highly condensed in the chromomeres on the axis, where genes are generally not expressed.

Giant chromosomes in the lampbrush form are useful model for studying chromosome organization and gene expression during meiotic prophase, since they allow the individual transcription units to be visualized.