Another entry in the Woven Together series talking about the fascinating development of a little person. This series deviates from the main content of the blog – how things go wrong – to tell how things go right. I hope you enjoy!
Meiosis. This word probably conjures up memories of high school biology class where you had to memorize a lot of meaningless terms, but today I want to show you how it is a romantic word. Yes, you read that right.
A couple comes together pretty intimately in the body of their child. She has Mom’s nose and Dad’s eyes, Mom’s chin and Dad’s smile. But meiosis allows the couple to come even closer in the cells of their grandchildren. Let’s dive in!
Meiosis (pronounced my – oh – sis) is a process of dividing a body cell in two, cutting the number of genes in half, so that it can become an egg cell or a sperm cell. ((Mitosis, a similar word, divides a cell in half but makes an exact copy of the genome, leaving the number of genes the same. In humans, everything except the egg and sperm cells divides by mitosis.)) Can you imagine the chaos if we didn’t cut the number of genes in half with each generation? Mom and Dad would have two copies of each gene, their baby would have 4, their grandchildren 8, great-grandchildren 16, and their great-great-great grandchildren would have 64!
In humans, even an extra copy of one chromosome can cause problems. ((such as Down syndrome, Trisomy 18, and others. Most trisomies are fatal early in pregnancy.)) So, meiosis allows us to keep the gene number to 2 with each generation. ((Strangely, polyploidy (more than two copies of each chromosome) is really common in plants. Some wheat has 6 copies of each chromosome, and strawberries have 8!))
In this story, we’ll be looking at Grandma and Grandpa, who had Mom, who married Dad and had Baby. Mom got one complete set of genes from Grandma and one complete set of genes from Grandpa.
As a reminder, genes are regions of DNA that code for proteins. Humans have over 20,000 genes. Genes live on chromosomes, connected strands of DNA, and each body cell has 46 chromosomes. If the entire collection of DNA, the genome, is an encyclopedia set, chromosomes are the individual volumes and genes are entries within the books.
So, of Mom’s 46 chromosomes, 23 came from Grandma and 23 from Grandpa. It’s like she got two “E” volumes of an encyclopedia, and each volume has an entry about eye color. ((These “entries” may be identical (Baby gets green eye information from both parents and his eyes are green) or different (Baby gets green eye information from one parent and brown eye information from another, and baby’s eyes are brown.) )) When Mom is ready to make an egg cell, she has to sort the library so that each egg cell only gets one copy of the “E” volume (Dad’s sperm cell will provide the other copy to Baby).
So, let’s talk about how she sorts out the library. A cell in a woman’s body ((called an oocyte)) starts with the normal 46 chromosomes.
The first step is for each chromosome to make an identical copy of itself in preparation for division, so we now have 92 (= 46 X 2). For simplicity, we’ll only follow one pair of chromosomes.
They line up, so that the two copies of chromosome 1 Mom got from Grandma are next to the two copies of chromosome 1 from Grandpa, and so on down the line. This group of four chromosome copies is called a tetrad.
Then, the inner arms of each tetrad wrap around each other and trade information. Some of the genes that were originally on Grandpa’s chromosome 1 jump over to be on Grandma’s chromosome 1, and vice versa. Usually there are one or two sites of crossing over per chromosome, and the exact location is random. When the tetrads are later pulled apart, the inner chromatids will each take pieces of their partner with them.
I find this beautiful. This mechanism is a biological revelation of “the two shall become one.” The couple that married, that became one flesh in the body of their daughter, has now become so intertwined in these chromosomes that their grandbaby won’t make any distinction between their respective genes.
Next, each tetrad is pulled in half into two separate cells. Each of those cells divides again so that there are 4 cells with only one copy of each chromosome (23 total chromosomes in each cell).
It’s random which copy of each chromosome goes into the four final cells. So, one cell might get Grandma’s unchanged copy of chromosome 1, a mixed copy of chromosome 2, Grandpa’s unchanged copy of chromosome 3, etc. Meiosis gives four gametes that are different from each other and different from the grandparents. ((In women, only one of each group of four cells survives to become the egg. The other three donate most of their cytoplasm to make one giant egg cell. Men make 4 sperm cells from each round of meiosis.))
One of these cells becomes the egg cell. That egg cell merges with a sperm cell of Dad’s, and thus Baby is begun!
Meiosis, that most romantic of words, mixes up the genes that go to baby in two ways. The chromosomes are sorted randomly ((Baby might get Grandma’s “A” volume and Grandpa’s “B” volume)), but the genes also get mixed up through crossing over ((Baby’s “C” volume might have some entries from Grandma and some from Grandpa)). It’s an effective way of giving our offspring access to the best genes from many different people.
So, when Baby is conceived, he gets from Mom some chromosomes straight from Grandma, some straight from Grandpa, and some that are mixed. From his dad, Baby gets chromosomes from his paternal grandparents: some straight, and some mixed. What a family tree!
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Sources & Further Reading
- Down the rabbit hole on polyploidy plants & hybrids.
- Cartoon description of meiosis (video makers after my own heart!). Great for classrooms.
- Less cartoon-y animation of how meiosis works.
And all my old biology textbooks…
- Alberts, Bruce, et al. Molecular Biology of the Cell (5th ed.). New York: Garland Science, 2008.
- Pierce, Benjamin A. Genetics: A Conceptual Approach (3rd ed.). New York: W.H. Freeman and Company, 2008.
- Purves, William K., David Sadava, Gordon H. Orians, and H. Craig Heller. Life: The Science of Biology (7th ed.). Sinauer Associates, Inc., 2004.
- Wolpert, Lewis. Principles of Development (3rd ed.). Oxford: Oxford University Press, 2007.