AP Biology 5.1 - Meiosis

This section of the AP Biology curriculum focuses on the physical processes that take place during meiosis. We’ll start by looking at why meiosis is necessary for diploid organisms to reproduce sexually. In doing so, we’ll learn the difference between a reductional division and an equational division – of which meiosis has one of each. Then, we’ll take a closer look at the differences between mitosis and meiosis and why multicellular organisms often require both. Finally, we’ll go through both meiosis I and meiosis II in extreme detail – examining exactly how the chromosomes are divided in each division and how this reduces the ploidy level of the resulting daughter cells. This information can get pretty complex, so we’ve animated every step of the process to make it simple!

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Heritable information provides for the continuity of life.

Explain how meiosis results in the transmission of chromosomes from one generation to the next.

Describe similarities and/or differences between the phases and outcomes of mitosis and meiosis.

Meiosis is a process that ensures the formation of haploid gamete cells in sexually reproducing diploid organisms —

  1. Meiosis results in daughter cells with half the number of chromosomes of the parent cell.
  2. Meiosis involves two rounds of a sequential series of steps (meiosis I and meiosis II).

Mitosis and meiosis are similar in the way chromosomes segregate but differ in the number of cells produced and the genetic content of the daughter cells.

5.1 Meiosis Overview

Plants produce pollen for 1 purpose: reproduction. Whether a bee carries pollen to another flower or the pollen is blown through the wind, each pollen granule contains sperm cells. These sperm cells can fertilize the egg cells in each flower they arrive at to produce new embryos – also known as seeds!

But, long before sperm cells are packaged up into pollen and sent out into the world, they must be created through the process of meiosis. Meiosis is like the process of mitosis, except that it reduces a single diploid cell into 4 haploid cells during two consecutive rounds of cell division. This effectively reduces the genome, allowing two cells to combine via fertilization into a new zygote. Meiosis allows for the production of specialized gametes like sperm and egg cells, and the process of sexual reproduction that they power. Knowing the steps and purpose of meiosis is required for the AP Biology test. So, stick with us as we go through the entire process of meiosis and how it works.

To understand the purpose of meiosis, let’s take a look at why cells divide in the first place. Most unicellular eukaryotes divide via the process of mitosis. During interphase of the cell cycle, the DNA is replicated. Through the process of mitosis, two genetically identical daughter cells are produced using each copy of the DNA.

While some multicellular organisms are able to reproduce asexually via the process of mitosis, most multicellular organisms utilize the process of sexual reproduction in order to increase their genetic diversity and reproduce at a higher rate. If each parent organism donated a cell with a full set of genetic material, every new organism would have twice the amount of DNA as its parents! After a few generations, this would mean a massive amount of DNA! So, this is why most sexually reproducing organisms are diploid as adults, but produce haploid gametes. While there are many variations on the ploidy levels in different organisms, this general process ensures that each species maintains a steady number of chromosomes.

Ploidy levels are determined by both the number of chromosome copies and the genetic content of the chromosomes
Ploidy levels are determined by both the number of chromosome copies and the genetic content of the chromosomes

The process of fertilization combines two haploid gamete cells into a new zygote, while the process of meiosis is what reduces diploid cells back into haploid gametes so DNA can be combined into the next generation of organisms. Together, these processes allow a population of organisms to maintain a high level of genetic variability while also maintaining the amount of genetic material in each cell.

Think about this… the process of mitosis is like a factory production line. Every cell comes out nearly identical, and for good reason. Mitosis is how an organism replaces damaged cells and how some organisms clone themselves through asexual reproduction. By contrast, meiosis is like splitting a deck of cards and shuffling each half. When you put the deck back together again, it will still contain the same number of cards. But, it will have many unique variations in the card order. That’s what keeps the game interesting! Meiosis is a process that splits and shuffles entire genomes, allowing organisms to make variable offspring.

To understand meiosis, it is important to first understand what is happening in the process of mitosis. Before mitosis or meiosis takes place, each chromosome is replicated. This produces a cell with sister chromatids of each chromosome – exact copies of each chromosome made through the process of DNA replication.

Sister chromatids aligned on the metaphase plate during mitosis
Sister chromatids aligned on the metaphase plate during mitosis

During mitosis, each of these chromatids is bound to its pair, and all the pairs line up on the metaphase plate during metaphase. As the sister chromatids are pulled apart during anaphase, each cell ends up with 1 copy of each parental chromosome, exactly like the parent cell that the process started with. Unless there are mutations that occur during this process (which rarely happens), each daughter cell is identical.

Homologous chromosomes align on the metaphase plate during meiosis I
Homologous chromosomes align on the metaphase plate during meiosis I

By contrast, the daughter cells created by meiosis are all unique. This is because meiosis is a process that includes two consecutive cell divisions, without a DNA replication event in the middle. In the first division, homologous chromosomes are separated. These are chromosomes that contain the same genetic information but from different parental sources (for example, chromosome 1 from your mom and chromosome 1 from your dad). The second meiotic division is very similar to mitosis since sister chromatids are separated into separate cells.

