Acentric fragment
Encyclopedia
An acentric fragment is a segment of a chromosome
that lacks a centromere
.
Because the centromere is the point of attachment for the mitotic apparatus, acentric fragments are not evenly distributed to the daughter cells in cell division (mitosis
and meiosis
). As a result one of the daughters will lack the acentric fragment.
Lack of the acentric fragment in one of the daughter cells may have deleterious consequences, depending on the function of the DNA in this region of the chromosome. In the case of a haploid organism or a gamete
, it will be fatal if essential DNA is contained in that DNA segment. In the case of a diploid organism, the daughter cell lacking the acetric fragment will show expression of any recessive genes found in the homologous chromosome. Developmental geneticists look to cells and cell lineages lacking unpaired chromosome segments produced this way as a means of identifying essential genes for specific functions.
Acentric fragments are commonly generated by chromosome-breaking events, such as irradiation
. Acentric fragments can also be produced when an inverted segment is present in one member of a chromosome pair. In that case, a crossover event will result in one chromosome with two centromeres and an acentric fragment. The acentric fragment will be lost as explained above, and chromosomes with two centromeres will break unevenly during mitosis, resulting in one daughter lacking essential genes.
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...
that lacks a centromere
Centromere
A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment...
.
Because the centromere is the point of attachment for the mitotic apparatus, acentric fragments are not evenly distributed to the daughter cells in cell division (mitosis
Mitosis
Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...
and meiosis
Meiosis
Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
). As a result one of the daughters will lack the acentric fragment.
Lack of the acentric fragment in one of the daughter cells may have deleterious consequences, depending on the function of the DNA in this region of the chromosome. In the case of a haploid organism or a gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...
, it will be fatal if essential DNA is contained in that DNA segment. In the case of a diploid organism, the daughter cell lacking the acetric fragment will show expression of any recessive genes found in the homologous chromosome. Developmental geneticists look to cells and cell lineages lacking unpaired chromosome segments produced this way as a means of identifying essential genes for specific functions.
Acentric fragments are commonly generated by chromosome-breaking events, such as irradiation
Irradiation
Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...
. Acentric fragments can also be produced when an inverted segment is present in one member of a chromosome pair. In that case, a crossover event will result in one chromosome with two centromeres and an acentric fragment. The acentric fragment will be lost as explained above, and chromosomes with two centromeres will break unevenly during mitosis, resulting in one daughter lacking essential genes.