Life Science 9-2

Life Science, Grade 7

Cell Division Review, and Where Taxonomy Came From

35 Minutes  |  Room 212  |  Mr. Gibbs

At a Glance

• 0:00 to 0:04  Bell work, silent

• 0:04 to 0:07  Diagnostic check, find out who is lost and where

• 0:07 to 0:22  Cell division, retaught a different way

• 0:22 to 0:33  Where taxonomy came from

• 0:33 to 0:35  Close

Materials: whiteboard only. Nothing to print, nothing to gather.

Do not repeat yesterday. If a student did not understand it the first time, hearing the same words louder will not help. Everything in the review section uses a different analogy, a different order, or a different way in. The recipe book runs through the whole thing and replaces yesterday’s charging cable.

Bell Work

4 minutes silent. Short today, because the review needs the time.

1.  How many chromosomes are in one of your body cells?

2.  Mitosis makes ______ cells. Meiosis makes ______ cells.

3.  What is ONE thing about yesterday’s lesson that you are still confused about? Write it down. This one is not for a grade and I want an honest answer.

1. Diagnostic Check

3 minutes. Find out where the class actually is before you start reteaching.

• Take answers to 1 and 2 fast: 46, and two, and four.

• Then question 3, and this is the important one: ask for hands and take five or six answers out loud. Write each confusion on the corner of the board as a short phrase. Do not answer any of them yet.

• Expect these five, because they are the usual ones:

◦ What a chromosome actually IS.

◦ Why the DNA gets copied before anything happens.

◦ Why meiosis divides twice instead of once.

◦ Where each one happens in the body.

◦ Which one makes identical cells and which one does not.

• Then tell them the plan: "Every one of those is on the board. By the end of the review I am going to cross each one off. If yours is still up there at the end, tell me."

Writing their confusions on the board and crossing them off does two things. It tells you exactly where to spend the fifteen minutes, and it tells a struggling student that being lost is a normal, fixable, out loud thing rather than something to hide.

2. Cell Division, Retaught

15 minutes. One analogy runs through the whole thing. Build the drawing on the board slowly and let them copy it.

Start over with the recipe book  (4 min)

• Set up the analogy and use it for everything that follows: imagine that inside the nucleus of every one of your cells is a giant cookbook. That cookbook has the instructions for making every single part of you. Bones, eyes, blood, hair, everything.

• That cookbook is the DNA. It is very, very long. About two meters if you stretched it out.

• Now the chromosome, and go slowly here: the book is not one huge volume. It is split into 46 separate volumes, and each volume gets closed up and bound tight so you can carry it around. ONE of those closed, bound volumes is a CHROMOSOME.

• Say what a chromosome IS, in one sentence, and write it on the board: a chromosome is a piece of your DNA, coiled up tight into a bundle. It is not something different from DNA. It IS DNA, packed up for moving.

• Check for the misconception directly: ask whether a chromosome and DNA are two different things. The answer is no. Same stuff, different shape. Most of the time your DNA is loose and stringy and you cannot see it. Only when the cell is about to divide does it wind up into chromosomes you could spot under a microscope.

• Cross the first item off the board.

Why the copy comes first  (3 min)

• Pose it as a problem, not a step: you have a 46 volume cookbook. You are about to split into two cells. Each new cell needs a COMPLETE set of 46 volumes, because each new cell has to be able to build anything.

• Ask the question: if you just split your 46 volumes down the middle and gave 23 to each new cell, what would go wrong? Let them answer. Neither cell would have the whole cookbook. Neither one could build everything.

• So: before ANY splitting happens, the cell goes to the photocopier and copies all 46 volumes. Now there are 92, sitting in matched pairs.

• Then the split: when the cell divides, each new cell gets one complete set of 46. Nobody is short a volume.

• Say the general rule: copy first, THEN divide. Every time. That is why the copying step comes before prophase.

• Cross the second item off the board.

Mitosis, drawn slowly  (4 min)

• Draw four boxes across the board and fill them in as you go. Tell them to copy it. A picture they drew themselves is worth more than one they watched you draw.

• BOX 1. One cell. Draw four chromosomes inside it, not 46, because 46 is impossible to draw. Say out loud that you are drawing four to stand for 46.

• BOX 2. The same cell, but now every chromosome has been copied. Draw eight, in four matched pairs, each pair joined in the middle.

• BOX 3. All four pairs lined up single file down the middle of the cell.

• BOX 4. The pairs pulled apart, four going left and four going right, and the cell pinching into two.

• Then count with them out loud: started with four. Ended with two cells, each with four. Same number. Identical to each other and identical to the original.

• Where it happens: body cells. Skin, muscle, bone, stomach. Ask what happens when they get a paper cut and let them say it heals. That is this, happening right now in their hands.

• Cross two more items off the board.

Meiosis, and the second division  (4 min)

• Do not start with the steps. Start with the problem, again: you have 46 chromosomes. Your mom has 46. Your dad has 46. If her cell brought all 46 and his cell brought all 46, you would have 92.

• Write the doubling on the board and let it get ridiculous: 46, then 92, then 184, then 368. Ask how many your great grandchildren would have. Ask if that is possible. It is not.

• So the answer: egg cells and sperm cells have to be made a DIFFERENT way, one that gives them only HALF. 23 instead of 46. Then 23 plus 23 equals 46, and every generation stays the same forever.

