Answer key · Unit 6

Cell division.

Each lesson’s practice questions, their correct answers and the mistakes they spot. Answers read off the Scene use the same setting the learner sees.

Misconception catalogue

Answer key 5

Lesson 1 of 5 · About 20 minutes

The cell cycle

Why cells divide, what a cell does before it divides, how to count chromosomes and chromatids, and why the two daughter cells match the mother cell.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Why cells divide
Practice · Choose
Question
Every day your body carries out billions of mitotic divisions. Which of these is mitosis not used for?

Correct answer
- Correct Making sperm and eggs
Setup, choices and feedback
The Scene at this step
Stage
5 %
Chromosome pairs
4
Show the spindle
Off
Choices
Shown in a random order.
- Correct Making sperm and eggs
- Wrong A child’s growth
  Misconception: Thinks sperm and eggs are made by mitosis.
  Feedback from the catalogue
  Mitosis makes cells identical to the mother cell, with $2n$ ⁠ chromosomes: for growth, for healing wounds and for replacing worn-out cells. Sex cells are made by meiosis, which divides twice and leaves each cell with $n$ ⁠ chromosomes, one copy of each pair. If eggs and sperm were made by mitosis, the number of chromosomes would double every generation.
- Wrong Closing a cut in the skin
  Misconception: Thinks sperm and eggs are made by mitosis.
  Feedback from the catalogue
  Mitosis makes cells identical to the mother cell, with $2n$ ⁠ chromosomes: for growth, for healing wounds and for replacing worn-out cells. Sex cells are made by meiosis, which divides twice and leaves each cell with $n$ ⁠ chromosomes, one copy of each pair. If eggs and sperm were made by mitosis, the number of chromosomes would double every generation.
- Wrong Replacing worn-out cells of the gut lining
  Misconception: Thinks sperm and eggs are made by mitosis.
  Feedback from the catalogue
  Mitosis makes cells identical to the mother cell, with $2n$ ⁠ chromosomes: for growth, for healing wounds and for replacing worn-out cells. Sex cells are made by meiosis, which divides twice and leaves each cell with $n$ ⁠ chromosomes, one copy of each pair. If eggs and sperm were made by mitosis, the number of chromosomes would double every generation.
Feedback for a right answer
Sex cells are made by meiosis, which leaves them half the chromosomes. Everything else, growth, healing and replacement, needs cells exactly like the cell they came from, and that is what mitosis gives.

Hint
Mitosis makes cells identical to the mother cell. Which of these cells should not be identical?

Worked solution
1. Mitosis makes two cells from one, with exactly the same chromosomes.
2. Growth, healing and replacing cells need exactly that: more cells of the same kind.
3. A sex cell needs only one copy of each pair, so that fertilisation gives a full number again. So it is made by meiosis.
Step 2 of 5
Open this step
Counting chromosomes
Practice · Calculate
Question
The cell in the Scene has $4$ ⁠ pairs of chromosomes, like a fruit fly, and it is in prophase: the chromosomes have coiled into X shapes. How many chromosomes does the cell hold?

Correct answer
8 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
22 %fixed
Chromosome pairs
4fixed
Show the spindle
Off
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Chromosomes in each cell, counted by centromeres (sisters count as two once they part)/nodes/cell/outputs/chromosomes
Wrong answers it recognises
- 4 ± 0.1
  Misconception: Counts the homologous pairs as the number of chromosomes.
  Feedback from the catalogue
  A homologous pair is two chromosomes: one from the mother and one from the father, the same size and with the same genes. In the Scene they are marked in two colours. A fruit fly has $4$ ⁠ pairs, that is $8$ ⁠ chromosomes, $2n = 8$ ⁠. A human has $23$ ⁠ pairs, $46$ ⁠ chromosomes.
- 16 ± 0.1
  Misconception: Counts each chromatid of an X-shaped chromosome as a chromosome of its own.
  Feedback from the catalogue
  An X shape is one chromosome that has already been copied: two identical sister chromatids joined at a centromere. Chromosomes are counted by their centromeres, so one X is one chromosome, holding two DNA molecules. Only in anaphase, when the sisters part and each gets a centromere of its own, does each one count as a chromosome.
Feedback for a right answer
$4$ ⁠ pairs, $8$ ⁠ chromosomes: $2n = 8$ ⁠. Count them in the Scene: four pink from the mother and four blue from the father.

