Meiosis I Stages: Prophase I, Metaphase I, Anaphase I & Telophase I Video

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  • 2:18 Interphase
  • 2:37 Prophase I
  • 4:26 Metaphase I
  • 4:50 Anaphase I
  • 5:10 Telophase I
  • 5:51 Lesson Summary
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Lesson Transcript
Instructor: Greg Chin
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!

Dividing Chromosomes

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.

During meiosis I, chromosomes in the cells are reduced by half
Reductional division

This is basically the problem facing a cell undergoing meiosis. Two 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

In the first step of meiosis I, homologs pair off and the nuclear membrane is broken down
First step of meiosis I

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.

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