Allelic exclusion
Encyclopedia
Allelic exclusion is a process by which only one allele
of a gene is expressed while the other allele is silenced. For most genes, the individual inherits one copy of each gene from each parent.
At least two distinct selection events can lead to allelic exclusion. On one hand, one allele of the gene
can be transcriptionally silent, which would result in the expression of only the second allele. On the other hand, both alleles can be transcribed, in which case posttrancriptional and posttranslational mechanisms will lead to the elimination of the protein
product of one allele.
Though the mechanism by which allelic exclusion occurs is not fully understood.
gene rearrangement of the genetic material from one chromosome results in the shutting down of rearrangement of genetic material from the second chromosome. If no successful rearrangement occurs, rearrangement of genetic material on the second chromosome takes place. If no successful rearrangement occurs on either chromosome, the cell dies. As a result of allelic exclusion, all the antigen receptors on an individual lymphocyte
will have the same amino acid sequence in the variable domain of the heavy chain protein. As the specificity of the antigen receptor is modulated by the variable domain of the light chain encoded by one of the immunoglobulin light chain loci, the specificities of B cells containing the same heavy chain recombination event can differ according to their light chain recombination event.
pA-methylation
helps for allelic exclusion in sensory neurons.
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
of a gene is expressed while the other allele is silenced. For most genes, the individual inherits one copy of each gene from each parent.
At least two distinct selection events can lead to allelic exclusion. On one hand, one allele of the 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...
can be transcriptionally silent, which would result in the expression of only the second allele. On the other hand, both alleles can be transcribed, in which case posttrancriptional and posttranslational mechanisms will lead to the elimination of the protein
Protein
Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
product of one allele.
Though the mechanism by which allelic exclusion occurs is not fully understood.
Allelic exclusion in B-Lymphocytes
Allelic exclusion has been observed most often in genes for cell surface receptors and has been extensively studied in immune cells such as B lymphocytes. In B lymphocytes, successful heavy chainHeavy chain
]The immunoglobulin heavy chain is the large polypeptide subunit of an antibody .A typical antibody is composed of two immunoglobulin heavy chains and two Ig light chains. Several different types of heavy chain exist that define the class or isotype of an antibody. These heavy chain types vary...
gene rearrangement of the genetic material from one chromosome results in the shutting down of rearrangement of genetic material from the second chromosome. If no successful rearrangement occurs, rearrangement of genetic material on the second chromosome takes place. If no successful rearrangement occurs on either chromosome, the cell dies. As a result of allelic exclusion, all the antigen receptors on an individual lymphocyte
Lymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...
will have the same amino acid sequence in the variable domain of the heavy chain protein. As the specificity of the antigen receptor is modulated by the variable domain of the light chain encoded by one of the immunoglobulin light chain loci, the specificities of B cells containing the same heavy chain recombination event can differ according to their light chain recombination event.
Allelic exclusion in sensory neurons
A recent study showed that CCytosine
Cytosine is one of the four main bases found in DNA and RNA, along with adenine, guanine, and thymine . It is a pyrimidine derivative, with a heterocyclic aromatic ring and two substituents attached . The nucleoside of cytosine is cytidine...
pA-methylation
DNA methylation
DNA methylation is a biochemical process that is important for normal development in higher organisms. It involves the addition of a methyl group to the 5 position of the cytosine pyrimidine ring or the number 6 nitrogen of the adenine purine ring...
helps for allelic exclusion in sensory neurons.
Further reading
- Cellular and Molecular Immunology (5th Ed.) Abbas AK, and Lichtman AH., Editor: Saunders, Philadelphia, 2003.