Hint
Each pair has one chromosome from the mother and one from the father.

Worked solution
1. A homologous pair is two chromosomes.
2. $4$ ⁠ pairs: $4 \times 2 = 8$ ⁠ chromosomes.
3. That is the diploid number, $2n = 8$ ⁠.
If you answered $4$ ⁠, you counted pairs. If you answered $16$ ⁠, you counted each chromatid as a chromosome.
Step 3 of 5
Open this step
Copied in advance
Practice · Calculate
Question
The cell copied its DNA in interphase, and now it is dividing. How many DNA molecules, that is chromatids, does it hold?

Correct answer
16 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
22 %fixed
Chromosome pairs
4fixed
Show the spindle
Off
Accepted within
± 0.1
Computed from the Scene at this step’s setting
DNA molecules (chromatids) in each cell/nodes/cell/outputs/chromatids
Wrong answers it recognises
- 8 ± 0.1
  Misconception: Thinks the DNA is copied during mitosis itself, so a cell in interphase holds single, uncopied chromosomes.
  Feedback from the catalogue
  The DNA is copied before mitosis, in the S phase of interphase. When mitosis begins, every chromosome is already two sister chromatids. So a fruit-fly cell ready to divide holds $8$ ⁠ chromosomes but $16$ ⁠ DNA molecules. Mitosis itself only shares out what was already copied.
- 4 ± 0.1
  Misconception: Counts the homologous pairs as the number of chromosomes.
  Feedback from the catalogue
  A homologous pair is two chromosomes: one from the mother and one from the father, the same size and with the same genes. In the Scene they are marked in two colours. A fruit fly has $4$ ⁠ pairs, that is $8$ ⁠ chromosomes, $2n = 8$ ⁠. A human has $23$ ⁠ pairs, $46$ ⁠ chromosomes.
Feedback for a right answer
$8$ ⁠ chromosomes, each two sister chromatids: $16$ ⁠. That is why the chromosomes look like an X in prophase.

Hint
After copying, how many chromatids is each chromosome made of?

Worked solution
1. The cell has $8$ ⁠ chromosomes.
2. The DNA was copied in interphase, so each chromosome is two chromatids.
3. $8 \times 2 = 16$ ⁠ DNA molecules.
If you answered $8$ ⁠, you thought the DNA is copied only during division itself.
Step 4 of 5
Open this step
One becomes two
Practice · Reach the target
Question
Move Stage until the tightening ring closes the cell off completely, and the Scene holds two separate cells. Press Check.

Goal
- 2 ± 0.1
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
Stage
42 %
Chromosome pairs
4
Show the spindle
Off
Read from the Scene at
Separate cells once the ring has pinched them apart: 1, 2, or 4 after meiosis/nodes/cell/outputs/cells
One setting that reaches it
Stage
97 %
Chromosome pairs
1
Feedback for a right answer
Two daughter cells, each with a nucleus of its own, at half the mother cell’s volume. Now each will grow, copy its DNA, and be able to divide again: that is the cell cycle.

Feedback for a wrong answer
Not yet. Even when the cell looks almost cut in two, it is still one cell until the ring closes.

Hint
Splitting the cell itself, cytokinesis, comes last.

Worked solution
1. Division starts in the nucleus: prophase, metaphase, anaphase and telophase.
2. Only after that, in cytokinesis, a ring of proteins tightens round the cell’s equator.
3. Move Stage almost to the end, above 97%, until the ring closes.
Step 5 of 5
Open this step
The daughters
Practice · Calculate
Question
Now the cell has $3$ ⁠ pairs, like a mosquito, and the division is over. How many chromosomes are in each of the two daughter cells?

