In our last installment about chromosomes, we discussed mitosis, the key process that maintains the proper chromosome number in somatic cells. But what happens when organisms want to reproduce? In organisms that reproduce sexually, or through the combination of genetic material from two individuals, we end up with a tricky situation. How do we take the cells from two organisms and create an offspring without changing the chromosome number? This is where meiosis comes into play.
Bacteria (and some eukaryotic organisms like yeast) reproduce asexually. This means that a single cell reproduces its DNA and other cellular components, expands the volume of the cell, and then splits into two, creating two identical organisms that are exactly the same as the parent. To increase the genetic diversity of the resulting organisms, many eukaryotic organisms — animals, protists, fungi, and more — choose to reproduce sexually, where two parents contribute genetic material to produce an offspring that is genetically different from either parent.
So, how does that work? Let’s take a look at Drosophila chromosomes to figure things out. In flies, the genetic material is present in eight chromosomes which can be sorted into four pairs. We call a cell with two complete sets of chromosomes diploid. Cells that only have one set of chromosomes (in the case of fly somatic cells, that is four chromosomes total) are haploid. Often times, we describe haploid cells as being “n”, where n is the number of chromosomes. Accordingly, we call diploid cells that two complete sets of four chromosomes “2n.” In order for sexual reproduction to be successful, each offspring needs to have one chromosome from each parent. This requires the production of haploid cells called gametes that facilitate reproduction. The process of meiosis creates these specialized cells.
The first phase of meiosis (called meiosis I) follows the same general sequence of steps that we see in mitosis; however, this phase of meiosis separates homologous chromosomes (that is, identical chromosome pairs) from one another. So, instead of separating eight chromosomes into individual chromatids, we are separating the four pairs of chromosomes.
- Prophase I: the long, relaxed threads of chromosomal DNA begin to condense into chromosomes. The homologous chromosomes pair along their lengths in a process called synapsis. At this point, homologous chromosomes begin to rearrange themselves in a process called “crossing over.” This mixes up the sequence of the chromosomes at random, creating genetic diversity. Simultaneously, the meiotic spindles begin to form around the centrosomes. The mitotic spindles begin to form and move to opposite poles of the cell. The nuclear membrane breaks down, and the mitotic spindles separate from one another as the microtubules extend their length.
- Metaphase I: The pairs of chromosomes line up at the center of the cell, equidistant from the cell poles. Some microtubules connect to the kinetochore of each homologous chromosome.
- Anaphase I: During anaphase, the proteins holding the homologous chromosomes together break down. The sister chromatid pairs move towards the spindles. Structural microtubules begin to lengthen, elongating the cell.
- Telophase I: At this time, cytokinesis occurs and two separate cells form. In some organisms, nuclei begin to reform. In other organisms, we move right to meiosis II.
The second phase of meiosis (meiosis II) is critical to create haploid cells. In this stage, the cells separate the sister chromatids from one another in a similar set of steps.
- Prophase II: Meiotic spindles form and move to opposite sides of the cells. The sister chromatids move towards the center of the cell.
- Metaphase I: The pairs of chromosomes line up at the center of the cell, equidistant from the cell poles. Some microtubules connect to the kinetochore of each sister chromatid.
- Anaphase I: During anaphase, the proteins holding the sister chromatids together break down. Each chromatid moves towards the spindle. Structural microtubules begin to lengthen, elongating the cell.
- Telophase I: At this time, the nuclei reforms. Cytokinesis forms two separate cells, each containing 23 sister chromatids. These haploid cells will develop into the gametes that are exchanged in sexual reproduction.
Now, different organisms have different sexual life cycles. Some organisms, like fungi, live most of their lives as haploid (n) organisms, with one set of chromosomes (Fig 3A). They only become diploid briefly after fertilization before undergoing meiosis and creating mature organisms. Other organisms, like humans, have a diploid life cycle, where the individual spends its live as a diploid individual that creates haploid gametes for reproduction (Fig 3A). Finally, other organisms will alternate generations between haploid and diploid forms, which display both mature haploid and diploid forms that undergo reproduction
So, meiosis creates haploid cells that allow for greater genetic diversity through genetic recombination. Now, what are the big differences between meiosis and mitosis? We’ll discuss more in our next blog post!