Lampbrush Chromosomes:
Lampbrush chromosomes are found in oocytes of many invertebrates and all vertebrates, except mammals; they have also been reported in human and rodent oocytes. But they have been the most extensively studied in amphibian oocytes. These chromosomes are most distinctly observed during the prolonged diplotene stage of oocytes. During diplotene, the homologous chromosomes begin to separate from each other, remaining in contact only at several points along their length. Each chromosome of a pair has several chromomeres distributed over its length; from each of a majority of the chromomeres generally a pair of lateral loops extends in the opposite directions perpendicular to the main axis of the chromosome. In some cases, more than one pair, even up to 9 pairs of loops may emerge from a single chromomere. These lateral loops give the chromosomes the appearance of a lampbrush which is the reason for their name ‘lamp-brush chromosomes.’
These chromosomes are extremely long, in some cases being 800-1000^ in length. The size of loops may range from an average of 9.5ja in frog to 200|i in newt. The pairs of loops are produced due to uncoiling of the two chromatin fibres (hence the two sister chromatids) present in a highly coiled state in the chromosomes; this makes their DNA available for transcription (RNA synthesis). Thus each loop represents one chromatid of a chromosome and is composed of one DNA double helix. One end of each loop is thinner (thin end) than the other end (thick end). There is extensive RNA synthesis at the thin ends of loops, while there is little or no RNA synthesis at the thick end. The chromatin fibre of the chromomere is progressively uncoiled towards the thin end of a loop; the DNA in this region supports active RNA synthesis but later becomes associated with RNA and protein to become markedly thicker. The DNA at the thick end of a loop is progressively withdrawn and reassembled into the chromomere. The number of pairs of loops gradually increases in meiosis till it reaches maximum in diplotene. As meiosis proceeds further, number of loops gradually decreases and the loops ultimately disappear due to disintegration rather than reabsorption back into the chromomere.
Loops represent the sites of gene action (transcription), and the function of lampbrush chromosomes is to produce the large numbers and quantities of proteins and RNA’s stored in eggs.