Episode 16 gets some swanky tunes for sex cell and fertilization discussions.
Episode 16 gets some swanky tunes for sex cell and fertilization discussions (1:00). PMAT is coming back into play with new terminology of haploid and homologous pairs. Melanie begins with a recAP of Meiosis (2:39) before contrasting the two cellular division pathways(4:33). There are two main contributors to genetic variation through meiotic pathways (3:14). Melanie recommends organizing this information in a table when studying!
The Question of the Day asks (5:30) “When do human egg cells complete Meiosis II?”
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap Mitosis vs. Meiosis
Lets Zoom out:
Unit 5 - Heredity
Topic - 5.1 and 5.2
Big idea - Information Storage and Transmission
Meiosis is the process by which organisms pass on half of their genes - not the best half, not the dominant half, but a random half of chromosomes will be distributed into gametes. In humans, these gametes are sex cells known as egg and sperm. The haploid cells (represented as n) will combine during fertilization to produce a new and unique diploid cell (represented as 2n). The algebraic placeholder of n stands for the quantity of chromosomes in a haploid cell. For humans, n = 23 and so 2n = 46, the number of chromosomes in body cells.
Lets Zoom in:
If you haven’t already done so, please go back and first listen to Episode 14 on the Cell Cycle and Mitosis. As methods of cell division, meiosis and mitosis have a lot in common. Lets recap meiosis first before drawing comparisons.
When observing the human karyotype, or mapping of chromosomes, it's important to recognize that they can be organized into 23 homologous pairs. These homologous chromosomes are a similar size and have similar genetic information. The chromosomes you have in each of your cells right now came from your parent’s gametes, with homologous chromosomes pairing back-up during fertilization. Following the same logic, the process of meiosis will be to separate the 23 homologous pairs into gametes. Meiosis occurs in sex organs, such as testes and ovaries, and will include the familiar steps of prophase, metaphase, anaphase and telophase. In order to reduce the chromosomes number in half, the cells will need to go through PMAT twice.
Prophase I begins with similar condensing of DNA into chromosomes and the disappearing of the nuclear membrane. Additionally, homologous chromosomes pair up, and an exchange of DNA may occur between them by crossing over. This event is called recombination and is one of the main contributors of genetic variation in gametes. In metaphase I, the homologous chromosomes will line up as pairs in a random order in the middle of the cell, with a spindle fiber attached to each centromere. This random arrangement, called independent assortment, is another contributor to genetic variation - where over 8 million different combinations are possible for humans. When spindle fibers shorten in Anaphase I, homologous pairs are separated. Because centromeres are still intact, our chromosomes are still in the familiar X shape. Telophase I and cytokinesis usually overlap, with nuclear membranes and uncondensing of DNA often skipped. Congratulations! You have now formed two haploid daughter cells, each with 23 chromosomes.
Meiosis I and Meiosis II are consecutive. There is no interphase between them. This makes sense because the cell has just gone through all this work to cut the chromosome number in half - it doesn’t need to synthesise more DNA! Meiosis II will also follow PMAT, with much similarity to Mitosis steps. Prophase II has spindle fibers attaching to centromeres, pulling chromosomes to the middle of the cell in metaphase II, separating sister chromatids in anaphase II, and lastly uncoiling into chromatin and reforming nuclear membranes in telophase. Division of the cytoplasm is the final step with cytokinesis and vuala! A total of 4 haploid daughter cells, each genetically unique from each other and from the original diploid parent cell.
OK - so let's rewind and look at the process of meiosis as compared to mitosis. I highly recommend organizing this information in a table or venn diagram. The purpose of mitosis is to grow, repair, and reproduce asexually while meiosis is to form gametes for sexual reproduction (think S of meiosis for sex cells). Mitosis creates two identical diploid cells through one division while meiosis forms four genetically unique haploid cells through two divisions. Meiosis involves crossing over and independent assortment while mitosis does not. Lastly, Mitosis involves the separation of sister chromatids only while Meiosis will first separate homologous paris and then separate sister chromatids.
To recap….
Cell division by meiosis allows for heritable information to be passed on while creating genetically unique gametes. Although meiosis and mitosis have many differences, each process follows the phases of PMAT.
Today’s Question of the day is about gametes.
Question: When do human egg cells complete meiosis II?