Askjagden's Guide to Biology: Meiosis

Askjagden's Guide to Biology: Meiosis

It seems that the words "reproduce sexually" even induces wattpad to rate this guide automatically PG-13. Please excuse the rating, because I'm not using the term "sexually" out of hand.

 There are three terms you must learn before you learn about meiosis. Here's an example found in every textbook: the fruit fly, or Drosophila. The fruit fly has 8 chromosomes. Half comes from one parent; half comes from the other. These different sets of chromosomes are called homologous. "Homo-" means same. Homologous chromosomes are two sets of chromosomes that are the same. In other words, one chromosome in one set corresponds to another chromosome in the other set. The same concept applies to all the chromosomes in both parents' chromosomes. Are you still confused? "Set" refers to the inheritance of chromosomes from one parent. When both sets are homologous, the cell is called diploid. The symbol for diploid is 2N. The human zygote's chromosmoes can be represented as 2N = 46, because humans have 46 chromosomes. All organisms that reproduce sexually have gametes. These gametes are called haploid, which means that they have one set of chromosomes each. For humans, there are two cells that are haploid: the two gametes, the ovum (or egg) and the sperm. Haploid is represented by the letter N. For human gametes, the chromosomes can be represented like this: N = 23.

Here's when meiosis comes in. Meiosis is the process of dividing a diploid cell into four haploid cells in a way that is quite similar to mitosis. You will find, however, that the two are quite different. Each chromosome replicates in a phase called interphase I. The cell grows and makes important molecules for it as well. Just like mitosis, interphase I comes prior to meiosis (or, in this case, meiosis I). Division begins in prophase I, in which centrioles line up in opposite sides of the cell and make spindle fibers. The nuclear envelope dissolves, and chromatin condenses into chromosomes, which makes the chromosomes visible. Here's one difference that happens in prophase I that doesn't happen in prophase: a tetrad forms. A tetrad is a structure made up of 4 chromatids. Note that I said chromatids, not chromosomes. Understand that chromosomes are made up of chromatids, just in case you need some reinforcement. Two chromosomes pair up in a tetrad. You will learn why in this guide. After prophase I comes metaphase I. In metaphase I, spindle fibers line up through the centromeres of the chromosomes. Next comes anaphase I. The spindle fibers pull the chromosomes towards the opposite ends  of the cell. Next comes telophase I. The nuclear envelopes reform. Meiosis I isn't finished. Finally comes cytokinesis, in which the cell divided into two cells.

Meiosis I is finished. But meiosis isn't. After meiosis I comes meiosis II. Meiosis II starts with prophase II. Meiosis II is different from meiosis I because meiosis I starts with 4 chromosomes in one cell, while meiosis II starts with 2 chromosomes in each cell (2 total). In prophase II, the nuclear envelopes dissolve. The centrioles line up on the opposite ends of the cell. Next comes metaphase I. The spindle fibers line up through the centromeres of the chromosomes. Next comes anaphase II. The chromosomes separate. Next comes telophase II. The nuclear envelopes reform around the chromosomes, 2 chromosomes in each. Cytokinesis finally divides the the two cells. Here, you can see that the result of meiosis is 4 haploid cells.

Here's the difference between mitosis and meiosis: while mitosis results in the production of two genetically diploid cells, meiosis results in four genetically different haploid cells.Why does meiosis produce genetically different cells? Here's how it works. Remember what I said about the tetrad? During the formation of the tetrads, chromatids "switch". In other words, two genetically different chrmoosomes will switch one chromatid. This process is called crossing-over, and results in genetic variety, unlike mitosis, which produces 2 genetically identical cells.

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