This week, we modeled meiosis. Thankfully, after our recent cell cycle video, we had our process for making the stop motion down. We used two colors of DNA, red and yellow, to distinguish the maternal and paternal genes. To represent the cell membrane, we used colored clay. Though this initially created a large enough cell, as it divided in two, and then four daughter cells, the resulting cells were problematically small. This was a reminder that a large number of small cells has a larger surface area (clay) to volume (cell size) ratio than one large cell. This made it more difficult to illustrate Anaphase II and Telophase II, because the chromosomes did not fit inside the cell. This project also demonstrated that, even though meiosis divides one cell into 4, each of the four has an entire half the number of chromosomes as the original. This is because each of the initial chromosomes is double-stranded, unlike the chromosomes in the 4 daughter cells. Here’s our video:
Our Meiosis Video
What is the function of meiosis? Meiosis splits a diploid cell, a cell with 2n number of chromosomes, into four haploid cells, which have n number of chromosomes. The haploid cells, called gametes, are used in reproduction,and in most cases must fuse with another gamete to create a new organism.
What events promote genetic variation during meiosis? In Prophase I when homologous chromosomes line up, certain corresponding pairs of DNA are switched between the chromosomes by enzymes, called crossing-over. This allows the genes an organism receives from its grandparents to be totally random: that is, if one gene is from their grandfather, the whole rest of the gene is not necessarily so. This allows natural selection to act on individual genes, instead of entire chromosomes.
What causes non-disjunction? Non-disjunction is caused by chromosomes that do not split during metaphase I or II.This can be because the centromeres did not break correctly following Metaphase II, leading haploid one cell to have n+1 chromosomes and its partners to have n-1.
Panda bears have 42 chromosomes compared to 74 chromosomes found in most bears. How could this occur? Explain in terms of non-disjunction. Non-disjunction creates gametes that have a different number of chromosomes than the previous n number. For example, if one gamete does not receive one of its chromosomes due to non-disjunction, it will create an organism with one less chromosome. So, among a bear population, non-disjunction could have eliminated certain chromosomes throughout the generations. This, along with other genetic variations, would create a new species, the panda bear.
What did you learn from this project? This project showed me just how many things can go wrong in meiosis. Ensuring that every chromosomes went to the right place, and that every spindle fiber was just the right length proved how complex of a process this is. We had a joke that if our group were in charge of all cell reproduction, no one would be healthy. Given that meiosis occurs so often in many organisms, this showed how complex the systems that regulate it must be (even if our diagrams are quite simple).