Crossing-over is the exchange of maternal and paternal chromosomes as it occurs during the prophase of meiosis. This piece exchange allows for the trait diversity of offspring. Errors in crossing-over cause diseases such as Wolf-Hirschhorn syndrome.
What is crossing-over?
Crossing-over is the exchange of maternal and paternal chromosomes that occurs during the prophase of meiosis. Crossing-over is the term genetics uses to describe chromosomal crossing over. During this process, there is an exchange of pieces from the maternal and paternal chromosomes. Chromosomes contain DNA that is packaged in proteins. Thus, during crossing-over, the DNA of the offspring is composed of the genetic material from both parents. This step is a part of meiosis, also known as maturity division. It is preceded by a break in the DNA strands caused by enzymes. The crossing-over occurs during prophase one of meiosis and occurs in the synaptonemal complex. This is a structure consisting of proteins, DNA and RNA. Light microscopically, crossing-over can be observed as two chromatids cross over each other. The recombination of the crossing-over is also called intrachromosomal recombination. It follows interchromosomal recombination, which results in a random arrangement of paternal and maternal gametes into zygotes.
Function and task
Genetics distinguishes between mitosis and meiosis. Mitosis is also called simple cell division. During this process, the paternal and maternal chromosome pairs are duplicated and distributed to daughter cells. This means that each of the two halves is assigned a homologous partner after simple division. Thus, the daughter cells again possess twice the chromosome set of the original cells. In a manner of speaking, mitosis doubles the DNA. Compared to this, meiosis is the mature division in which the germ cells are formed. In the first step of meiosis, the paired gene copies are doubled again. A fourfold gene copy set is created. All existing chromosomes are thus present in two chromatids each, i.e. in DNA double strands including the associated proteins. The process of DNA duplication is accompanied by recombination. Alleles are exchanged and the genetic material undergoes a new arrangement. This new arrangement during interchromosomal recombination is left to chance. Thus, during prophase one, the corresponding chromosomes of the mother and father initially attach to each other at random. This attachment is also called zygotene and results in the synaptonemal complex of proteins, DNA and RNA. Zygotene is followed by a phase of mating, also known as pachytene. The structure thus generated is called bivalent because of the duplication of chromosomes. Sometimes it is also called a tetrad, because now there are four chromatids each. Finally, special enzymes generate a break in the DNA strands. This break takes place where the individual chromatids overlap. Intrachomosomal recombination now assembles the chromosome breakages crosswise. Therefore, this process is also referred to as chromosomal crossing-over. Crossing-over thus involves the exchange of entire chromosomal regions between two different chromosomes. Without the crossing-over, no new trait combinations can arise for the offspring. Recombination is only the basis for adaptations to changing environmental conditions. Thus, intrachromosomal recombination is an important component of evolution. Meiosis and intrachromosomal crossing-over are followed by mitosis.
Diseases and disorders
Errors in crossing-over are a significant factor in disease. For example, when two chromosome sequences are very similar during tetrad formation, these sequences may be superimposed even though they are not homologous. This is then referred to as inaequal crossing-over. In this phenomenon, one strand is often lost and another is duplicated. In this context, there is also talk of deletion and duplication. Some errors in crossing-over have no disease value, but others result in combinations that produce non-survivable individuals. In other cases, the survivability is given again, but disease value is connected with the faulty crossing-over.Diseases associated with crossing-over are either chromosomal aberrations or chromosomal dysplasias. As a consequence of such aberrations or dysplasias, for example, congenital diseases such as Huntington’s disease may develop. Huntington’s disease is a hereditary disease of the brain that causes disturbed affects, loss of control over muscles, delusions, and dementia or cognitive impairment. The disease results from an unnaturally high accumulation of glutamine residues encoded by certain genes. Cri-du-chat syndrome is also due to a defect in crossing over. This disease is an example of a hereditary disease resulting from a deletion. In this case, the short arm of chromosome five is deleted. Affected individuals are mentally impaired and emit unusual sounds. They often have a very small head and widely spaced eyes. Wolf-Hirschhorn syndrome is also caused by a defect in crossing over. In this case, part of the short arm of chromosome four is lost. As a result, severe mental retardation and growth retardation occur. A less severe disease caused by errors in crossing-over is red-green vision deficiency. In this disease, genes for certain light sensitivities are missing due to faulty recombination. Thus, the affected person cannot perceive the colors of the corresponding light range. This phenomenon is passed on in X-linked recessive inheritance.