Correct answer
6 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
100 %fixed
Chromosome pairs
3fixed
Show the spindle
On
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Chromosomes in each cell, counted by centromeres (sisters count as two once they part)/nodes/cell/outputs/chromosomes
Wrong answers it recognises
- 3 ± 0.1
  Misconception: Thinks each daughter of mitosis gets half the parent's chromosomes.
  Feedback from the catalogue
  Mitosis keeps the number. Before division every chromosome was copied, and in anaphase the two sisters of each chromosome are pulled to opposite poles. So each daughter gets one copy of every chromosome: the same $2n$ ⁠ chromosomes as the mother cell, exactly the same information. What is split in two is the copied DNA, not the number of chromosomes.
- 12 ± 0.1
  Misconception: Thinks the chromosome count in the cell stays 2n during anaphase.
  Feedback
  The mother cell holds $12$ ⁠ chromosomes only in anaphase, when the sisters part. When the cell pinches in two, they are shared half and half between the two cells.
Feedback for a right answer
$6$ ⁠ in each cell, as in the mother cell: $2n = 6$ ⁠. Mitosis does not change the number of chromosomes, and the two daughters are identical to each other and to the cell they came from.

Hint
How many chromosomes did the mother cell have, before dividing?

Worked solution
1. The mother cell has $3$ ⁠ pairs, that is $6$ ⁠ chromosomes, each two chromatids.
2. In anaphase the two sisters of each chromosome are pulled to opposite poles.
3. Each daughter gets one sister of every chromosome: $6$ ⁠ chromosomes.
If you answered $3$ ⁠, you thought mitosis halves the number of chromosomes. That happens only in meiosis.

Lesson 2 of 5 · About 25 minutes

Mitosis step by step

The five phases of mitosis by what you see in the cell: prophase, metaphase, anaphase, telophase and cytokinesis, and how many chromosomes the cell holds in each.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Prophase
Practice · Reach the target
Question
Move Stage to metaphase: the chromosomes are lined up along the cell’s equator, and spindle fibres reach them from both poles. Press Check.

Goal
- 2 ± 0.1
  At the start of the step: 0
Setup, choices and feedback
The Scene at this step
Stage
0 %
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Read from the Scene at
Mitosis: 0 interphase, 1 prophase, 2 metaphase, 3 anaphase, 4 telophase, 5 cytokinesis; meiosis: 0 interphase, 1–4 prophase I to telophase I, 5–8 prophase II to telophase II/nodes/cell/outputs/phase
One setting that reaches it
Stage
30 %
Chromosome pairs
1
Feedback for a right answer
At metaphase every chromosome is tied by fibres to both poles, one sister to each. The cell checks they are all attached, and only then goes on.

Feedback for a wrong answer
Not yet. Look for the moment when all the chromosomes stand in one line across the middle.

Hint
First the chromosomes coil into X shapes and the nucleus breaks up. Only after that do they line up.

Worked solution
1. Interphase: loose threads in the nucleus.
2. Prophase: the chromosomes coil into X shapes, the nucleus breaks up and the centrosomes move to the poles.
3. Metaphase: the chromosomes in a line on the equator. That happens roughly between 30% and 46% of Stage.
Step 2 of 5
Open this step
Metaphase
Practice · Predict, then look
Question
The chromosomes are lined up on the equator. What will happen next?

Correct answer
- Correct Each X will split, and its two sisters will be dragged to opposite poles
Setup, choices and feedback
The Scene at this step
Stage
42 %fixed
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Choices
- Correct Each X will split, and its two sisters will be dragged to opposite poles
- Wrong The mother’s and the father’s chromosome of each pair will be pulled apart
  Feedback
  That is what happens in the first division of meiosis. In mitosis each chromosome splits into its two sisters, so each daughter gets both the pink and the blue.
- Wrong The cell will pinch in two at once
  Misconception: Thinks each daughter of mitosis gets half the parent's chromosomes.
  Feedback from the catalogue
  Mitosis keeps the number. Before division every chromosome was copied, and in anaphase the two sisters of each chromosome are pulled to opposite poles. So each daughter gets one copy of every chromosome: the same $2n$ ⁠ chromosomes as the mother cell, exactly the same information. What is split in two is the copied DNA, not the number of chromosomes.
Once the prediction is locked in, the Scene changes
Stage
55 %fixed
The Scene then shows: 3 · Mitosis: 0 interphase, 1 prophase, 2 metaphase, 3 anaphase, 4 telophase, 5 cytokinesis; meiosis: 0 interphase, 1–4 prophase I to telophase I, 5–8 prophase II to telophase II/nodes/cell/outputs/phase
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Anaphase: the centromere of every chromosome comes apart, and the spindle fibres shorten and pull each sister to its pole. A full set gathers at each pole.

