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Quiz 13 · Part II: The Cell and the Organism

Cell Division

Two identical, or four that are half and all different

The Guide

Two ways to make a new cell

Every new cell is made by an older cell dividing. Your body does this every day, to grow and to replace cells that are worn out or injured. This quiz is about how cells divide. (Scientists count “cells come only from cells” as one of the three parts of cell theory, which Quiz 9 covers.) There are two routes, they produce very different results, and nearly every question on this topic turns on telling them apart.

Both begin the same way. Before a cell divides at all, it copies its DNA, so that there is a full set to hand on. The DNA is carried on structures called chromosomes, and counting them is how you tell the two routes apart.

Mitosis and meiosis compared One body cell holding four chromosomes divides two different ways. By mitosis it becomes two cells, each holding four chromosomes, for growth and repair. By meiosis it becomes four cells, each holding two chromosomes, for making sex cells. The two mitosis cells are labeled diploid and the four meiosis cells are labeled haploid. The drawing uses four chromosomes for legibility; a human body cell has forty-six. One body cell four chromosomes MITOSIS for growth and repair MEIOSIS for making sex cells Two cells four chromosomes each diploid Four cells two chromosomes each haploid Drawn with four. A human body cell has 46.

Mitosis — for building and maintaining the body

An ordinary body cell — skin, muscle, liver, bone — divides once and becomes two. Each of the two carries the same chromosomes the parent had, in the same number, so each can do the same work. The name for an ordinary body cell is a somatic cell.

Cells like these carry two complete sets of chromosomes, one set inherited from each parent of the organism. The word for carrying two sets is diploid. Every cell in your body is diploid except for the sex cells.

This is how you grew from a single cell, how a cut closes over, and how worn-out cells get replaced. It happens constantly, everywhere in the body.

Meiosis — for making sex cells

A cell in the ovaries or the testes, or in the parts of a flower that make pollen and ovules, divides twice and becomes four. A cell set aside for this job is called a germ cell — nothing to do with germs that make you ill; the older sense of the word, as in the germ of an idea, something that a whole thing grows from.

Each of the four carries one set of chromosomes instead of two — half of what the parent cell had. The word for carrying one set is haploid, and the first syllable is a fair reminder: hap, half.

These four are the sex cells: eggs and sperm in animals, pollen and ovules in plants. And no two of them are alike, which is the part that matters in Part IV and Part V.

 MitosisMeiosis
What it is forGrowth, repair, replacementMaking sex cells
Starts froma somatic cella germ cell
Dividesoncetwice
Cells produced24
Chromosome sets in eachtwo — diploidone — haploid
The new cells areidentical to the parentall different from each other

One thing the diagram simplifies. It shows a single cell branching two ways, which makes the comparison easy to see and is not quite true. A somatic cell only ever divides by mitosis. Meiosis happens only in germ cells, which are set aside early for that one purpose. Read the right-hand branch as starting from a germ cell, not from the skin cell on your hand.

And one thing the words simplify. One set or two sets is the whole story in animals, and not in everything. Plants often carry more: bread wheat has six sets, most potatoes four, and a banana three, which is why it has no seeds worth the name. Some tissues inside an animal double up on purpose as well, including cells in your own liver. None of this is on the test and none of it changes anything above. It is here only so that haploid and diploid do not look like a complete pair the way odd and even do.

Why the halving is not optional. A human body cell carries 46 chromosomes. If an egg carried 46 and a sperm carried 46, the child would start life with 92, and that child's children with 184. Within a few generations the number would be impossibly large. Halving before fertilization is what holds the number steady from one generation to the next.

How the chromosome number stays at 46 across a generation Two body cells, each holding 46 chromosomes. Meiosis halves the number, giving an egg with 23 and a sperm with 23. Fertilization joins them, and the new cell holds 46 again. 46 body cell 46 body cell meiosis halves it 23 egg 23 sperm fertilization 46 the new cell

Why the four are all different. Chromosomes come in pairs, one of each pair from each parent. When meiosis splits them up, which member of each pair ends up in which cell is settled at random, and the pairs also trade pieces with one another before they separate. So every egg and every sperm carries a mix that has never existed before. That is where variety between brothers and sisters comes from, and it is the raw material for everything in Part V.

Worked Examples

Two questions, worked through

Example 1. A sunflower grows six inches in a week. Which process made the new cells?

Ask what the new cells have to be. They have to be sunflower stem cells, with a full set of chromosomes, able to do the job the cells beside them are doing. Only one of the two routes produces cells like the ones it started with.

Example 2. A fruit fly body cell has 8 chromosomes. How many are in a fly sperm cell, and how many in a fertilized fly egg?

Sperm are sex cells, so they come from meiosis, so they carry half: 4. Fertilization joins a sperm to an egg, and the egg also carries 4. Four and four make 8.

The Quiz

Ten questions

Answer all ten, then press the button at the bottom. Nothing is timed.

Diagram — questions 1 to 5

Use the comparison diagram from the Guide above: one body cell holding four chromosomes, dividing by mitosis on the left and by meiosis on the right.

1.According to the diagram, how many cells does mitosis produce from one cell?

2.How many cells does meiosis produce from one cell?

3.Compared with the parent cell, what does each cell from meiosis carry?

4.Which process does a somatic cell use for growth and repair?

5.A cut on your hand heals over with new skin. Which process built those cells?

Diagram — questions 6 and 7

Use the generation diagram from the Guide above: two body cells of 46 chromosomes, an egg and a sperm of 23 each, and the new cell they make together.

6.Why must meiosis cut the chromosome count in half?

7.Suppose sex cells were not halved first. What would follow?

Passage — questions 8 to 10

A strawberry grower increases her stock two ways. She pegs down runners — side shoots that root where they touch the soil and grow into new plants. She also saves seeds from the ripe fruit and raises plants from those.

