When you hear the term sexual reproduction, you might not immediately picture cell division (unless you are already a cell biology aficionado). However, a specific type of cell division named meiosis is crucial for sexual reproduction to work because it creates gametes, or sex cells, suitable for this type of reproduction.
Scientists and science teachers sometimes call meiosis reduction division. This is because the germ cells destined to become gametes must reduce their number of chromosomes before they divide to produce those sex cells, such as sperm or egg cells in humans or spore cells in plants.
This reduction division maintains the correct number of chromosomes from one generation to the next and also ensures genetic diversity for the offspring.
Cell Division and Simple Eukaryotes
Cell division, which includes both mitosis and meiosis, simply enables a parent cell to divide into two (or more) daughter cells. This division makes it possible for cells to reproduce, either sexually or asexually.
Single-celled eukaryotic organisms, such as amoebas and yeast, use mitosis to divide into daughter cells that are identical to the parent cell during asexual reproduction. Since these daughter cells are exact replicas of the parent cell, genetic diversity is minimal.
Related content: S phase
Cell Division and More Complex Eukaryotes
In more complex eukaryotes who use sexual reproduction, such as humans, mitosis also plays important roles. These include cell growth and tissue healing.
When your body needs to grow or replace the skin cells it sloughs off all the time, the cells at that site will undergo mitosis to replace lost cells or add bulk. In the case of wound healing, the cells at the edges of the damaged tissue will undergo mitosis to close up the injury.
The process of meiosis, on the other hand, is the way complex eukaryotic organisms make gametes in order to reproduce sexually. Since this cell program shuffles the genetic information encoded in the chromosomes, the daughter cells are genetically unique rather than identical copies of the parent cells (or the other daughter cells).
This uniqueness might make some daughter cells more fit to survive.
Chromosomes and Reduction
Your chromosomes are a form of your DNA that is packaged by wrapping the strands of genetic material around specialized proteins called histones. Each chromosome contains hundreds or thousands of genes, which code for the traits that make you different from other people. Humans usually have 23 pairs of chromosomes, or 46 total chromosomes in every DNA-containing cell of the body.
In order for the math to work when producing gametes, the parent diploid cells with 46 chromosomes each must reduce their set of chromosomes by half to become haploid daughter cells with 23 chromosomes each.
Sperm and egg cells must be haploid cells since they will come together to make a new human during fertilization, essentially combining the chromosomes they carry.
Chromosome Math and Genetic Disorders
If the number of chromosomes in these cells were not reduced by meiosis, the resulting offspring would have 92 chromosomes instead of 46, and the next generation would have 184 and so on. Conserving the number of chromosomes from one generation to the next is important because it makes it possible for each generation to use the same cell programs.
Even one extra (or missing) chromosome can cause serious genetic disorders.
For example, Down syndrome occurs when there is an extra copy of chromosome 21, giving people with this disorder 47 chromosomes rather than 46.
While errors can and do occur during meiosis, the basic program of reducing the number of chromosomes before dividing into gametes ensures that most offspring wind up with the correct number of chromosomes.
Phases of Meiosis
Meiosis includes two phases, called meiosis I and meiosis II, which occur in sequence. Meiosis I produces two haploid daughter cells with unique chromatids, which are the precursors to chromosomes.
Meiosis II, is somewhat similar to mitosis because it simply divides those two haploid daughter cells from the first phase into four haploid daughter cells. However, mitosis occurs in all somatic cells whereas meiosis only takes place in reproductive tissues, such as the testes and ovaries in humans.
Each of the phases of meiosis includes subphases. For meiosis I, these are prophase I, metaphase I, anaphase I and telophase I. For meiosis II, these are prophase II, metaphase II, anaphase II and telophase II.
What Happens During Meiosis I?
To make sense of the nuts and bolts of meiosis II, it’s helpful to have a basic understanding of meiosis I since the second phase of meiosis builds on the first. Through a series of regulated steps laid out in the subphases, meiosis I pulls the paired chromosomes, called homologous chromosomes, of the parent cell to opposite sides of the cell until each pole contains a cluster of 23 chromosomes. At this point, the cell divides into two.
Each of these reduced chromosomes comprises two sister strands, called sister chromatids, held together by a centromere. It’s easiest to picture these in their condensed versions, which you can imagine as looking somewhat like butterflies. The left set of wings (one chromatid) and the right set of wings (the second chromatid) connect at the body (the centromere).
Meiosis I also includes the three mechanisms that ensure genetic diversity of the offspring. During crossing over, the homologous chromosomes exchange small regions of DNA. Later, random segregation ensures that the two versions of the genes from these chromosomes shuffle randomly and independently into the gametes.