Hint
Each X is two identical chromatids. Each daughter needs one copy of each.

Worked solution
1. At metaphase every chromosome is tied to both poles, a sister to each side.
2. In anaphase the link between the sisters lets go, and the fibres pull them to opposite poles.
3. After the reveal the Scene is at phase $3$ ⁠, anaphase.
Separating homologous pairs belongs to meiosis, not mitosis.
Step 3 of 5
Open this step
Anaphase
Practice · Calculate
Question
The cell has $3$ ⁠ pairs, $2n = 6$ ⁠, and it is in the middle of anaphase. How many chromosomes does the whole cell hold at this moment?

Correct answer
12 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
55 %fixed
Chromosome pairs
3fixed
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Chromosomes in each cell, counted by centromeres (sisters count as two once they part)/nodes/cell/outputs/chromosomes
Wrong answers it recognises
- 6 ± 0.1
  Misconception: Thinks the chromosome count in the cell stays 2n during anaphase.
  Feedback from the catalogue
  In anaphase every chromosome splits: the two sister chromatids part, and each now has a centromere of its own, so each is a chromosome. Until the cell pinches in two it therefore holds twice as many chromosomes, $4n$ ⁠. A human cell in anaphase holds $92$ ⁠ chromosomes, and each of the two daughters, once it has split, $46$ ⁠.
- 3 ± 0.1
  Misconception: Thinks each daughter of mitosis gets half the parent's chromosomes.
  Feedback from the catalogue
  Mitosis keeps the number. Before division every chromosome was copied, and in anaphase the two sisters of each chromosome are pulled to opposite poles. So each daughter gets one copy of every chromosome: the same $2n$ ⁠ chromosomes as the mother cell, exactly the same information. What is split in two is the copied DNA, not the number of chromosomes.
Feedback for a right answer
$12$ ⁠: each of the $6$ ⁠ chromosomes has split into two sisters, each now with a centromere of its own. There are $6$ ⁠ at each pole, and once the cell pinches in two, $6$ ⁠ in each daughter.

Hint
Chromosomes are counted by their centromeres. How many centromeres are there once each X has split?

Worked solution
1. Before anaphase: $6$ ⁠ chromosomes, each two chromatids, $12$ ⁠ chromatids.
2. In anaphase each chromatid gets a centromere of its own and becomes a chromosome.
3. In the whole cell: $12$ ⁠ chromosomes, $6$ ⁠ at each pole.
If you answered $6$ ⁠, you counted as if the sisters were still joined.
Step 4 of 5
Open this step
Telophase
Practice · Reach the target
Question
Move Stage to telophase: at each pole a new nuclear envelope forms round the set that arrived there, and the chromosomes begin to loosen. Press Check.

Goal
- 4 ± 0.1
  At the start of the step: 3
Setup, choices and feedback
The Scene at this step
Stage
55 %
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Read from the Scene at
Mitosis: 0 interphase, 1 prophase, 2 metaphase, 3 anaphase, 4 telophase, 5 cytokinesis; meiosis: 0 interphase, 1–4 prophase I to telophase I, 5–8 prophase II to telophase II/nodes/cell/outputs/phase
One setting that reaches it
Stage
62 %
Chromosome pairs
1
Feedback for a right answer
Two nuclei in one cell: the nucleus has divided, and the cell not yet. Cytokinesis begins right after.

Feedback for a wrong answer
Not yet. Look for two whole nuclei, one at each side, in a cell not yet cut in two.

Hint
Telophase comes after the sisters reach the poles, and before the cell pinches in two.

Worked solution
1. Anaphase: the sisters are dragged to the poles.
2. Telophase: a nuclear envelope is built round each set, and the chromosomes loosen.
3. That happens roughly between 62% and 78% of Stage.
Step 5 of 5
Open this step
A human cell
Practice · Calculate
Question
A human skin cell holds $46$ ⁠ chromosomes, $23$ ⁠ pairs. It divides by mitosis. How many chromosomes will each daughter cell have?

