How does meiosis keep track of homologs and reduce the genome by half? Know these answers and more as we navigate the steps of meiosis I.
This lesson will be tetradical!
We’ve learned thus far that meiosis is a special type of cell division that makes gamete cells for sexual reproduction. The most important distinction between meiosis and mitosis is that the daughter cell chromosome number must be reduced.To put meiosis into everyday terms, let’s consider the chromosomes as cookbooks again for a minute. If mitosis was like some crazy person making copies of every one of his cookbooks, meiosis is like two roommates trying to split a cookbook collection.
Say that you and I have amassed a bunch of cookbooks, but now we’re going our separate ways. Luckily, we have two editions of each cookbook, so at least we can both get a version of the recipes in those books. Now, if we have a lot of cookbooks, and those cookbooks are kind of poorly organized, it’ll be easiest to identify both editions of each cookbook before tossing any cookbooks into our piles. This will ensure that neither of us gets two editions of the same cookbook or no version at all.
|cell divisions occur during meiosis, and the events of the first meiotic division (meiosis I) address the problem of identifying and separating pairs of homologous chromosomes. Meiosis I is sometimes referred to as a reductional division because it reduces the number of chromosomes in the cell by half.The meiotic I cell division consists of 4 basic steps: prophase I, metaphase I, anaphase I and telophase I. Although many components used during mitosis are also utilized in meiosis I, as we trace the events of meiosis I, note the significant modifications that are necessary to achieve a reductional division.
Just like mitosis, the first meiotic division is preceded by interphase. During interphase, the chromosomes replicate, meaning that each chromosome is composed of two chromatids prior to meiosis I. During this cell cycle step, the centrosomes also duplicate.
Prophase I is the first step in meiosis I. And a lot happens during this step, some of which is a departure from what we saw in mitosis. During prophase I, the chromatin condenses. The centrosomes move to opposite poles and begin to produce spindle fibers. Homologous chromosomes search for their partner homolog in a process known as pairing.Crossing-over occurs between homologous chromosomes.
Note that genetic information is exchanged between homologous chromosomes as a result of a crossover event. More importantly, the crossover event also serves to hold the homologs together, so they can be properly oriented on the meiotic spindle.When the homologs become physically linked together, the entire structure is referred to as a tetrad, and this makes sense because tetra means four in Greek and there are four chromatids in a tetrad. Also, note that the name ‘crossing over’ stems from the fact that the arms of the homologs appear to cross over each other in forming this structure.Kinetochore proteins assemble on the outermost chromatid that faces each spindle pole. Meiotic spindle attaches to the kinetochore. As in mitosis, our little kinetochore friend is busy working those winches on each side of the tetrad.
By adjusting the length of microtubules on each side, the kinetochore positions the tetrad in the middle of the cell. By the end of prophase I, the nuclear membrane has broken down and the meiotic spindle apparatus has been fully assembled.
Metaphase I is the second step in meiosis I. By metaphase I, the tetrads are aligned at the metaphase plate. Note that the physical linkage between the homologous chromosomes and the tension produced by the meiotic spindle keep the tetrads positioned in the middle of the cell.
Anaphase I is the third step in meiosis I.
It begins with the release of the physical connection between homologs. No longer physically linked, the spindle pole tension can pull the homologs to their respective spindle pole.
Telophase I is the fourth and final step of meiosis I.
During telophase I, the nuclear membrane reforms and the chromosomes decondense in most organisms. Telophase I is also accompanied by the division of the mother cell into two daughter cells. This marks the end of meiosis I.
To review, meiosis is a special type of cell division that makes gamete cells for sexual reproduction. Meiosis I is the first of the two meiotic divisions during which the number of chromosomes in the cell is reduced by half.Prophase I is the first step in meiosis I.
During prophase I, the chromatin condenses, centrosomes move to opposite poles, and a spindle apparatus forms as in mitosis. Unlike mitosis, crossing-over physically links the homologous chromosomes and the resulting tetrad, rather than the individual chromosomes, is aligned at the metaphase plate. A tetrad is the structure formed when two homologous chromosomes become linked during a crossover event.Metaphase I is the second step in meiosis I. At metaphase I, the tetrads are aligned at the metaphase plate.Anaphase I is the third step in meiosis I.
During anaphase I, the physical connection between homologs is broken, allowing spindle pole tension to pull each homolog to its respective pole.Telophase I is the fourth and final step of meiosis I. During telophase I of most organisms, the nuclear membrane reforms and the chromosomes decondense. This is also followed by cytokinesis.Finally, here’s a lyrical reminder of the major ideas of this lesson:How to achieve sex reproduction?The first division is reductionCrossing over links DNASo homologs no more may strayCut the tetrad to let them looseAnd so the genome does reduce
By the end of this lesson, you should be able to: