Stages of Meiosis I and II Mechanisms of Genetic Variation

Concept 6: In meiosis, two rounds of cell division commence; Meiosis 1 and Meiosis 2. The cells go through the “PMAT” stage the same in the two, but both rounds go through each stage differently. 1.​ In Meiosis 1, there is one whole cell, and it is ready to divide through meiosis. ​ - Prophase 1: The chromosomes condense, they go into homologous pairs. Crossing over occurs and chromosomes exchange genetic information. The nuclear membrane breaks down - Metaphase 1: The chromosomes are lined up by the spindle fibres in their homologous pairs(IMPORTANT: Homologous chromosomes can line up on any side, this is the process independent assortment and is one of the factors of genetic variation. - Anaphase 1: Homologous chromosomes are pulled to either pole of the cell(not chromatids). - Telophase 1: Chromosomes are on each pole of the cell, and nuclear membranes start to form on each side. 2.​ In Meiosis 2, after cytokinesis, there are two daughter haploid cells that are formed and ready to be split into 4. -​ Prophase 2: Chromosomes condense and nuclear membrane breaks down again, Homologous pairs are not formed. -​ Metaphase 2: Chromosomes are lined up single file in the equator of the cell, held together by the spindle fibres. -​ Anaphase 2: Sister chromatids are separated onto opposite sides of the cell. -​ Telophase 2/cytokinesis: The Nuclear membrane is formed around the two sides of the cells and they are divided into four genetically varying cells. Learning task A: The pool noodle analogy of meiosis is a great tool to use to understand the process. In our example, there are four different pool noodles with two different sizes. -​ -​ -​ -​ -​ -​ -​ -​ In the first stage(after “interphase”), the pool noodles are paired up with noodles with the same size and letters written in tape(representing genes). Areas of the two pool noodles are switched and swapped between the pairs(representing crossing over). Next, the noodle pairs are lined up beside each other at the middle of the cell(their order being random→Independent assortment). After, representing anaphase, the pairs of noodles split onto opposite sides, the noodles are still whole and not split yet. To complete the first stage of PMAT, the noodles start forming a barrier(nuclear membrane) around them on each side. Prophase II​ Each cell has a few single pool noodles (still joined in the middle). No pairing happens. The noodles get ready to split again. Metaphase II​ Noodles line up single file down the center of each cell. The order is random. Anaphase II​ Each noodle is split in half down the middle. The halves (sister chromatids) move to opposite sides. Telophase II​ A barrier (nuclear membrane) forms around the noodle halves. The cells divide again, forming four unique cells with one half-noodle each. -​ Concept 7/Learning task B: In Meiosis, the ploidy number of a cell, which means the sets of chromosomes, halves from 2n, to n(N refers to the number of unique chromosomes). -​ For example, in most human cells, n would equal 23, as we have 23 unique chromosomes, and the coefficient of n would be 2, to make 2n. This is because humans have 2 sets of chromosomes, therefore we have diploid body cells. -​ Karyotypes are pictures of a cell, with all of its chromosomes lined up by Homologous pairs. It is used to detect additional chromosomes and/or to check for genetic mutations and syndromes. -​ RECAP: N tells us how many unique chromosomes are in a cell The coefficient(2n) tells us how many sets of chromosomes there are C tells us how many molecules/DNA there are in the cell. Concept 8: During meiosis, technically every cell is made differently, and this is due to two factors, independent assortment and crossing over. -​ -​ Independent assortment consists of homologous chromosomes to be in random orders when lined up in metaphase 1. This allows for gametes to be produced with many different combinations of chromosomes. Crossing over is when two paired homologous chromosomes exchange genetic information, this happens in prophase 1. Crossing over allows for chromosomes to be genetically unique and allows chromosomes in gametes to vary. Learning task C: This image represents pairs of homologous chromosomes for each person. -​ For the girl on the left, there are mostly one or two colour schemes. This indicates that her parents’ genes are probably similar and her parents most likely have similar origins. -​ For the man on the right, we can see many different colours throughout the karyotype of his chromosomes. This indicates that the man’s parents have unsimilar heritage and very different DNA and genetic information.