Prophase I is the first stage of meiosis I, and it’s divided into five sub-stages. Crossing over occurs during the pachytene substage of prophase I, when the homologous chromosomes pair up and exchange genetic material. This exchange of genetic material results in the formation of recombinant chromosomes, which are unique and different from the original chromosomes.
This article explains how recombination or crossing over occurs during meiosis and why it’s important.
Crossing Over During Meiosis
Meiosis is a special type of cell division that occurs in our body, which results in the formation of sex cells, i.e., sperm cells and egg cells. This process is essential for sexual reproduction and the production of genetically diverse offspring.
Meiosis involves two rounds of cell division (meiosis I and II), leading to the formation of four haploid daughter cells from a single diploid cell, in which the chromosome numbers are reduced by half.
Crossing over is a crucial process that occurs during meiosis, which is responsible for creating genetic diversity.
During Which Phase of Meiosis Does Crossing Over of Chromosomes Occur?
Crossing over occurs during prophase I of meiosis I, which is the longest stage of meiosis. Prophase I is divided into five sub-stages, namely leptotene, zygotene, pachytene, diplotene, and diakinesis.
During zygotene, the homologous chromosomes pair and begin to undergo synapsis, where they become tightly associated, forming a structure called the bivalent or tetrad.
Synapsis facilitates crossing over between homologous chromosomes, which occurs during pachytene, where the bivalents begin to separate but remain connected at regions called chiasmata. {1}
Once the process of crossing over is complete, the homologous chromosomes separate.
The Process of Crossing Over
Haploid cells contain half the number of chromosomes found in diploid cells — the normal body cells found in tissues and other organs also called somatic cells. During meiosis, the homologous chromosomes, which are pairs of chromosomes that contain the same genes at the same loci but can have different versions of those genes, undergo a process known as synapsis, where they line up next to each other.
This pairing is followed by a process called crossing over, where genetic material is exchanged between homologous chromosomes. Crossing over is essential for genetic diversity, as it leads to the creation of new combinations of genetic information that were not present in either parent.
The process is divided into five stages — leptotene, zygotene, pachytene, diplotene, and diakinesis. During the pachytene stage, homologous chromosomes pair up and form a bivalent, which is a structure consisting of two chromosomes that are connected by a protein complex called the synaptonemal complex.
The synaptonemal complex holds the homologous chromosomes tightly together, which allows the exchange of genetic material to occur. The exchange of genetic material between homologous chromosomes occurs when they break and rejoin with each other.
The breakage and rejoining of the chromatids within a chromosome occur at specific sites called chiasmata, which are visible under a microscope as physical connections between the homologous chromosomes. The exchange of genetic material between homologous chromosomes at chiasmata creates new combinations of alleles, which are different versions of a gene that are inherited from each parent.
The frequency of crossing over varies among different organisms and different regions of the chromosome. {2}
However, some regions of the chromosome are more prone to crossing over than others, which can lead to the uneven distribution of genetic material. Several factors can influence the frequency of crossing over, including the length of the chromosome, the distance between genes, and the presence of genetic markers.
Genetic markers are identifiable traits that are controlled by specific genes and can be used to track the inheritance of those genes across generations. Abnormalities in the process of crossing over can result in genetic disorders, such as Down syndrome, which is caused by an extra copy of chromosome 21.
Down syndrome is usually caused by a failure of the homologous chromosomes to separate correctly during meiosis I, which leads to the presence of an extra chromosome in the resulting gametes.
The Importance of Crossing Over
By exchanging genetic material between homologous chromosomes, crossing over generates new combinations of alleles, which are different versions of a gene that are inherited from each parent. The creation of new combinations of alleles through crossing over is critical for the survival of species, as it increases the genetic variability of populations.
This genetic diversity allows populations to adapt to changing environments, resist diseases, and avoid inbreeding depression, which is the reduction in fitness that occurs when closely related individuals mate and produce offspring.
Crossing over also plays a crucial role in the evolution of species. By generating new combinations of alleles, crossing over creates genetic variation that can be acted upon by natural selection.
Natural selection is the process by which individuals with advantageous traits are more likely to survive and reproduce, while individuals with less favorable traits are less likely to do so.
As a result of natural selection, the frequency of alleles in a population can change over time, leading to the evolution of new species.
In the classic example of the peppered moth, the frequency of dark-colored alleles increased in populations of moths living in industrial areas, as they were better camouflaged against pollution-darkened trees, which increased their survival rates.
Crossing over is also important for genetic mapping, which is the process of determining the relative positions of genes on a chromosome.
By tracking the inheritance of genetic markers — identifiable traits that are controlled by specific genes — geneticists can identify regions of the chromosome more prone to crossing over and map the location of genes associated with specific traits or diseases.
Frequently Asked Questions
Sources
1 – Gottlieb, S. F., Gulani, A., Tegay, D. H. StatPearls, “Genetics, Meiosis.” StatPearls Publishing, 2023.
2 – School of Biological Sciences, University of Sydney: Crossing over – Genetics.