Correct answer
46 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
100 %
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Accepted within
± 0.1
A fixed value in the question
Wrong answers it recognises
- 23 ± 0.1
  Misconception: Thinks each daughter of mitosis gets half the parent's chromosomes.
  Feedback from the catalogue
  Mitosis keeps the number. Before division every chromosome was copied, and in anaphase the two sisters of each chromosome are pulled to opposite poles. So each daughter gets one copy of every chromosome: the same $2n$ ⁠ chromosomes as the mother cell, exactly the same information. What is split in two is the copied DNA, not the number of chromosomes.
- 92 ± 0.1
  Misconception: Thinks the chromosome count in the cell stays 2n during anaphase.
  Feedback
  The cell holds $92$ ⁠ chromosomes only in anaphase. Once it splits, each daughter gets half of them.
Feedback for a right answer
$46$ ⁠, as in the mother cell. Every cell in the body, apart from the sex cells and the red blood cells, which have no nucleus, carries the same $46$ ⁠ chromosomes, because they all came by mitosis from one fertilised egg.

Hint
What happens with $3$ ⁠ or $4$ ⁠ pairs in the Scene happens with $23$ ⁠ too.

Worked solution
1. Before division each of the $46$ ⁠ chromosomes was copied into two sisters.
2. In anaphase the sisters part, $46$ ⁠ to each pole.
3. Each daughter gets $46$ ⁠ chromosomes.
If you answered $23$ ⁠, you mixed it up with meiosis, where the number of chromosomes is halved.

Lesson 3 of 5 · About 25 minutes

Meiosis

How sex cells are made: two divisions after one copying, homologous pairs that part, four haploid cells, and why no two sex cells are alike.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Half a set
Practice · Choose
Question
Why do sex cells need only half the chromosomes?

Correct answer
- Correct So that at fertilisation, when egg and sperm join, a full number results again
Setup, choices and feedback
The Scene at this step
Stage
0 %
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Choices
Shown in a random order.
- Correct So that at fertilisation, when egg and sperm join, a full number results again
- Wrong So that the cells are smaller and lighter
  Feedback
  The egg is one of the largest cells in the body. The reason is not size but arithmetic: what happens to the number of chromosomes at fertilisation.
- Wrong They don’t: eggs and sperm hold $46$ ⁠ chromosomes, like every cell
  Misconception: Thinks a sperm or egg holds the full diploid set, 2n.
  Feedback from the catalogue
  A sex cell is haploid: it holds $n$ ⁠ chromosomes, one of each pair. A human egg and sperm hold $23$ ⁠ chromosomes each, and at fertilisation they join into a fertilised egg with $46$ ⁠. That is how the number stays the same from generation to generation.
Feedback for a right answer
$23 + 23 = 46$ ⁠. If each sex cell carried $46$ ⁠, the next generation would have $92$ ⁠, and the one after $184$ ⁠. Meiosis brings the number back to half, and fertilisation makes it whole.

Hint
What would happen to the number of chromosomes if an egg of $46$ ⁠ joined a sperm of $46$ ⁠?

Worked solution
1. At fertilisation the egg and sperm join into one cell, and their chromosomes add up.
2. For the child to have $46$ ⁠, like the parents, each must give $23$ ⁠.
3. So sex cells are made by meiosis, which leaves each cell one copy of each pair.
Step 2 of 5
Open this step
The first division
Practice · Calculate
Question
The cell has $4$ ⁠ pairs, $2n = 8$ ⁠. The first division of meiosis is over, and there are now two cells. How many chromosomes are in each of them?

Correct answer
4 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
56 %fixed
Chromosome pairs
4fixed
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Chromosomes in each cell, counted by centromeres (sisters count as two once they part)/nodes/cell/outputs/chromosomes
Wrong answers it recognises
- 8 ± 0.1
  Misconception: Thinks the first division of meiosis keeps the chromosome number, as mitosis does.
  Feedback from the catalogue
  In the first division of meiosis it is not the sisters that part but the pairs: the two homologous chromosomes of each pair, each still two chromatids, are pulled to opposite poles. So each of the two cells gets $n$ ⁠ chromosomes, one of each pair. That is the reduction division. Only in the second division do the sisters part, as in mitosis.
- 2 ± 0.1
  Misconception: Counts each chromatid of an X-shaped chromosome as a chromosome of its own.
  Feedback
  Each chromosome is still two sisters joined together. They part only in the second division, and until then each X counts as one chromosome.
Feedback for a right answer
$4$ ⁠: one from each pair, each still an X of two sisters. The number of chromosomes is halved already in the first division, which is why it is called the reduction division.

