The Concept of Homologous Chromosomes
Homologous chromosomes contain one set of maternal and one set of paternal chromosomes; they are called “homologous” because they have genes that code for the same genetic trait located at corresponding areas. For example, if the maternal chromosome has information that codes for a certain inherited trait located within itself, the paternal chromosome holds its own info for the same trait in the analogous space. The locations of the genes provide a reference point to where the chromosome pairs can align correctly. This fact is very important when it comes to the sharing of genetic material between the two chromosomes, but before we discuss meiotic errors, we must first understand meiosis itself.
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Homologous chromosomes contain one set of maternal and one set of paternal chromosomes; they are called “homologous” because they have genes that code for the same genetic trait located at corresponding areas. For example, if the maternal chromosome has information that codes for a certain inherited trait located within itself, the paternal chromosome holds its own info for the same trait in the analogous space. The locations of the genes provide a reference point to where the chromosome pairs can align correctly. This fact is very important when it comes to the sharing of genetic material between the two chromosomes, but before we discuss meiotic errors, we must first understand meiosis itself.
Click on the button to review meiosis concepts.
Chromosome Reduction in Meiosis
Meiosis reduces chromosomes through a few related steps as cell division occurs. Starting in Prophase I, the homologous chromosomes are brought closer to their partners through synapsis, sections of the genetic material cross over, and their DNA is exchanged through recombination. During Metaphase I, the newly combined chromosomes align themselves on the metaphase plate, and are attached to the poles of the cell by fibers tied to their centromere. In Anaphase I, single pairs of sister chromatids separate from each other as they move to opposite poles; and finally during Telophase and Cytokinesis I, the cell divides in half, leaving the number of chromosomes split in two as well. These steps preformed in Meiosis I are what cause the reduction of chromosomes in a cell, however, it is important to note that these steps preformed in Meiosis II have the same effect on the cell, dividing it further.
Meiosis reduces chromosomes through a few related steps as cell division occurs. Starting in Prophase I, the homologous chromosomes are brought closer to their partners through synapsis, sections of the genetic material cross over, and their DNA is exchanged through recombination. During Metaphase I, the newly combined chromosomes align themselves on the metaphase plate, and are attached to the poles of the cell by fibers tied to their centromere. In Anaphase I, single pairs of sister chromatids separate from each other as they move to opposite poles; and finally during Telophase and Cytokinesis I, the cell divides in half, leaving the number of chromosomes split in two as well. These steps preformed in Meiosis I are what cause the reduction of chromosomes in a cell, however, it is important to note that these steps preformed in Meiosis II have the same effect on the cell, dividing it further.
Meiotic Errors
Despite years of evolution working to perfect Meiosis, errors can take place during cell division and cause abnormalities in chromosome conditions. In humans, for example, proper meiosis results in 23 chromosomes per sperm or egg. Faults in cell division can lead to an improper yield of genetic information, and can harm the reproductive processes of organisms. The most common meiotic error is nondisjunction, this occurs during Anaphase I and causes homologous chromosomes to separate unequally to their respective poles. Therefore, once the cell divides during Telophase and Cytokinesis I, the distribution of homologous chromosomes is uneven. A cell with an abnormal amount of chromosomes is called an aneuploid. If a cell has an extra chromosome, this case is called a trisomy; and if a cell is missing a chromosome, its called a monosomy. Once fertilized, monosomic and trisomic cells lead to a higher risk of birth defects, genetic disorders, or even miscarriage.
Despite years of evolution working to perfect Meiosis, errors can take place during cell division and cause abnormalities in chromosome conditions. In humans, for example, proper meiosis results in 23 chromosomes per sperm or egg. Faults in cell division can lead to an improper yield of genetic information, and can harm the reproductive processes of organisms. The most common meiotic error is nondisjunction, this occurs during Anaphase I and causes homologous chromosomes to separate unequally to their respective poles. Therefore, once the cell divides during Telophase and Cytokinesis I, the distribution of homologous chromosomes is uneven. A cell with an abnormal amount of chromosomes is called an aneuploid. If a cell has an extra chromosome, this case is called a trisomy; and if a cell is missing a chromosome, its called a monosomy. Once fertilized, monosomic and trisomic cells lead to a higher risk of birth defects, genetic disorders, or even miscarriage.