• Now the question they asked, why divide twice, and answer it with the cookbook: you start with 46 volumes, then you COPY them like always, so now you have 92. To get down to 23, you have to cut in half TWICE. 92, then 46, then 23. One division is not enough. That is why meiosis divides twice and mitosis divides once.

• Draw it as one line on the board: 92 arrow 46 arrow 23. Three numbers, two arrows, two divisions. Point at each arrow and say "that is one division."

• And the count at the end: you started with one cell and finished with FOUR, and each one has half a set.

• Cross the last item off the board.

Teach your neighbor  (quick check, 2 min)

• The instruction: "Turn to the person beside you. One of you explain mitosis in three sentences. The other explain meiosis in three sentences. Go."

• Circulate and listen. You will learn more about who understands this in ninety seconds than from any quiz. Students who were lost yesterday will either be able to do it or will visibly stall, and you will see which.

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

• Handback question: "Voices off. Look at the board. Is anything still up there that you are confused about?" If a hand goes up, take it now.

3. Where Taxonomy Came From

11 minutes. Start by reminding them they already found the problem themselves.

You already discovered the problem  (2 min)

• Call back to the One Square Meter lab: remember when you went outside, counted everything in your square, and then made up your OWN way to sort what you found?

• Then the part that matters: remember that every single group found something that did NOT fit their groups, or fit into two of them at once? And remember that no two groups made the same categories?

• Land it: that is not because you did it wrong. That is exactly the problem scientists had for about two thousand years. Everybody had their own system, and none of them agreed with each other.

• Name the subject: TAXONOMY is the science of classifying living things and giving them names. Today is where it came from.

Why common names do not work  (3 min)

• Give the best example, and let them enjoy it: there is an animal in North America called a mountain lion. It is also called a cougar. It is also called a puma. It is also called a panther. It is also called a catamount. Five names, ONE animal.

• Now flip the problem: in America a buzzard is a vulture. In England a buzzard is a hawk. Same word, two completely different birds. Ask what happens if an American scientist writes to a British scientist about buzzards.

• And a third kind of failure, which they will like: a starfish is not a fish. A jellyfish is not a fish. A koala bear is not a bear. Common names lie.

• State the need: so scientists needed ONE system, with ONE name per living thing, that works in every country and every language.

Aristotle, the first big try  (2 min)

• About 2,300 years ago in Greece, a man named Aristotle made the first serious attempt. He split all living things into two groups, PLANTS and ANIMALS. Then he sorted animals by where they lived: land, water, or air.

• Ask the class to break it: "Where does a penguin go?" It is a bird, but it swims. "Where does a whale go?" It lives in water, but it breathes air. "What about a frog?"

• Give him credit anyway: his system had real problems, but he was the first to try to organize all of life on purpose, and his system was used for about 2,000 years.

Linnaeus, and the system we still use  (4 min)

• Introduce him: in the 1700s, a Swedish scientist named CARL LINNAEUS built the system we still use today, almost 300 years later. He had two big ideas.

• IDEA ONE, group by what things ARE, not where they live: Linnaeus sorted living things by their shared characteristics, their body structures. And he made the groups NESTED, one inside another, from very broad down to very specific.

• IDEA TWO, give everything a two part name: every living thing gets a name made of two words, and it is always in LATIN. The first word is the GENUS and it is capitalized. The second word is the SPECIES and it is lowercase.

• Give them theirs: humans are Homo sapiens. Every person on this planet, in every country, is Homo sapiens. A scientist in Japan, in Brazil, in Kenya, writes the exact same two words.

• Ask why Latin, and let them guess before you answer: because it is nobody’s everyday language anymore. It does not belong to one country, so no country is favored, and it does not change over time the way living languages do.

And here is the ladder connection

• Put the taxonomy levels on the board, broad down to specific: DOMAIN, KINGDOM, PHYLUM, CLASS, ORDER, FAMILY, GENUS, SPECIES.

• Give them the sentence for remembering it: "Dear King Philip Came Over For Good Soup." Have them write it down.

• Be honest about the history: Linnaeus did not start with all eight of these. He had fewer. Scientists have added levels since then, and DOMAIN was not added until the 1990s, which is more recent than your parents’ childhood. The system is still being adjusted.

• Now make the connection, and this is the point of putting the two together: draw the organizational ladder beside it. Ask what the two have in common.

• The answer, and pull it out of them: both are LADDERS. Both have levels. And in both, every level FITS INSIDE the one above it. A cell is part of a tissue, which is part of an organ. A species is part of a genus, which is part of a family.

• Then the difference: the organizational ladder is about how one living thing is BUILT, from small parts up to whole systems. The taxonomy ladder is about how living things are RELATED to each other, from huge broad groups down to one specific kind.

• One is a ladder of PARTS. The other is a ladder of GROUPS. Say it that way and write both on the board.

4. Close

2 minutes.

• Point back at the crossed off list from the beginning of class and ask one more time whether anything is left. Take it if there is.

• One question to end on: "If you found a brand new animal in the woods tomorrow, what would you have to figure out before you could name it?" Take one or two answers. What it is LIKE, what it is related to. That is next.

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

Objectives

• Students will explain that a chromosome is DNA coiled up tightly, not a separate structure.

• Students will explain why DNA is copied before a cell divides.

• Students will explain why meiosis requires two divisions and mitosis requires one.

• Students will state that taxonomy is the science of classifying and naming living things.

• Students will explain why common names are inadequate and why scientists use two part Latin names.

• Students will compare the taxonomy levels to the levels of organization and identify both as nested hierarchies.