Hint
In the first division, what parts: the sisters, or the two chromosomes of each pair?

Worked solution
1. In prophase I the homologous chromosomes join up in pairs.
2. In anaphase I the pairs part: one chromosome of each pair to each pole, and the sisters stay joined.
3. In each cell: $4$ ⁠ chromosomes, $n = 4$ ⁠, each two chromatids.
If you answered $8$ ⁠, you thought the first division separates sisters, as mitosis does.
Step 3 of 5
Open this step
The second division
Practice · Predict, then look
Question
Meiosis runs to the end. How many cells will come from the cell that began it?

Correct answer
- Correct Four
Setup, choices and feedback
The Scene at this step
Stage
30 %fixed
Chromosome pairs
4
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Choices
- Correct Four
- Wrong Two
  Misconception: Thinks meiosis is a single division that makes two cells.
  Feedback from the catalogue
  Meiosis is two divisions in a row, with one copying of the DNA before them. The first separates the homologous pairs, and the second separates the sister chromatids. So four cells come from one, each with $n$ ⁠ chromosomes.
- Wrong Eight
  Feedback
  Two divisions, not three: one cell makes two, and each of those makes two more.
Once the prediction is locked in, the Scene changes
Stage
100 %fixed
The Scene then shows: 4 · Separate cells once the ring has pinched them apart: 1, 2, or 4 after meiosis/nodes/cell/outputs/cells
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Four cells, from two divisions. In a man all four become sperm. In a woman only one becomes an egg, and the other three, small ones, wither.

Hint
How many divisions are there in meiosis?

Worked solution
1. First division: the cell splits in two, and the homologous pairs part.
2. Second division: each of the two cells splits again, and the sisters part.
3. $2 \times 2 = 4$ ⁠ cells. After the reveal the Scene shows $4$ ⁠.
Step 4 of 5
Open this step
The sex cells
Practice · Calculate
Question
Meiosis is over. How many chromosomes are in each of the four cells?

Correct answer
4 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
100 %fixed
Chromosome pairs
4fixed
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Chromosomes in each cell, counted by centromeres (sisters count as two once they part)/nodes/cell/outputs/chromosomes
Wrong answers it recognises
- 8 ± 0.1
  Misconception: Thinks a sperm or egg holds the full diploid set, 2n.
  Feedback from the catalogue
  A sex cell is haploid: it holds $n$ ⁠ chromosomes, one of each pair. A human egg and sperm hold $23$ ⁠ chromosomes each, and at fertilisation they join into a fertilised egg with $46$ ⁠. That is how the number stays the same from generation to generation.
- 2 ± 0.1
  Misconception: Thinks the first division of meiosis keeps the chromosome number, as mitosis does.
  Feedback
  The number is halved only once, in the first division. The second division separates sisters, like mitosis, and does not halve it again.
Feedback for a right answer
$4$ ⁠ in each cell, one from each pair, and now each a single chromatid. $n = 4$ ⁠: half a body cell’s number.

Hint
The second division separates the two sisters of each X. Does it change the number of chromosomes in each cell?

Worked solution
1. After the first division: $4$ ⁠ chromosomes in each cell, each two chromatids.
2. In the second division the sisters part, one going to each cell.
3. In each of the four cells: $4$ ⁠ chromosomes, $n = 4$ ⁠.
If you answered $8$ ⁠, you thought a sex cell holds a double set, like a body cell.
Step 5 of 5
Open this step
New combinations
Practice · Calculate
Question
A mosquito has $3$ ⁠ pairs. At metaphase I each pair lines up one of two ways, pink on top or blue on top, regardless of the other pairs. How many different combinations of the mother’s and the father’s chromosomes can the sex cells get, leaving crossing over aside?

