Life Science 9-1

Life Science, Grade 7

Cell Growth and Division

Mitosis and Meiosis  |  35 Minutes  |  Room 212  |  Mr. Gibbs

At a Glance

• 0:00 to 0:05  Bell work, silent

• 0:05 to 0:08  Bell work reviewed, and the bridge

• 0:08 to 0:13  Why cells divide at all

• 0:13 to 0:20  Mitosis, the copy machine

• 0:20 to 0:27  Meiosis, and the halving problem

• 0:27 to 0:34  Activity, the Chromosome Line Up and a quick check

• 0:34 to 0:35  Close

Materials: whiteboard, an old charging cable or a pair of earbuds, and clear floor space at the front for eight students. Nothing to print.

Bell Work

5 minutes silent. On the board before the bell.

1.  Which organelle in a cell holds the instructions?

2.  Name the four letters of DNA. Which one pairs with which?

3.  What is the THIRD part of cell theory?

4.  You started out as ONE single cell. You now have trillions of them. Where did all the extra cells come from?

1. Bell Work Reviewed

3 minutes. Questions 3 and 4 answer each other, so take 3 first.

• 1: the nucleus.

• 2: A, T, C, G. A pairs with T, C pairs with G.

• 3: all cells come from cells that already existed. Make them say the whole thing.

• 4, and connect it straight to 3: every one of those trillions of cells came from a cell that split in two. Then that cell split. Then those split. Over and over, from one cell, for years. Today is HOW that happens.

2. Why Cells Divide at All

5 minutes.

• Give the three reasons and keep them short:

◦ GROWTH. You get bigger by making MORE cells, not by inflating the ones you have.

◦ REPAIR. You cut your finger and it heals. New cells filled the gap.

◦ REPLACEMENT. Cells wear out and die. Your skin replaces itself constantly, and dandruff is dead skin cells flaking off.

• Now ask the question that makes them think: why not just make your cells BIGGER instead of making more of them? Take a couple of guesses.

• Answer it using last class: everything a cell needs has to come IN through the membrane, and all the waste has to go OUT through it. If a cell gets too big, the inside is too far from the edge and the membrane cannot keep up. So the cell splits instead.

That connects directly to the cell transport lesson. A cell that grows too large cannot get food in and waste out fast enough. Say it out loud so they see the two lessons are the same subject.

Chromosomes, and why DNA coils up

• Bridge from last week: they already know DNA is a long twisted ladder inside the nucleus. Ask how long the DNA in ONE of their cells would be if you stretched it out. About two meters. Taller than anyone in the room.

• Then the problem: if you had to move two meters of thread out of one room and into another without tangling it, what would you do first? Wind it up.

DEMONSTRATE:  Hold up a tangled charging cable or a pair of earbuds. Ask what happens when you shove them in a pocket loose. Then wind one up neatly. That is what a cell does before it divides. The DNA coils up tight so it can be moved without tangling.

• Name the result: when DNA coils up tight like that, each coiled up piece is called a CHROMOSOME. You cannot even see them most of the time. They only show up when a cell is getting ready to divide.

• Give them the number: a human body cell has 46 chromosomes. Write 46 on the board and circle it. It matters in about ten minutes.

3. Mitosis, the Copy Machine

7 minutes. Draw it on the board as a four step strip.

• Say what it produces first, before any steps: MITOSIS is when one cell makes TWO cells that are exactly the same as the original. Same instructions, same chromosome number, identical twins.

• Give the number: the original cell has 46 chromosomes. Each new cell has 46. Nothing is lost and nothing is gained.

• Where it happens: in your body cells. Skin, muscle, bone, stomach lining. This is the growth, repair, and replacement machine.

The steps, told as a story

• FIRST, the DNA gets copied. Before anything else happens, the cell makes a complete second copy of every chromosome. Now there are two of each, stuck together in the middle.

• PROPHASE. The DNA coils up into chromosomes you could actually see. The wall around the nucleus breaks down and gets out of the way.

• METAPHASE. All the chromosomes line up single file down the middle of the cell. Think of a line painted down the center.

• ANAPHASE. The pairs get pulled apart. One of each goes to one end of the cell, the other goes to the other end.

• TELOPHASE. A new nucleus forms around each set, and then the cell pinches in the middle and splits into two.

• The result, said one more time: two cells. Both identical. Both with 46 chromosomes. Copy machine.

