Lampbrush Chromosomes

Lamp brush chromosomes are so called because they appear like the brushes once used for cleaning glass chimneys of kerosene lamps. Lampbrush chromosomes are found in animal oocytes which grow greatly during prophase meiosis I and which produce a lot of RNA.

Structure.

During early prophase lampbrush are in the form of a pair of homologous chromosomes having a few points of contact or chiasmata. From each chromomere arises a pair of lateral loops.

1. Chromosomal axis. Each chromosome of the pair of homologous chromosomes consists of two chromatids, which are represented by axial filaments. Thus the pair of homologous chromosomes has four filaments in all. The axial filaments and the chromosomeres consist of DNA.

2. Chromomeres. At certain points along their length the axial filaments become tightly coiled. These points are the chromomeres. The choromomeres are found in pair, one chromomere for each filament.



3. Loops. Loops are of two main types, typical and special. Most of the loops are typical. Each typical loop consists of a central axis from which are given off RNA fibrils of progressively increasing lengths. This makes the loop markedly thicker on one side. The special loops have a marked asymmetry and have granules at the end of the fibrils.



Each loop consists of an axial fiber which is covered with a matrix. When the loop is treated with deoxyribonuclease it is broken down, indicating that5 it consists of DNA. When treated with ribonuclease (an enzyme which breaks down RNA), trypsin and pepsin, the matrix of the loop is removed. From this it can be concluded that the matrix consists of RNA and protein.

These granules are probably large molecules of the enzyme RNA polymerase, which synthesizes RNA. Arising from these RNA polymerase molecules are fine fibrils of RNA-protein.