Lampbrush chromosome
Encyclopedia
Lampbrush chromosomes are a special form of chromosomes that are found in the growing oocytes
(immature eggs) of most animals, except mammals. Lampbrush chromosomes of tailed and tailless amphibians, birds and insects are described best of all. Chromosomes
transform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcription
of many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids. Lampbrush chromosomes are clearly visible even in the light microscope
, where they are seen to be organized into a series of chromomeres with large chromatin loops extended laterally. Amphibian and avian lampbrush chromosomes can be microsurgically isolated form oocyte nucleus
(germinal vesicle) with either forceeps or needles.
A given loop always contains the same DNA sequence, and it remains extended in the same manner as the oocytes
grows. 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. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop. 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.
It is thought that the interphase chromosomes of all eukaryotes
are similarly arranged in loops. Although these loops are normally too small and fragile to be easily observed in a light microscope
, other methods can be used to infer their presence. For example, it has become possible to assess the frequency with which two loci along an interphase chromosome are paired with each other, thus revealing candidates for the sites on chromatin that form the closely apposed bases of loop structures. These experiments and others suggest that the DNA in human chromosomes is organized into loops of different lengths. A typical loop might contain between 50,000 and 200,000 nucleotide pairs of DNA, although loops of a million nucleotide pairs have also been suggested.
Giant chromosomes in the lampbrush form are useful model for studying chromosome
organization, genome
function and gene
expression during meiotic prophase, since they allow the individual transcription units to be visualized. Moreover lampbrush chromosomes are widely used for high-resolution mapping of DNA sequences and construction of detail cytological maps of individual chromosomes.
For more information see
http://www.projects.ex.ac.uk/lampbrush/
Oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
(immature eggs) of most animals, except mammals. Lampbrush chromosomes of tailed and tailless amphibians, birds and insects are described best of all. Chromosomes
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
transform into the lampbrush form during the diplotene stage of meiotic prophase I due to an active transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
of many genes. They are highly extended meiotic half-bivalents, each consisting of 2 sister chromatids. Lampbrush chromosomes are clearly visible even in the light microscope
Optical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
, where they are seen to be organized into a series of chromomeres with large chromatin loops extended laterally. Amphibian and avian lampbrush chromosomes can be microsurgically isolated form oocyte nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
(germinal vesicle) with either forceeps or needles.
A given loop always contains the same DNA sequence, and it remains extended in the same manner as the oocytes
Oocyte
An oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
grows. These chromosomes are producing large amounts of RNA for the oocyte, and most of the genes
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
present in the DNA loops are being actively expressed. Each lateral loop contains one or several transcription units with polarized RNP-matrix coating the DNA axis of the loop. 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.
It is thought that the interphase chromosomes of all eukaryotes
Eukaryote
A eukaryote is an organism whose cells contain complex structures enclosed within membranes. Eukaryotes may more formally be referred to as the taxon Eukarya or Eukaryota. The defining membrane-bound structure that sets eukaryotic cells apart from prokaryotic cells is the nucleus, or nuclear...
are similarly arranged in loops. Although these loops are normally too small and fragile to be easily observed in a light microscope
Optical microscope
The optical microscope, often referred to as the "light microscope", is a type of microscope which uses visible light and a system of lenses to magnify images of small samples. Optical microscopes are the oldest design of microscope and were possibly designed in their present compound form in the...
, other methods can be used to infer their presence. For example, it has become possible to assess the frequency with which two loci along an interphase chromosome are paired with each other, thus revealing candidates for the sites on chromatin that form the closely apposed bases of loop structures. These experiments and others suggest that the DNA in human chromosomes is organized into loops of different lengths. A typical loop might contain between 50,000 and 200,000 nucleotide pairs of DNA, although loops of a million nucleotide pairs have also been suggested.
Giant chromosomes in the lampbrush form are useful model for studying chromosome
Chromosome
A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
organization, genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
function and gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
expression during meiotic prophase, since they allow the individual transcription units to be visualized. Moreover lampbrush chromosomes are widely used for high-resolution mapping of DNA sequences and construction of detail cytological maps of individual chromosomes.
For more information see
http://www.projects.ex.ac.uk/lampbrush/
External links
- A site introducing a remarkable form of chromosome that exists during the extended diplotene phase of meiosis in the growing ovarian eggs of all animals except mammals. Compiled and edited by Herbert Macgregor, School of Biosciences, University of Exeter.
- Methods for making lampbrush chromosome preparations from oocytes of birds