The four phase names are here because most seventh grade tests ask for them, but do not spend the period on memorizing them. If a student can tell you the STORY, copy the DNA, line up, get pulled apart, split, they understand mitosis. The names can be drilled later.

4. Meiosis, and the Halving Problem

7 minutes. Set up the problem BEFORE you give the answer. This is the best part of the lesson.

• Start with a question, not a definition: you got half your instructions from your mom and half from your dad. Her egg cell and his sperm cell joined together and became the one cell you started as.

• Now the trap: if a body cell has 46 chromosomes, and mom’s cell brought 46, and dad’s cell brought 46, how many would YOU have? Let them do it. 92.

• Push it one more generation: and how many would YOUR kids have? 184. And theirs? 368. Ask if that can possibly be right. It cannot. Something has to cut the number in half BEFORE the two cells join.

• Now name the solution: that something is MEIOSIS. It is a special kind of cell division that makes SEX CELLS, meaning egg cells and sperm cells, and it gives each one only HALF the chromosomes. 23 instead of 46.

• Then close the loop: 23 from mom plus 23 from dad equals 46. Back to normal, every single generation, forever.

The four differences, side by side on the board

• Draw two columns and fill them in together:

◦ MITOSIS makes TWO cells. MEIOSIS makes FOUR.

◦ MITOSIS keeps the full number, 46. MEIOSIS cuts it in half, to 23.

◦ MITOSIS makes cells that are IDENTICAL. MEIOSIS makes cells that are all DIFFERENT from each other.

◦ MITOSIS is for body cells, for growth and repair. MEIOSIS is ONLY for making egg and sperm cells.

• One more sentence, and it is worth saying: because meiosis mixes the instructions up, no two of those cells are alike. That is why brothers and sisters are not identical even though they have the same parents.

5. Activity, The Chromosome Line Up

7 minutes. Eight students at the front, everyone else seated with a job.

• The seated job, given first: "Watch and be ready to tell me how many cells we ended up with, and how many chromosomes each one has." Everyone has something to do.

Round 1, mitosis  (2 min)

• Bring up EIGHT students. Pair them off into four pairs. Each PAIR is one chromosome that has already been copied, so the two partners are identical and hold hands.

• Have all four pairs line up single file down the middle of the front of the room. That is metaphase.

• On your signal, each pair lets go and the partners walk to opposite sides. That is anaphase.

• Draw an imaginary line between them. Two groups of four. Ask the class: how many cells? Two. How many chromosomes each? Four. Same as we started with.

Round 2, meiosis  (3 min)

• Reset the same eight students into four pairs, but now arrange them as TWO groups of two pairs. Tell them the pairs that match are the ones from mom and from dad.

• FIRST DIVISION: the matching pairs separate. Whole pairs, still holding hands, walk to opposite sides. Now there are two groups, each with two pairs.

• SECOND DIVISION: now the partners in each pair let go and separate again. Now there are FOUR groups.

• Ask the class: how many cells? Four. How many chromosomes each? Two. Half of what we started with. Point at it and let them see it.

Keep the same eight students for both rounds so the class can see that the difference is the number of divisions, not the number of people. If eight is too many bodies for your floor space, use six as three pairs. The arithmetic still works.

Round 3, quick check  (2 min)

• Everyone seated. The rule: I say a statement. Hold up ONE finger for mitosis, TWO for meiosis, THREE for both. Fingers at your chest, no calling out.

• Makes two cells. (1)

• Makes four cells. (2)

• Copies the DNA first. (3)

• Makes egg and sperm cells. (2)

• Heals a cut on your finger. (1)

• Cuts the chromosome number in half. (2)

• Makes cells that are identical to each other. (1)

• Happens inside your body. (3)  Expect disagreement on this one, and it is a good one to argue about.

RESET.  "Reset in three, two, one." Wait in silence until every student has finished before you say anything else.

• Handback question: "Voices off. Why does meiosis have to cut the number in half?"

6. Close

1 minute.

• One sentence from a student: the difference between mitosis and meiosis.

• Point back at the bell work: you started as one cell. Mitosis is how you got to trillions. Meiosis is how that one cell got made in the first place.

• Name the reading and the due date. Dismiss by procedure, not by the bell.

Objectives

• Students will give three reasons cells divide, and explain why cells divide instead of growing larger.

• Students will explain that a chromosome is tightly coiled DNA and state that human body cells have 46.

• Students will describe mitosis as producing two identical cells with the full chromosome number.

• Students will explain why meiosis must cut the chromosome number in half.

• Students will state at least three differences between mitosis and meiosis.