When Two Become One: Understanding the Process of Meiosis

“Where do babies come from?”  This is a popular question among children (and one that parents dread), but it does open up the discussion regarding the biology behind the creation of life.  As adults you should already be aware of how making babies occurs, but you probably are not aware of how the sperm and egg cells come into existence.  Today’s blog post is devoted to explaining the process of meiosis and how this process lends to the genetic diversity that exists in any given population.

Meiosis is a “specialized cell division that involves in  two cellular divisions and results in four daughter cells, each containing half of the original number of chromosomes (aka gamates)” (Jurmain et al. 2014).  In layman’s terms meiosis is the cellular division that leads to the creation of the reproductive cells (or gametes), which are the sperm and egg cells.  The gametes each carry half of the genetic material of the original (aka parent) cells, which is integral to genetic diversity in a population. 

As previously discussed each individual carries a specific number of genes which make up who and what he or she is (in a manner of speaking).  Our genes code for various biological characteristics, such as eye color, nose shape, presence or absence of hitchhiker thumb, etc.  Our genetic compositions are provided by each of our parents, with each providing half of the genes that are contained in the offspring.  There are variations of each gene, known as alleles, that exist in a population and each has an order of hierarchy in regards to expression (please see this post for more information).  Individuals can therefore carry the same two variations/alleles for a gene (e.g. two hitchhiker thumb alleles), or they can carry two different variations/alleles for a gene (e.g. one hitchhiker thumb allele and one non-hitchhiker thumb allele).  This is important because what is passed down to offspring and what is not is what leads to genetic variation that exists in a population.  

Figure 1: Steps of Meiosis

Several steps must occur for meiosis to occur (Figure 1).  First, the cells located in the reproductive organs must replicate the chromosomes within itself.  In humans there are 46 chromosomes, so these 46 double and become 92.  These 92 chromosomes are exact copies of the original 46 chromosomes, which divide equally along both sides of the cell.  The cell then divides itself in two, thereby creating two clones (exact duplicates) of the original parent cell.  Each of these clones carries the same 46 chromosomes of the parent cell.  If this process stopped here it would conclude mitosis (the simple cellular division that produces identical daughter cells), but meiosis has to continue since it results in gamete cells, each carrying half of the genetic material of the parent.  The process of meiosis continues as the cloned daughter cells undergo chromatic division again, but this time without first replicating as they had done previously.  The 46 chromosomes contained in each of the daughter cells divide into 23 chromosomes each, moving to the sides of the cell.  The daughter cells pinch down the middle and produce four daughter cells, each containing 23 chromosomes (half of the genetic material of the parent).

So where do babies come into play?  Each of these daughter cells that contain 23 chromosomes forms into either an egg cell (in females) or sperm cell (in males).  The union of the sperm and egg (typically though sexual reproduction) causes the fusion of the chromosomes contained in both to form  a complete organism, which in the case of humans is 23 + 23 = 46, or a human being.  In other words, two become one (Figure 2).  Note, for more information on meiosis please watch this video.

Figure 2: Where Babies Come From :P

References Cited

Farabee, MJ.  2007.  "Cell Division: Meiosis and Sexual Reproduction."  Website. 

Jurmain, R; Kilgore, L; Trevathan, W; Ciochon, RL.  2014.  Introduction to Physical Anthropology.  Wadsworth Publishing.

The Biology Project.  2004.  "Meiosis Tutorial".  Website.