compare and contrast mitosis and meiosis

He also proposed a model to explain the phenomenon, involving shifting the position of Ab10 toward the meiotic spindle poles in anaphase. Rhoades showed that Ab10 preferentially segregates into the egg in female meiosis. Ab10 contains an extra DNA segment, termed a knob, that includes a repetitive DNA sequence. Geneticist Marcus Rhoades introduced the concept of meiotic drive in 1942 based on observations of abnormal chromosome 10 (Ab10) in maize.

The chromosomal races on Madeira Island and elsewhere show how drive can lead to rapid karyotype change and reproductive barriers between populations that have accumulated different sets of fusions. White: “It may be that the very few chromosomal rearrangements which play a critical role in speciation through the ability to generate powerful isolating mechanisms are precisely those which happen to possess a segregational advantage in the female meiosis.” Rb fusions are an example of such a rearrangement that can generate a segregational advantage (i.e., drive) through centromere expansion. Meiotic drive of Rb fusions illustrates an idea proposed more than 50 years ago in a paper by zoologist Michael J. Thus, it seemed that newly formed Rb fusions could result in larger centromeres that would drive and become fixed in natural populations.Ĭentromere drive depends on a combination of asymmetries in female meiosis. Centromere DNA is typically highly repetitive, and we found that larger centromeres have more of the satellite repeats characteristic of mouse centromeres and more centromere proteins associated with that DNA. Focusing on the centromere-the part of each chromosome that interacts with spindle microtubules to direct segregation in mitosis or meiosis-we found that the structure’s size determines the direction of biased segregation, with bigger centromeres preferentially segregating into the egg. The Madeira mice suggest that fusion chromosomes can also drive unequal inheritance.īecause Rb fusions are easy to identify morphologically, and because mouse oocytes are an established model system, studying these fusions in mice provided an entry for my lab at the University of Pennsylvania to investigate the cell biology of meiotic drive, starting in 2010. Scientists have known for decades, however, that selfish genes can subvert Mendelian segregation to increase their frequency in the next generation, a phenomenon known as meiotic drive.

This law explains the 3:1 ratio of phenotypes that Mendel observed in his classic studies of heredity. As we learn in high school biology, if a diploid individual carries two different alleles of a gene (i.e., is heterozygous), then either allele is equally likely to end up in a haploid gamete.

We usually think of the chromosome segregation machinery as ensuring unbiased, random segregation. But their accumulation in the populations of Madeira Island and in multiple other isolated mouse populations elsewhere is likely due to another influencing factor: the preferential segregation of the Rb fusion into the egg rather than into the discarded polar bodies that form during female meiosis. The so-called Robertsonian (Rb) fusions that led to these rapid karyotype changes are relatively common chromosomal rearrangements.

Compare and contrast mitosis and meiosis