Independent assortment makes sure that sister chromatids wind up in separate gametes. Altogether, these mechanisms shuffle the genetic deck to produce many possible combinations of genes.
What Happens in Meiosis II, Prophase II?
With meiosis I completed, meiosis II takes over. During the first phase of meiosis II, called prophase II, the cell gets the machinery it needs for cell division ready to work. First, two areas of the cell’s nucleus, the nucleolus and nuclear envelope, dissolve.
Then, the sister chromatids condense, which means they dehydrate and change shape to become more compact. They now appear thicker, shorter and more organized than they do in their uncondensed state, called chromatin.
The cell’s centrosomes, or microtubule organizing centers, migrate to opposite sides of the cell and form a spindle between them. These centers produce and organize microtubules, which are protein filaments that play a wide variety of roles in the cell.
During prophase II, these microtubules form the spindle fibers that will eventually perform important transportation functions during later stages of meiosis II.
What Happens in Meiosis II, Metaphase II?
The second phase, called metaphase II, is all about moving the sister chromatids into proper position for cell division. To do this, those spindle fibers attach to the centromere, which is the specialized region of DNA holding the sister chromatids together like a belt, or the body of that butterfly you imagined where the left and right wings are the sister chromatids.
Once connected to the centromere, the spindle fibers use their localization mechanisms to push the sister chromatids into the center of the cell. Once they arrive in the center, the spindle fibers continue to push the sister chromatids until they line up along the midline of the cell.
What Happens in Meiosis II, Anaphase II?
Now that the sister chromatids are lined up along the midline, attached at the centromere to the spindle fibers, the work of dividing them into daughter cells can begin. The ends of the spindle fibers that aren’t attached to the sister chromatids are anchored to the centrosomes located at each side of the cell.
The spindle fibers begin to contract, cranking the sister chromatids apart until they separate. During this time, the contraction of the spindle fibers at the centrosomes acts like a reel, pulling the sister chromatids apart from each other and also dragging them toward opposites sides of the cell. Scientists now call the sister chromatids sister chromosomes, destined for separate cells.
What Happens in Meiosis II, Telophase II?
Now that the spindle fibers have successfully divided the sister chromatids into separate sister chromosomes and transported them to opposite sides of the cell, the cell itself is ready to divide. First, the chromosomes decondense and return to their normal, thread-like state as chromatin. Since the spindle fibers have performed their jobs, they are no longer necessary, so the spindle disassembles.
All that is left for the cell to do now is split into two through a mechanism called cytokinesis. To do this, the nuclear envelope forms again and creates an indentation down the center of the cell, called a cleavage furrow. The way the cell determines where to draw this furrow remains unclear and the subject of heated debate among scientists who study cytokinesis.
A protein complex called the actin-myosin contractile ring causes the cell membrane (and cell wall in plant cells) to grow along the cytokinesis furrow, pinching the cell into two. If the cleavage furrow formed at the correct location, with the sister chromosomes segregated into separate sides, the sister chromosomes are now in separate cells.
These are now four haploid daughter cells that contain unique, varied genetic information that you know as sperm cells or egg cells (or spore cells in plants).
When Does Meiosis Happen in Humans?
One of the most interesting aspects of meiosis is when it occurs in humans, which varies based on the person’s sex assignment. For male humans past the onset of puberty, meiosis takes place continually and produces four haploid sperm cells per round, each ready to fertilize an egg cell and produce offspring if given the opportunity.
When it comes to female humans, the timeline for meiosis is different, more complicated and much stranger. Unlike male humans who continually produce sperm cells from puberty until death, female humans are born with a lifetime supply of eggs already inside their ovarian tissues.
Wait, What? Stop and Start Meiosis
It’s a bit mind blowing, but female humans undergo a portion of meiosis I while they are still fetuses themselves. This produces egg cells inside the ovaries of the fetus, and then meiosis essentially goes offline until it's triggered by hormone production at puberty.
At that time, meiosis resumes briefly but then halts yet again at the metaphase II stage of meiosis II. It only starts back up and completes the program if the egg is fertilized.
While the whole meiosis program produces four functional sperm cells for male humans, it only makes one functional egg cell for female humans and three extraneous cells called polar bodies.
As you can see, sexual reproduction involves much more than sperm meets egg. It's actually a super complicated set of cell division programs working together to ensure that each potential offspring has the right number of chromosomes and a unique chance of survival, thanks to genetic shuffling.