The plants grown from runners are identical to the plant they came from: same fruit size, same flavor, same everything. The plants grown from seed vary. Some bear larger fruit and some smaller, and a few shrug off a leaf fungus that damages the rest.

8.Which of the grower's two methods produces plants identical to the parent?

9.Why do the seed-grown plants vary?

10.A new fungus reaches the farm. Which planting is at greater risk of being lost entirely?

Send this line to your teacher

The line records which questions you missed and which answer you chose. That is more useful to your teacher than the score, because it shows where a question went wrong. If a question felt unclear even though you got it right, add its number with a question mark — for example 5? — before you send it.

Score ______ / 10    Missed — write the question number and the letter you chose:
______________________________________________________________

The Key

Answers, and the trap in each one

1. B — two. Count the cells on the left-hand branch of the diagram. D is the count on the other branch. The two numbers are the fastest way to tell the processes apart, and the fastest way to mix them up.
2. D — four. Count the cells on the right-hand branch. Meiosis divides twice, which is why four come out of it. B is the mitosis count. If you had to guess which process gives more cells, the answer is meiosis — but each one is smaller in what it carries.
3. A — one set, haploid. The parent holds four chromosomes, which is two sets of two. Each meiosis cell holds two chromosomes, which is one set. One set is haploid. B is a true statement about the wrong process — the cells from mitosis are diploid. The two words look alike and describe opposite outcomes, so it is worth fixing the pair now. A memory aid: think of hap- as half. Sex cells are haploid; every other cell in your body is diploid.
4. C — mitosis. A somatic cell is an ordinary body cell, and ordinary body cells divide only one way. New ones have to do the job of the cells around them, which means carrying both full sets of chromosomes. B is the other branch, and its cells could not do the work — a sex cell carries only one set of chromosomes, and it is made for one job: to join with a sex cell from the other parent. Meiosis makes cells for one purpose only.
5. D — mitosis, because the new cells must match. Healing is repair, and repair needs replacements identical to what was lost. Skin cells are body cells. A gives the right kind of reason — a lot of cells are needed — attached to the wrong process. Quantity is not what separates these two. What separates them is whether the new cells match the old, and how many chromosomes they end up with.
6. A — so the number stays the same across generations. Two halves make a whole. 23 from the egg plus 23 from the sperm gives 46, which is where the parents started. The halving exists to be undone by fertilization. C sounds close and points the wrong way. Offspring do not end up with fewer chromosomes than their parents; they end up with exactly as many. Only the sex cells carry half, and only until they meet.
7. D — twice their parents' number. 46 from one cell plus 46 from the other gives 92. The generation after that would start at 184. The doubling compounds and there is no way back. A is what actually happens because the halving exists, so it is the right answer to a different question. Read what the question supposes: it removes the halving and asks what follows.
8. C — the runners. A runner grows from the parent plant by mitosis, so every cell in it is a copy. The passage says so plainly: same size, same flavor, same everything. A whole field of runner plants is one plant, repeated. Gardeners have been making copies of living things for thousands of years without a word for it.
9. B — meiosis gives each seed a different mix. Seeds come from pollen and ovules, and those come from meiosis, which shuffles the chromosomes so that no two are alike. The variation was built in before the seed was planted. D names something real that would also produce differences — soil does vary — but it explains the wrong kind of difference. Better soil gives a bigger plant. It does not give a plant that can shrug off a fungus while its neighbor cannot. That difference was inherited.
10. A — the runners. Every runner plant is identical, so whatever the fungus can do to one it can do to all of them. Among the seed-grown plants a few already resist, and those few survive to carry on. C treats the risk as being about how the fungus travels. It is not. It is about whether anything in the field is different enough to survive. Hold on to this item. Part V, on evolution, comes back to this idea, and it is not hypothetical. Ireland's potato crop in the 1840s was grown from cuttings, nearly all of one kind, and when a disease arrived that this kind could not resist, there was no plant in the country that could.

Your Score

What the number means

8 to 10Solid. Check topic 13 on your map. That completes Part II.
6 or 7Close. Read the whole key, then take this again in a few days before you check the box.
5 or fewerWorth another pass. Draw the comparison from memory — one cell at the top, two branches, and the counts underneath. It is a small drawing and it carries the whole quiz.

Misses on 1, 2, or 3 are the counts, and the drawing fixes those faster than the words do. Misses on 4 or 5 mean the two processes are still trading places — the test is always whether the new cells have to match. Misses on 6 or 7 are the halving arithmetic, which is worth working out on paper once with any numbers you like. A miss on 9 or 10 is the one to sit with: both are about where variation comes from, and Part V cannot be understood without them.

That is Part II

Six quizzes, and they build in one line. Living things are made of cells. Cells have parts, and each part has a job. The boundary decides what crosses. Sunlight is packed into sugar, and the sugar is broken open again for energy. And cells make more cells — two ways, for two purposes.

Part III turns to the human body: the systems, how they keep you steady, and what happens when they fail. It is the largest run in the series and the one your own life gives you the most to compare against.

The plate for this quiz

The gallery image is one of Walther Flemming's drawings of dividing cells, published in 1882. He stained salamander tissue with dyes that happened to cling to threads inside the nucleus, and under the microscope he watched those threads double, line up along the middle, and get pulled apart to opposite ends. He named the threads for the way they took the color — chromatin, from the Greek for color — and he named the process mitosis.

He had no idea what the threads were for. Heredity was not on his mind, Mendel's work was unread and nearly forgotten, and the word gene did not exist. He drew what he saw, accurately, and it took another twenty years for anyone to work out that he had been watching inheritance being handed on. The plates are public domain by age.