Correct answer
8 ± 0.1
Setup, choices and feedback
The Scene at this step
Stage
22 %
Chromosome pairs
3fixed
Whole cell / Spindle and chromosomes
Spindle and chromosomes
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Kinds of gamete independent assortment alone can make, 2 to the number of pairs (crossing over makes more)/nodes/cell/outputs/assortments
Wrong answers it recognises
- 6 ± 0.1
  Misconception: Adds the choices of the pairs instead of multiplying them when counting kinds of gamete.
  Feedback from the catalogue
  Each pair lines up at metaphase I one of two ways round, regardless of the other pairs, like a coin toss. So multiply: with $3$ ⁠ pairs there are $2 \times 2 \times 2 = 2^3 = 8$ ⁠ combinations, not $2 + 2 + 2 = 6$ ⁠. In a human, with $23$ ⁠ pairs, there are $2^{23}$ ⁠, more than eight million kinds of sex cell, even before crossing over.
- 3 ± 0.1
  Misconception: Adds the choices of the pairs instead of multiplying them when counting kinds of gamete.
  Feedback from the catalogue
  Each pair lines up at metaphase I one of two ways round, regardless of the other pairs, like a coin toss. So multiply: with $3$ ⁠ pairs there are $2 \times 2 \times 2 = 2^3 = 8$ ⁠ combinations, not $2 + 2 + 2 = 6$ ⁠. In a human, with $23$ ⁠ pairs, there are $2^{23}$ ⁠, more than eight million kinds of sex cell, even before crossing over.
Feedback for a right answer
$2 \times 2 \times 2 = 8$ ⁠ combinations. In a human, with $23$ ⁠ pairs, $2^{23}$ ⁠: more than eight million. And with crossing over, which swaps stretches between the chromosomes of a pair, almost every sex cell is one of a kind.

Hint
It is like tossing $3$ ⁠ coins: how many possible outcomes are there?

Worked solution
1. Each pair has two choices, independent of the other pairs.
2. By the multiplication rule: $2 \times 2 \times 2 = 2^3$ ⁠.
3. $2^3 = 8$ ⁠ combinations.
If you answered $6$ ⁠, you added the pairs’ choices instead of multiplying them.

Lesson 4 of 5 · About 20 minutes

Stem cells

From fertilised egg to blastocyst: cleavage divisions that do not grow the embryo, the inner cell mass, and totipotent, pluripotent and multipotent stem cells.

Open the lesson

The 5 steps and their answers

Step 1 of 5
Open this step
Dividing without growing
Practice · Predict, then look
Question
A fertilised egg divides into $16$ ⁠ cells over the first four days. What will happen to the size of the whole embryo?

Correct answer
- Correct It will stay about the size of the egg
Setup, choices and feedback
The Scene at this step
Time since fertilisation
0.5 daysfixed
Cut open
Whole embryofixed
Choices
- Correct It will stay about the size of the egg
- Wrong It will grow $16$ ⁠ times, a new cell for each division
  Misconception: Thinks the early embryo grows as its cells divide.
  Feedback from the catalogue
  In its first days the embryo does not feed or grow: it is still inside the egg’s clear shell, with only what the egg held. Each division cuts the cells in two, so the cells get smaller and smaller and the whole embryo stays the size of the egg, about 0.14 mm. These divisions are called cleavage.
- Wrong It will grow a little, about twice
  Misconception: Thinks the early embryo grows as its cells divide.
  Feedback from the catalogue
  In its first days the embryo does not feed or grow: it is still inside the egg’s clear shell, with only what the egg held. Each division cuts the cells in two, so the cells get smaller and smaller and the whole embryo stays the size of the egg, about 0.14 mm. These divisions are called cleavage.
Once the prediction is locked in, the Scene changes
Time since fertilisation
4 daysfixed
The Scene then shows: 0.14 mm · Greatest length of what is drawn: the ball to day 7, then the embryo/nodes/embryo/outputs/length
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
About 0.14 mm, as on the first day. The embryo does not feed yet, so each division just cuts the cells in two: $16$ ⁠ small cells instead of one large one.

Hint
What are the new cells made of? Does the embryo have anywhere to get material from?

Worked solution
1. The embryo is inside the clear shell, and has no source of food yet.
2. Each division cuts a cell in two without it growing first: cleavage.
3. So after four rounds there are $16$ ⁠ cells, each $\frac{1}{16}$ ⁠ of the egg’s volume, and the whole embryo is the same size.
Only after it implants in the womb, and gets food from the mother’s blood, does it start to grow.
Step 2 of 5
Open this step
Counting cells
Practice · Calculate
Question
Three days after fertilisation. How many cells does the embryo in the Scene have?

