Cell replication and division are processes vital to the growth and reproduction of all living organisms. Prior to dividing, cells must first duplicate their genetic material or DNA. Crossing over, otherwise known as genetic recombination, refers to the swapping of DNA segments between two chromosomes during replication. Following this replication, cells separate or divide and the two resulting cells are known as daughter cells. One consequence of genetic recombination is the production of daughter cells that are different than the original parent cell, thereby introducing genetic variability.
Alleles
Higher organisms, such as plants and animals, have two copies of each chromosome, referred to as chromosome pairs. This means that cells carry two copies of each gene, although the copies may not be identical as they may contain different alleles. Alleles are simply different forms of a gene that codes for the same trait. Biology Online cites the example of a gene that codes for eye color in which one allele codes for brown eyes and another for blue. The eye color that appears depends on the dominant allele. Crossing over usually occurs between chromosome sections containing alleles for the same gene.
Types of Cell Division
Two distinct methods of DNA replication and cell division are used in higher organisms--mitosis and meiosis. During mitosis, cells replicate themselves fully and separate, resulting in the formation of two copies of the original cell. Meiosis is a separate process that is only utilized when forming gametes, which are known as egg and sperm cells. Meiosis begins with a cell containing chromosome pairs and ends with daughter cells that have only a single copy of each chromosome. When sperm and egg cells combine during reproduction, their chromosomes combine to form the chromosome pairs for the embryo.
Crossover Process
Chromosomal DNA normally exists in a tightly wound, super coiled state known as the double helix. During mitosis and meiosis, chromosomes must be unwound in order to be copied. This is accomplished by a group of enzymes that make small breaks in the DNA, causing the coil to lose its structure. DNA is then copied and reattached by a second group of enzymes. According to "Origins of Sex," chromosomes are lined up in close proximity to one another during the fragmented state and occasionally swap segments of DNA while it is being copied and reattached, in essence recombining their genes.
Frequency
Crossing over occurs most commonly between segments of DNA of the similar size and location on the chromosome. It does not occur uniformly, as areas of more or less frequent gene swapping have been observed. These areas are termed hot and cold spots. The "Origins of Sex" also reports that recombination events are more frequent in meiosis than mitosis and may even be more common in females than males.
Benefits of Variability
Genetic variability is essential to the continuing viability of individual species as well as entire ecosystems. According to "Origins of Sex," without the variability that leads to biodiversity, entire populations of species might be decimated by famine, drought or a single disease. Ecosystems themselves rely on the balances created by the species that live within them and without genetic variability these balances could be easily thrown off.
