5.14:
Lampbrush Chromosomes
Amphibians, like many other vertebrates, produce female gametes, or ova, which mature and develop within the ovary.
At one particular developmental stage, meiotic prophase-I, homologous replicated chromosome pairs display a distinct morphology- an enormously extended structure, so gigantic that it can be easily observed under a light microscope
These very unusual chromosomes are called ‘lampbrush chromosomes’ due to their resemblance to the brushes used in earlier times to clean kerosene lamps. Due to their enormous size, they are ideal models for studying chromosomes.
These giant chromosomes are organized into a series of large lateral, uncoiled DNA loops all along the chromosome axis. In addition to these extended loops, dense, thick coiled loops are observed along the chromosomal axis, which constitute the majority of DNA.
These two domains also differ greatly in the level of transcriptional activity. The genes in the lateral loops are actively transcribed, which gives them a fine fibrillar appearance.
In contrast, the genes on the condensed chromosome axis are generally not expressed.
Although they were first described in amphibians, these looped chromatin domains are now believed to occur in the interphase chromosomes of all eukaryotes.
Due to their small size and fragile nature, the looped chromatin can’t be observed using a light microscope in most eukaryotes. Their presence has been inferred using modern DNA technologies such as ‘Chromosome Conformation Capture.'
5.14:
Lampbrush Chromosomes
In 1882, Flemming observed lampbrush chromosomes (LBC) in salamander eggs. Later in 1892, Rückert observed LBCs in shark egg cells and coined the term "lampbrush chromosomes" because they looked like brushes used to clean kerosene lamps.
LBCs are made up of two pairs of conjugating homologous chromatids. Each chromatid consists of alternatively positioned regions of condensed-inactive chromatin and loosely placed-active side loops, which can be contracted and extended. The loops resemble the puffs of polytene chromosomes. The polytene puffs are composed of several parallel chromatids, whereas the loops of LBC consist of a single, double helix.
During the diplotene stage of meiosis prophase, LBCs decondense forming large chromosomes, approximately 30 times larger than regular mitotic chromosomes. The average length of LBC loops is 10-15 µm. In some cases, loops can be as large as 50-100 µm. Polymerase II transcribes the largest loops, and the smallest loops are transcribed by polymerase III.
LBCs are present in the oocytes of lower vertebrae, invertebrates, and birds. LBCs in all these organisms share similar structures and functions. Comparative genome studies of LBCs have shown that the length of side loop increases with the C-value, which is the total DNA content in the haploid set of an organism.
LBCs are studied for over a hundred years, yet, only a general structural idea of LBC is known. Recently, LBCs are used as model structures to study cytogenetic analysis and epigenetic regulation of chromatin structure and gene expression.
Suggested Reading
- Molecular Biology of Cell, Alberts, 6th edition, Pages 207-208
- Andraszek, Katarzyna, and Elżbieta Smalec. "Structure and functions of lampbrush chromosomes." BioTechnologia (2011).