How Crossing Over and Chromosomal Mutations Effect Genetic Diversity By Michael Downes May 22, 2012
Meiosis In order to genetically combine two cells and still have the correct number of chromosomes, you need to create two cells containing only 23 pairs of chromosomes instead of the normal 46 pairs found in regular body cells. Each new or split cell is called haploid (23 chromosomes) instead of diploid (46 chromosomes). The 46 Figure 1 shows the separation of Sister Chromatids during meiosis II chromosomes in a diploid cell represent one set of 23 chromosomes from the mother and another set of 23 chromosomes from the father. A homologous chromosome is a chromosomes pair: one from the mother and one from the father. Each pair has versions of the same genes, although the versions are not exactly alike, since each comes from a different parent. There are also homologous chromosomes that contain sister chromatids. Sister chromotids are chromosomes that stay attached along the centromere during the first phase of meiosis. There are still two sets, one from the mother and one from the father. Together the two sister chromotids still create a homologous chromosome pair. During the first part of meiosis, meiosis I, the chromosomes line up by duplicate pairs along the cell equator (metaphase I), and are then separated into two sets (anaphase I), which become two separate cells. During this first meiosis, sister chromatids are not separated. In meiosis II, sister chromatids line up along the cell equator (metaphase II) and are then separated into two sets (anaphase II), creating two new cells. Figure 2 shows two sister chromatids, each part of a homologous chromosome. It shows that sister chromatids are identical stands of DNA joined by a centromere.
Crossing Over Sometimes, before sister chromatids divide, the genes from one sister cross over into the other sister’s space. If this happens, when the sister’s divide, each division gets pieces of both sisters. In this illustration, some genes from the maternal sister (red) cross over into the paternal sister’s space (orange). This creates two different sisters than were there before. How Crossing Over Impacts Genetic Diversity Let’s look at an example of how crossing over effects genetic diversity. Let’s say the two sister chromatids were made up of the following sets of genes, A, B & C. On the paternal side the genes are dominate A, dominate B, and dominate C on one strand (left to right), and recessive a, recessive b, and recessive c on the other strand (right to left). Since sister chromatids Figure 3 Shows how the legs of two sister chromatids, are identical, the maternal side will be both part of the same homologous chromosome, have crossed over each other. When the sisters split, they have exactly the same: ABC left to right, swapped legs. and abc right to left. When these strands are separated, they will be as follows: two ABC (one maternal, one paternal), and two abc (one maternal, one paternal). So there are really only two unique sets of gene traits being passed on. Instead, let’s assume the sister chromatids crossed over, such that on the paternal side, the left to right strand of ABC, losses the lower gene (C), but gains the same gene from the maternal side (c). The same in reverse happens with the maternal sister. The right to left set of abc, losses the lower c, and it is replaced by a C. This means that when the sister chromatids split, instead of two ABC and two abc, there will be one ABc and one abc paternal, and one ABC and one abC maternal. There are now four unique sets of genes to combine in reproduction instead of just two. This doubles the genetic diversity of the potential offspring.
Genetic Mutation A mutation is a change in an organism’s DNA. There are many types of genetic mutation. Mutations that affect a specific gene happen during replication during mitosis. Mutations that affect entire chromosomes happen during meiosis. Affected chromosomes are passed on to offspring and affect the genetic diversity of a species. Gene Duplication One type of mutation that affects the entire chromosome is called gene duplication. This sort of mutation happens during the second part of meiosis with sister chromatids. The top half of the picture below shows an example of gene duplication. Like figure 3 above, two sister chromatids have crossed over their legs. But unlike figure 3, the line-up of the two legs is not symmetrical. When the sisters split with the unevenly crossed legs, the swap is not even. One sister gets a longer leg and one a shorter leg. The sister with the longer leg now has duplicate genes, while the sister with the shorter leg is missing a gene. Using our previous example of genes ABC and abc as the two parts of each sister, after the uneven split, the results are four strands: AB, abc (for the left sister), abcc, ABC (for the right sister). So, instead of one set of ABC and one set of abc, four unique strands are created. These strands will be passed on to offspring and increase the genetic diversity of the group. Gene Translocation Gene translocation is another way that chromosomes may change, put this happens in non-homologous chromosomes, which are not part of reproduction. These affect the organism itself, but they are not passed on to offspring. Figure 4 shows an example of gene duplication with sister chromatids on the top, and gene translocation of regular chromosomes on the bottom.
References: Figure 1: biology.iupui.edu, meiosis2cropped.jpg Figure 2: academic.brooklyn.cunyu.edu, homologous-chrom1669-1.jpg Figure 3: phschool.com, crossovr.gif Figure 4: Biology text, Holt McDougal, Stephen Nowicki, chapter 8.7, pages 253, bottom of page. Remainder of information taken from Biology textbook by Stephen Nowicki, published by Holt McDougal.