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Meiosis: Comparison to Mitosis, Crossing Over & Process

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  • 0:34 Mitosis vs. Meiosis
  • 2:36 Crossing Over
  • 4:24 Meiosis
  • 5:52 Summary
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Lesson Transcript
Instructor: Greg Chin
Welcome to meiosis, a realm of division doubled for reproduction untroubled. You'll get a preview of both divisions of meiosis and see what it takes to go from a diploid cell to a haploid gamete.

The Problem of Meiosis

Now we know that meiosis is the process that's going to produce gametes for sexual reproduction. Each gamete possesses a single copy of each chromosome. That means that when two gametes fuse, they produce a cell with two copies of each chromosome. The question we need to answer now is how is the organism going to make sure that each gamete gets exactly one copy of each homolog?

Mitosis vs. Meiosis

Let's reexamine mitosis. How the cell dealt with chromosomes during mitosis might give us a clue as to how they're handled in meiosis. Let's consider an example where we have a diploid organism with two chromosomes. And let's make it easier on ourselves to follow these chromologs and chromosomes by using different colors. We'll make one chromosome purple and the other one orange. Note the each homologous chromosome is a similar shade of the same color, signifying that they are similar but probably not identical versions of the same genes.

DNA replication prior to mitosis produced two copies of each chromosome stuck together at the centromere. Since the chromatids are exact copies, they're the same color. The mitotic spindle apparatus lines up each chromosome in the middle of the cell. By positioning each homolog in the middle of the cell, division can ensure that each daughter cell receives one copy of each homolog in the form of a chromatid. So, in our example, each daughter cell needs to receive four chromatids by the end of mitosis, each of which is represented by the four different colors.

The physical result of two homologous chromosomes linking together looks like a cross.
Crossing Over

Now, in contrast, the goal of meiosis is to reduce the genome by half. Rather than getting one copy of each distinct color, the daughter cells only need one copy of each basic color. In our example, that means that each gamete should possess one copy of the orange chromosome and one copy of the purple chromosome. It doesn't really matter if the chromosome is a light or dark version.

Meiotic Recombination

For this to happen, the cell needs a way of linking homologous chromosomes in a physical structure, similar to the way that chromatids are held together at the centromere during mitosis. This physical connection occurs as a result of homologous recombination, which is also referred to as crossing over because the physical structure that results looks like a cross. This crossing over is a process in which genetic material is exchanged between homologous chromosomes. Physically linking the homologs is a similar strategy to the one used in mitosis with the crossover structure playing a similar role to that of the centromere. By physically holding the homologs together during meiosis, the homologs act like one physical structure. The cell can then use spindle microtubules to orient the homologs until every homologous pair is ready for division.

As a result of meiotic recombination, genetic information is exchanged between homologs.
Exchange of genetic information

Once each homolog pair is oriented properly at the metaphase plate, the physical connection can be broken as each homolog segregates toward opposite spindle poles. It's important to note that the description of meiotic recombination provided here is a very simplified version of the actual events and participating proteins.

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