There Is an Easy Way to Understand Mitosis and Meiosis


mitosis, meiosis mitosis, meiosis
Mitosis is the division of a single cell into two cells (as shown here), each with its own nucleus and the same number and kind of chromosomes as the parent cell. Meiosis occurs when a single cell divides twice to produce four cells containing half the original amount of genetic information. Juhari Muhade/Getty Images

In the beginning, you were just some genetic material. In order to make you, your biological mom and dad both had to participate in an effort to pitch in one gamete each — a sperm cell and an egg cell, each with 23 chromosomes. Both of these sex cells contained everything needed — genetically speaking — to make a unique human, the likes of which has never been seen on this planet before. That's you.

As you're probably aware, in order to make "you", and not "half Paul and half Diane," some complicated genetic juju had to go down — the 23 chromosomes from each of your parents' gametes had to join forces to make your one-and-only genome (your complete set of DNA), which has been held in the nucleus of almost every one of your cells since you were a wee zygote, or fertilized egg cell. They just keep replicating this information again and again and again.

This process — the one where your cells (the nuclei of which contain all 46 of those original chromosomes your parents gave you on day one) divide over and over to make new ones when the old ones get tired or damaged — is called mitosis. Mitosis happens when you make new fingernail cells or even when you grow a cancerous tumor. Mitosis is such a workhorse you might not know it has a sister process called meiosis, which is equally important, but not as common.

What's the Difference?

"The key to understanding the difference between mitosis and meiosis is not in the steps, but in the final products of each," says Brandon Jackson, assistant professor in the Department of Biological and Environmental Sciences at Virginia's Longwood University. "Mitosis results in two identical 'daughter' cells, each with two versions of every gene — one version from each parent, just like every cell in the body. Meiosis results in four cells called gametes — sex cells — but each has only one version of each gene. This way, when sperm and egg fuse during fertilization, the resulting zygote is back to having two versions of each gene."

So, that's easy enough to remember: If cells are dividing, it's almost always through mitosis, unless the product is a gamete that's planning to meet up with another gamete to make a new organism. In this case, each cell can only have 23 chromosomes instead of the normal 46. So, some shuffling needs to happen in order to make sure each sex cell has half the chromosomes of a normal cell.

It's difficult to describe the differences between the processes of mitosis and meiosis without using terms like 'homologous recombination' and "cytokinesis," which are confusing. It helps to stop thinking about cell division in terms of chromosomes for a moment and, start thinking about sentences.

"Mitosis versus meiosis is my students' nemesis!" says Jackson. "But since DNA is a lot like words strung together to make sentences, we can use words to analogize these events."

One exercise Jackson does in his biology classes involves taking two sentences and calling them "chromosomes." (For the sake of this article, we made Sentence 1 bold to make it easy to follow its path through the processes of mitosis and meiosis.) Both these sentences describe basically the same idea, but Sentence 1 (an egg cell, with 23 chromosomes) comes from the female parent (in bold), and Sentence 2 (a sperm cell, also with 23 chromosomes) comes from the male parent.

Sentence 1: Imagine a rabbit hiding in the bushes.
Sentence 2: Conceptualize a hare cloaked in vegetation.

Both mitosis and meiosis start from here and duplicate the DNA, giving us two of each sentence.

Imagine a rabbit hiding in the bushes.
Imagine a rabbit hiding in the bushes.
Conceptualize a hare cloaked in vegetation.
Conceptualize a hare cloaked in vegetation.

The next step of mitosis separates the duplicates, and then sorts them back out to create twin cells that each contain genetic material inherited from both mother and father. Those can later make duplicates of themselves that are pretty much exactly like the duplicates your red blood cells or liver cells made last year or 20 years ago.

Imagine a rabbit hiding in the bushes.
Conceptualize a hare cloaked in vegetation.
Imagine a rabbit hiding in the bushes.
Conceptualize a hare cloaked in vegetation.

The first stage of Meiosis, (scientifically known as Meiosis I), takes the duplicated DNA that marks the beginning of the mitosis process, copies it, which results in two daughter cells, each containing with full sets of chromosomes and then shuffles them up like a deck of cards:

Conceptualize a rabbit hiding in the vegetation.
Imagine a hare cloaked in bushes.
Imagine a rabbit cloaked in bushes.
Conceptualize a hare hiding in the vegetation.

The first step (scientifically known as Meiosis I) is when a single cell is copied resulting in two daughter cells, each containing a full set of chromosomes.

Conceptualize a rabbit hiding in the vegetation.
Imagine a hare cloaked in bushes.
Imagine a rabbit cloaked in bushes.
Conceptualize a hare hiding in the vegetation.

The second step (scientifically known as Meiosis II) then separates the new daughter cells, putting each into its own cell, leaving four cells with different DNA in each.

Conceptualize a rabbit hiding in the vegetation.
Imagine a hare cloaked in bushes.
Imagine a rabbit cloaked in bushes.
Conceptualize a hare hiding in the vegetation.

"Each sentence says the same thing, but with different versions of each word — each version being an allele, in DNA speak," says Jackson. "Each allele is a mix of words from the male and female parents."

Phew! Meiosis seems like a whole lot of work! Why go through the hassle when you could just do some quick mitosis and be done with it?

"Variation!" says Jackson. "This is the first part of sexual reproduction, the point of which is to increase genetic variation, and this increases an organism's ability to continue to adapt to a changing world."

Let's say the last gamete above (those are the "sentences" formed by meiosis) fertilizes another gamete that says,

Consider a bunny disguised by weeds.

That would make a new cell and organism with the following DNA profile:

Conceptualize a hare hiding in the vegetation.
Consider a bunny disguised by weeds.

Not only is that different than our parent cell, the one we started with, but it's different than either of the grandparents. And if you have dozens of these sentences — humans have 23 pairs of "sentences," after all — and each sentence has thousands of words, every meiosis and fertilization event results in genetic combinations that have probably never existed.

Which is, of course, why you're so special.