Sock Mitosis (Cell Division Part 1)

This year, my teenager is taking AP Biology. Though he took Biology in 9th grade, he’s forgotten a lot of the details. Including the differences between mitosis and meiosis. Yes, I quizzed him. I’m evil that way.

He bore my heavy sigh of disappointment with steely resignation. You see, he could have taken AP Chemistry, but instead deliberately chose Biology knowing his mother is a built-in tutor. I not only taught Anatomy and Physiology, I also taught Introductory Biology to non-biology majors. I have tricks up my sleeve. Or, rather, in the sock drawer.

Sock drawer?

You heard me.

Mitosis and Meiosis not only sound the same, but they share many features. All those chromosomes zipping around, going this way and that way. My former students were confused, so I began hunting around for a better way to cement the similarities and differences in their minds.

Enter the hands-on demonstration.

The day of the mitosis lecture, I arrived with a bag full of socks and clips (magnetic for the giant whiteboard in the front of the classroom) and began to solicit volunteers. They came reluctantly, eyeing the pile of laundry on the desk before them with raised eyebrows and half smiles. Here’s what they endured.

Just in case, a quick review. Humans have 46 chromosomes. That’s our diploid (2n) number. 23 come from mom, and the other 23 come from dad.

• 22 pairs are autosomal chromosomes (general chromosomes)
• one pair are the sex chromosomes (XX or XY)
• for a total count of 23 pairs

Let’s begin by looking at DNA in the form of a karyotype, a convenient way to lay out chromosomes. From big to small, we pair them up and take a close look. Perhaps you remember this from high school biology? Look to the left. This is a human karyotype of an individual with Down’s Syndrome.

See how he has an extra #21 chromosome? That’s what happens with Down’s syndrome, something goes wrong during cell division and an extra chromosome hangs around (we’ll cover the how and why in part 2 since it’s an error that occurs during meiosis). And I said ‘he’. Take a close look. Sex chromosomes always come last on a karyotype. X chromosomes are large. Y chromosomes are small. See how I know this individual is male?

For simplicity, we’re going to work with a species that has less chromosomes. Sock puppets have 4 pairs of chromosomes. 6 autosomes (3 from mom, 3 from dad) and one pair of sex chromosomes (1 from mom, 1 from dad). That’s 8 chromosomes (4 pair). Their sex chromosomes are just like ours, XX=female and XY=male.

So. Below are Jill and Joe’s karyotypes where each sock represents a single chromosome. We call those chromosomes that are of the same size (one from mom and one from dad) homologous chromosomes.

Can you tell Jill’s karyotype from Joe’s? Did you find the small Y chromosome? That’s how you know it’s Joe’s karyotype.

With me?

Good. Moving on.

For MITOSIS, we’ll work with Jill’s chromosomes.

In mitosis, the DNA of a cell duplicates, then separates to form two identical cells. This is a basic function that allows us to replicate existing cells so that we can grow and/or replace cells that have worn out. It’s a process necessary for day-to-day existence.

Mitosis has a number of stages it progresses through.

Interphase > Prophase > Prometaphase > Metaphase > Anaphase > Telophase > Daughter Cells

I’ve listed the bare bone characteristics of each here (trust me, there’s plenty more), but for the purposes of this demonstration, we’re going to follow the DNA, aka socks.

INTERPHASE

• Chromosomes duplicate (not yet condensed)

PROPHASE

• DNA condenses into chromosomes [Imagine someone using yarn (chromatin) to knit (condense the chromatin) into socks] to appear as identical sister chromatids joined together at the centromere
• Mitotic spindle forms

Let’s pause for a moment here.

Sister chromatids are identical.

Identical. I’m repeating that because so, so many students forget this. A chromosome duplicates and is attached to its identical copy at the centromere.

PROMETAPHASE

• Nuclear envelope breaks down
• Microtubules ‘capture’ chromosomes at the kinetochore (located on the centromere)

METAPHASE

• Chromosomes line up on the metaphase plate
• Kinetochore of each sister chromatid attaches to a microtubule from opposite poles

We’ve reached another sticking point that becomes super relevant when we move into meiosis. Lock this down in your head now. Sister chromatids line up on the metaphase plate independently. Take a look back at that karyotype for Jill. Look at autosomal pair #3 (shrimp and octopus). The octopus chromosome (that came from Jill’s mom) has duplicated and lined up on the metaphase plate independently from the duplicated shrimp chromosome (that came from Jill’s dad). Look at the metaphase plate, see how they’re lining up? Mom and dad’s chromosomes have nothing to do with each other right now. They are NOT stuck to each other.

ANAPHASE

• Sister chromatids (chromosomes) separate from each other and are pulled to opposite poles

TELOPHASE and CYTOKINESIS result in DAUGHTER CELLS

• Mitotic spindle breaks down
• Cell cytoplasm divides
• Chromosomes de-condense
• Two cells (daughter cells) result, each is identical to the original cell

See how the word identical is in red? That’s because it’s a super important point. The whole reason for mitosis is to make new cells that are exactly like the one that came before. The body has a whole bunch of checkpoints built in to make sure exact replicas of the DNA are made and then separated 50/50. If something goes wrong, the process might stop and attempt a repair, but if it can’t be fixed, the cell will do a kind of autodestruct on itself. If this safety check fails and an imperfect cell is allowed to live, it leaves us (or Jill) at risk for cancer.

And that, dear readers, is Mitosis at its most basic.

Mitosis is nice and simple, right? Things were going just peachy in your biology class and then… the teacher started talking about Meiosis and ruined everything.

Except Meiosis is sexy. 😉 It’s how we make egg and sperm and propagate our socks.

Er. Species. (Though seriously, socks are always up to something in the wash, disappearing without warning. So you know, who knows what they’re up to…)

Next time, we’ll cover MEIOSIS!

 

READ THE MEIOSIS POST!