Now, let’s consider how these two different processes affect the ploidy level of cells. Ploidy can be measured two different ways – by the genetic content (denoted n) or by the number of copies of each chromosome (denoted c). When a cell enters G1 phase and before it replicates its DNA in S phase, the cell is 2n and 2c. In other words, the cell has two copies of the genetic code (or two alleles for each gene) and two copies of each gene. There are two homologous chromosomes from each parent, which can carry different alleles and also constitute separate copies of the same chromosome.

Let’s see what happens during mitosis. After the DNA is replicated, there is twice as much DNA material. This means that there are 4 copies of each chromosome or 4c (2 homologs, each duplicated). However, there is still a maximum of 2 different alleles per gene – the maternal and paternal contributions have simply been duplicated. Therefore, the genetic content ploidy stays diploid or 2n. Since the sister chromatids are separated, each cell ends up with the same parental contributions but half of the 4c chromosomes. This creates two daughter cells that are 2n and 2c – the same as the original parent cell. This is known as an equational division since the duplicated chromosomes are equally divided and the genetic content is not changed.

By contrast, meiosis has two divisions: a reductional division that decreases the genetic content, followed by an equational division that separates sister chromatids just like mitosis. Let’s see what this does to ploidy. As with mitosis, the process of meiosis starts after the DNA has been replicated, leaving the cell 2n and 4c. During meiosis I, the homologous chromosomes from maternal and paternal sources. This leaves each daughter cell with a maximum of 1 allele for each gene (1n) and with 2 copies of the homolog chromosome it received (2c). Then, an equational division takes place that separates these two copies, leaving the cells haploid (1n) with only 1 copy of each chromosome (1c).

The stages of meiosis are divided into two parts, meiosis I and meiosis II
The stages of meiosis are divided into two parts, meiosis I and meiosis II

This is how meiosis creates haploid cells that can be used for sexual reproduction. When two of these cells combine, it will lead back to the original diploid cell with 2n and 2c.

So, we’ve covered the general process of meiosis and how it is different than mitosis. Now, let’s look at the specific stages in meiosis. First off, you should note that the entire process of meiosis has two distinct cell division events. The first is known as meiosis I.

The stages of meiosis I are similar to the stages of mitosis. The nuclear envelope dissolves in prophase, the chromosomes align on the metaphase plate in metaphase, they are pulled apart in anaphase, and the nuclei reform in telophase before cytokinesis fully separates the two daughter cells.

The most important difference between mitosis and meiosis happens at metaphase. In metaphase of meiosis I, homologous chromosomes are separated. In metaphase of mitosis, sister chromatids are separated.

In mitosis, this results in the separation of the duplicated genome, reducing 4c back to 2c. In meiosis I, on the other hand, the genetic content is reduced because the two alleles of each gene are separated. This reduces the daughter cells from 2n to n.

Meiosis I also contains another very important event that typically occurs during prophase I: crossing over. Crossing over occurs as the two homologous chromosomes line up next to each other. Since they contain the same genes in the same structure, parts of each non-sister chromatid can swap places. This is an important process that increases genetic variation, and we will cover it more in section 5.2.

So, we’ve reviewed the reductional division that takes place during meiosis I. Now, let’s take a look at the second division of meiosis, known as meiosis II. During meiosis II, the sister chromatids in each cell are divided into two new daughter cells. Let’s see how this works, in detail.

After the first cellular division of meiosis I, the two daughter cells enter a short phase known as interkinesis. During this phase, the cell rests, reestablishes its energy stores for the next division, and starts organizing the chromosomes for the next cellular division. It is important to note that the DNA is not replicated during this phase, unlike during interphase of the normal cell cycle.

Pollen contains gamete cells created by plants via meiosis for the purpose of sexual reproduction
Pollen contains gamete cells created by plants via meiosis for the purpose of sexual reproduction

Instead, the cells immediately enter prophase II. Spindle fibers stretch across the cells, attach to the chromosomes, and start dragging them toward the middle of the cell. The chromosomes reach the metaphase plate as the cells reach metaphase II. Remember, the difference between metaphase II and metaphase I is that during metaphase I the homologous chromosomes were being divided. In metaphase II, the duplicated sister chromatids are being separated into individual chromosomes.

During anaphase II, these sister chromatids are fully separated into individual chromosomes. As the cell enters telophase II and cytokinesis, the process of meiosis is completed with 4 new daughter cells that are fully haploid and only have 1 copy of each chromosome.

Since the sister chromatids are dividing, meiosis II is a lot like mitosis. However, since the homologous chromosomes have already been separated during meiosis I, the cells enter meiosis II with a different level of ploidy than the cells entering mitosis. Cells entering mitosis and meiosis I are 2n diploid and have 4 copies of each chromosome. By contrast, cells entering meiosis II are 1n haploid and have only 2 copies of each chromosome. In the next section, we’ll analyze how the processes of meiosis I and II increase the genetic variability of each subsequent generation.