The 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.
They were described in detail in shark oocytes by R. Ruckert in 1892. Thorpe (1984) and Burns and Bottino (1989) preferred the term test tube brush chromosomes for them. However, due to recent investigations of Gall and coworkers (1962, 1983), the structure of these exceptionally large-sized chromosomes has been interpreted in functional terms, i.e., now they are merely visualized as means of “turning on and turning off” of the genes.
The lampbrush chromosomes occur at the diplotene stage of meiotic prophase in the primary oocytes of all animal species, both vertebrates, and invertebrates. Thus, they have been described in Sagitta (Chaetognatha), Sepia (Mollusca), Echinaster (Echinodermata), and in several species of insects, sharks, amphibians, reptiles, birds, and mammals (humans).
Lampbrush chromosomes are also found in spermatocytes of several species, the giant nucleus of Acetabularia, and even in plants (Grun, 1958). Generally, they are smaller and “hairy” in invertebrates than vertebrates.
Lamphrush chromosomes are best visualized in salamander oocytes because they have a high DNA content. For example, the largest chromosome having a length up to 1 mm have been observed in urodele amphibian.
Thus, lampbrush chromosomes are much larger (longer) than the polytene chromosomes of insects. Since the lampbrush chromosomes are found in the prolonged diplotene stage of meiotic prophase I, they are present in the form of bivalents in which the maternal and paternal chromosomes are held together by chiasmata, at those sites where crossing over has previously occurred.
The paired homologues are not condensed as usual chromosomes would be; instead, they are very long and stretched out. Each bivalent has four chromatids, two in each homologue. The axis of each homologue consists of a row of granules or chromomeres from which lateral loops extend.
The loops are always symmetrical, each chromosome having two of them, one for each chromatid. The loops can be categorized by size, thickness, and other morphological characteristics. Each loop appears at a constant position in the chromosome; this fact helps in the chromosome mapping.
There are about 10,000 loops per chromosome set or haploid set (e.g., oocytes of slamander Triturus). Each loop has an axis that is made of a single DNA molecule that is unfolded from the chromosome for intense RNA synthesis.
Thus, about 5 to 10 % of the DNA exists in the lateral loops, the rest being tightly condensed in the chromomeres which are transcriptionally inactive. The centromeres of the chromosomes bear no loops.
Each loop of lampbrush chromosomes is found to perform intense transcription of hnRNA or heterogeneous RNA molecules (i.e., precursors of mRNA molecules for various ribosomal proteins and for five types of histone proteins).
Electron microscopy of the loops has shown that RNA polymerase enzyme molecules are attached to the principal axis (DNA) of the loop from which RNA fibrils of increasing length extend. As transcription continues along the DNA strand of the loop, the fibrils of RNA (i.e., hnRNA) lengthen.
Proteins get associated with these RNA fibrils as they are formed and ultimately ribonucleoprotein product is released. Thus, each lateral loop is covered by a matrix that consists of RNA transcripts with hnRNA-binding proteins attached to them.
Generally, this matrix is asymmetrical, being thicker at one end of the loop than at the other. RNA synthesis starts at the thinner end and progresses toward the thicker end. Preparations spread for electron microscopy exhibit the typical ‘Christmas tree” images with nascent ribonucleoprotein chains attached perpendicularly to the DNA axis.
Many of the loops correspond to a single transcriptional unit (or single-gene) and they are transcribed continuously from end to end; they form a continuous thin-thick matrix. However, other loops contain several units of transcription (or many genes); such loops include an extended section of chromatin that is not transcribed at all.
For example, Gall et al., (1981) found that in the American newt Notophthalmus viridescens, clusters of the five histone genes are tandemly repeated in the genome but separated by about 50,000 base pairs of repeats of a 225-base pair satellite DNA.
Further, the number of pairs of loops gradually increases during meiosis till it reaches a maximum in diplotene. Such a lampbrush stage may persist for months or years as the primary oocyte builds up a supply of mRNA molecules and other materials required for its ultimate development into a new individual.
As meiosis proceeds further, the number of loops gradually decreases and the loops ultimately disappear either due to disintegration or by reabsorption back into the chromosome. For example, the addition of histone proteins to the lampbrush chromosomes stops the synthesis of RNA on the loops and causes the loops to retract into the chromosomes. Certain hypotheses regarding the nature of loops. The loop of the lampbrush chromosomes can be viewed in the following two ways:
- It may be static, unchanging in length, and constantly exposing the same stretch of chromosome fiber.
- It may be dynamic, with new loop material spinning out of one side of a chromomere and returning to a condensed state on the other side. This is called the spinning out and retraction hypothesis. It means that 100% of the genome can be expressed during the lampbrush stage.
Recent, DNA-RNA hybridization experiments have rejected the spinning out and retraction hypothesis.
Master and slave hypothesis.
Callan and Llyod (1960) suggested that each loop pair and thus each chromomere is associated with the activity of one specific gene. Their master and slave hypothesis were postulated to explain the large size of the chromomere and of lampbrush loop; presently this hypothesis has become obsolete, but still holds interest.
This hypothesis postulates that each loop consists not of one gene, but of a number of duplicate copies, linearly arranged, of one gene. There is a “master” copy at each chromomere and information is transferred from this to each of the “slave” copies which are matched against it to ensure that they are all identical to the master.
The master copy of the gene does not take part in RNA synthesis, but the slave copies of the gene existing in the loop have a role in transcription. The advantage of having a number of duplicate copies of a gene is that a higher rate of RNA synthesis is possible.
The study of both polytene and lampbrush chromosomes provided evidence that eukaryotic gene activity is regulated at the level of RNA synthesis (or transcription). Lampbrush chromosomes also show the possible way of gene amplification that is required during the growth phase of oocytes.