Which processes increase variation during meiosis?

Principles of Biology

Adapted by Dr. Angela Hodgson

39 Meiosis and Sexual Reproduction

Mechanisms that Increase Genetic Variation

The evolution of life on planet Earth is a dynamic process that is a direct result of genetic variation. Mutations in an organism's DNA produce changes in genes. When these changes occur, they may provide either a beneficial phenotypic change or a nonbeneficial one. Over time, processes such as natural selection favor organisms with mutations that are beneficial. If these changes provide enough of a benefit in the sexual reproduction of an organism, the mutations will become more prevalent in the population as a whole.

Figure 12: The sea anemone (order Actiniaria) reproduces via asexual reproduction.

In organisms like the sea anemone that reproduce without meiosis, the opportunity for introducing genetic variation into a population is very limited.

Courtesy of Laszlo Ilyes. Some rights reserved.

One significant advantage for genetic variation produced by sexual reproduction over the consistency of asexual reproduction is seen with viral disease. In many species that produce asexually, such as the sea anemone, a single virus may have devastating effects on a population (Figure 12). In sexually reproducing organisms with a varied gene pool, a virus will likely have a less detrimental effect because some of the genetic variations that arise may provide some degree of resistance to the virus. In humans, for example, gene variants that confer resistance to viruses have probably been favored by natural selection. For example, a study in 2010 from the University of Milan analyzed 52 populations worldwide and identified 139 human genes that modulate susceptibility to viral infections.

Independent assortment produces new combinations of alleles.

How is genetic variation generated? There are several points during sexual reproduction at which genetic variation can increase. In meiosis I, crossing over during prophase and independent assortment during anaphase creates sets of chromosomes with new combinations of alleles. Genetic variation is also introduced by random fertilization of the gametes produced by meiosis. Any of the genetically unique sperm generated by a male may fertilize the genetically unique egg produced by a female.

During metaphase I, the homologous pairs of chromosomes are aligned along the metaphase plate. The orientation of the homologous pairs is random and is different for every cell that undergoes meiosis. In humans, this arrangement involves 23 different tetrads. Each tetrad contains one maternal and one paternal pair of sister chromatids. In one cell, for example, the tetrad corresponding to human chromosome 1 may align so that its paternal sister chromatids face toward one pole while the maternal sister chromatids are facing toward the other pole. And there is a 50% chance that the opposite orientation will occur during metaphase I in another cell.

Calculating the Probability of Genetic Variation

The number of different gamete types resulting from independent assortment of the homologs is calculated using the formula 2n where n = the number of chromosomes in a haploid cell of a given species. How many different combinations are possible in a fruit fly with 4 different chromosomes and a human with 23 different chromosomes?

In the fruit fly n = 4, so there are 24 different assortments possible = 2 x 2 x 2 x 2 = 16 possible chromosome combinations in the gametes.

In humans, however, where n is a much larger 23, there are far more possibilities. In fact, there are almost 8.4 million (223) different ways that maternal and paternal chromosomes could be assorted in gametes. This results in a massive amount of variation in the gametes that form at the end of meiosis II.

Crossing-over involves switching sections of DNA between two non-sister chromatids.

Recombinant chromosomes are made of DNA that has been randomly transferred between two non-sister chromatids of two homologous chromosomes. Crossing-over occurs early during prophase I while the homologous chromosome pairs begin to loosely bind to each other. While they are connected, the two non-sister chromatids switch sections of DNA at specific points. Usually one to three crossover events occur per chromosome depending on the size and species (Figure 13).

Random fertilization increases genetic diversity.

When a male gamete and a female gamete finally meet, each is the result of an immense number of genetic possibilities created during independent assortment and crossing over. Human diploid cells have 23 pairs of chromosomes. Because of independent assortment during meiosis I, there are 223, or 8.4 million possible gametes that may be created even if crossing over didn't occur. In humans, there are about 2 to 3 crossovers per chromosome, and different crossovers occur in each meiotic division, so there is the potential to produce an enormous number of unique gametes.

Errors during meiosis.

The process of meiosis is highly regulated. Many different molecules and proteins are responsible for regulating the steps of meiosis as they occur. If any mistakes occur during the replication of chromosomes, it may have drastically detrimental effects on the resulting offspring. Errors may also occur during chromosome segregation. The term nondisjunction is used to describe the abnormal separation of chromosomes, resulting in the wrong number of chromosomes going to each gamete. There are many different human disorders that are the direct result of the incorrect separation and movement of homologous chromosome pairs. Disorders that are a direct result of nondisjunction include Down syndrome, Klinefelter syndrome, and XYY syndrome.

Updated April 05, 2018

By David H. Nguyen, Ph.D.

The advantage of sexual reproduction is that it generates genetic diversity, which makes a population of mating organisms better able to survive environmental pressures. Meiosis is the process of producing gametes, which are sperm cells and egg cells. Gametes have only half the number of chromosomes that normal cells have, because a sperm and an egg fuse to form a cell that has the full number of chromosomes. Genetic diversity arises due to the shuffling of chromosomes during meiosis.

A man produces sperm and a woman produces eggs because their reproductive cells undergo meiosis. Meiosis starts with one cell that has the full number of chromosomes specific to each organism -- human cells have 46 chromosomes. It ends with four cells, called gametes, that each have half the full number of chromosomes. Meiosis is a multi-step process in which a cell makes a copy of each strand of DNA, called a chromosome, and then divides twice. Each time it divides, it cuts its DNA content in half. In humans, a cell goes from having 46 strands of DNA, and then 96 after each is copied. The first division of meiosis cuts 96 in half into 46. The second division cuts 46 into 23, which is the number of chromosomes in a sperm or an egg.

At the beginning of meiosis, the chromosomes condense from long strands into short, thick finger-like structures. In humans, condensed chromosomes look like an X. Half of the 46 chromosomes in a human cell came from the mother, while the other 23 are similar but came from the father -- they form 23 pairs, like 23 pairs of non-identical twins. Chromosomes that form a pair are called homologous chromosomes. During the early part of meiosis, the homologous chromosomes pair up with their non-identical twins and exchange regions of DNA. This process is called crossing over, and results in a shuffling of DNA regions between two homologous chromosomes. Chromosomes are purposely broken and rejoined in new combinations.

Meiosis not only shuffles regions of DNA between homologous chromosomes, it shuffles whole chromosomes among the four gametes that result at the end. The distribution of chromosomes among four gametes is called random segregation. If the process of “crossing over” is like tearing blue cards and red cards apart, and then taping the pieces together to get striped cards, then “random segregation” is combining a red deck and a blue deck, shuffling them, and then randomly dividing them into four decks. Random segregation produces four decks of cards that contain different combinations of blue and red cards.

The third way that meiosis generates genetic diversity is through the separation of homologous chromosomes into the gametes. As described above, homologous chromosomes are like pairs of non-identical twins. One chromosome of the pair came from mom, the other from dad. Each homologous chromosome can contain the same genes, or slightly different versions of the same gene -- which is why they are like non-identical twins and not identical twins. Independent assortment describes the process in which the two homologous chromosomes of a pair must go into separate gametes. This ensures that each gamete can have only one of two homologous chromosomes, meaning each can have only one version of a gene, though the original cell might have had two slightly different versions of a gene.