A gene is defined as a unit of heredity that is passed along from one generation to the next, according to MedicineNet.com. Genes comprised short sequences of deoxyribonucleic acid, or DNA, and are arranged along chromosomes. Chromosomes are simply long sequences of DNA made up of many genes. Geneticists define "crossing over" as a process by which a pair of chromosomes aligns closely to each other and swap segments of DNA-containing genes during replication. Crossing over is also known as genetic recombination.
There are two different DNA replication processes that occur in plants and animals. The first, mitosis, occurs when a cell replicates to form two copies of itself. The second replication process is called meiosis and occurs only in the creation of sperm or egg cells. Meiosis starts with one cell containing pairs of chromosomes and ends with two cells containing single copies of each chromosome. When a sperm and an egg combine to form a zygote, as an embryo is called in its early stages, they form chromosome pairs. Crossing over occurs during both mitosis and meiosis, though the frequency is much higher in meiosis, according to Science Gateway.
Crossing Over Biology: Alleles
The number of chromosomes an organism has varies among species; humans have 23 pairs, or 46 chromosomes in total. The pairs consist of two copies of each chromosome; however, the copies may not be identical as they often contain different alleles. An allele is an alternative form of a gene, according to Access Science. For example, a DNA segment on each chromosome section may code for eye color, although one chromosome may code for brown eyes and the other for blue eyes. Which eye color is expressed will depend on which gene is dominant. Crossing over occurs most often between different alleles coding for the same gene.
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Mechanics of Crossing Over
Chromosomes normally exist in a compacted, super-coiled state. During mitosis and meiosis, they must be unwound to allow for replication to occur. This happens when enzymes make breaks at several points along the chromosomes, allowing them to unwind and be copied. Following replication, another set of enzymes reattaches the broken fragments of DNA. Chromosome pairs line up in close proximity to one another during these processes. In the unwound and fragmented phase, DNA segments of equal sizes may be swapped and then re-glued, forming a chromosome with a different combination of alleles than it started with, according to Science Gateway.
Frequency of Crossing Over: Meiosis
According to the Origins of Sex, the frequency of crossing over within an individual’s genome is not consistent. There are hot spots, so named as they cross over with greater frequency than average, as well as cold spots that rarely recombine. In humans, a difference has also been noted among sexes, with the average male having crossover events approximately 57 times during meiosis, while in females it is estimated to occur 75 times during the same phase.
A benefit of crossing over is that it maintains genetic diversity within a population, allowing for millions of different genetic combinations to be passed from parents to offspring. Genetic variability is very important to the long-term survival of a species. Without crossing over, meiosis and mitosis cannot produce the genetic diversity necessary for populations to survive adverse conditions, such as drought or disease.