Correct answer
8 ± 0.1
Setup, choices and feedback
The Scene at this step
Time since fertilisation
3 daysfixed
Cut open
Whole embryo
Accepted within
± 0.1
Computed from the Scene at this step’s setting
Cells in the embryo while they are few enough to count, to day 7; 0 after/nodes/embryo/outputs/cells
Wrong answers it recognises
- 3 ± 0.1
  Misconception: Thinks the early embryo grows as its cells divide.
  Feedback
  Each division doubles the number of cells, rather than adding one. Count them in the Scene.
Feedback for a right answer
$8$ ⁠ cells: three rounds of division, $1 \to 2 \to 4 \to 8$ ⁠. On day four, after another round, they pack into a tight ball, the morula.

Hint
Count the cells inside the clear shell. Each round of division doubles them.

Worked solution
1. The fertilised egg is one cell.
2. About once a day all the cells divide: $1, 2, 4, 8$ ⁠.
3. By day three three rounds have passed: $2^3 = 8$ ⁠ cells.
Step 3 of 5
Open this step
A hollow ball
Practice · Reach the target
Question
Move Time since fertilisation until the embryo becomes a blastocyst: a hollow ball of cells, with a clump of inner cells at one side inside it. Press Check.

Goal
- 3 ± 0.1
  At the start of the step: 1
Setup, choices and feedback
The Scene at this step
Time since fertilisation
2 days
Cut open
Whole embryo
Read from the Scene at
0 fertilised egg, 1 cleavage, 2 morula, 3 blastocyst, 4 two-layer disc, 5 gastrulation, 6 neural tube, 7 body plan/nodes/embryo/outputs/stage
One setting that reaches it
Time since fertilisation
4.5 days
Feedback for a right answer
A blastocyst, around day five. At this stage the embryo hatches from its shell and implants in the wall of the womb. The whole body will be built from the inner cell mass, and the placenta from the outer cells.

Feedback for a wrong answer
Not yet. Look for a cavity inside the ball.

Hint
First there is a morula, a solid ball. Then a cavity opens inside it.

Worked solution
1. Days $1$ ⁠ to $3$ ⁠: cleavage, $2$ ⁠, $4$ ⁠ and $8$ ⁠ cells.
2. Around day $4$ ⁠: a morula, a solid, packed ball.
3. From about day $4.5$ ⁠ a cavity opens inside it: a blastocyst, with an inner cell mass at one side.
Step 4 of 5
Open this step
What a cell can still become
Practice · Predict, then look
Question
The cells of the blastocyst’s inner cell mass can become any kind of cell in the body, but not placenta. What are stem cells like that called?

Correct answer
- Correct Pluripotent
Setup, choices and feedback
The Scene at this step
Time since fertilisation
5.5 daysfixed
Cut open
Whole embryofixed
Choices
- Wrong Totipotent
  Misconception: Thinks every stem cell can become any kind of cell.
  Feedback from the catalogue
  Stem cells differ in what they can still become. The fertilised egg and the cells of the first days are totipotent: both the body and the placenta can come from them. The cells of the inner cell mass in the blastocyst are pluripotent: any cell type of the body, but not placenta. And adult stem cells, such as those in bone marrow, are multipotent: only a few related kinds, for instance the kinds of blood cell.
- Correct Pluripotent
- Wrong Multipotent
  Misconception: Thinks every stem cell can become any kind of cell.
  Feedback from the catalogue
  Stem cells differ in what they can still become. The fertilised egg and the cells of the first days are totipotent: both the body and the placenta can come from them. The cells of the inner cell mass in the blastocyst are pluripotent: any cell type of the body, but not placenta. And adult stem cells, such as those in bone marrow, are multipotent: only a few related kinds, for instance the kinds of blood cell.
Once the prediction is locked in, the Scene changes
Cut open
Cut open, by fatefixed
The Scene then shows: 2 · What the most versatile cells can still make: 3 totipotent, 2 pluripotent, 1 multipotent/nodes/embryo/outputs/potency
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Pluripotent: they can make the whole body, but no longer the placenta, which the outer cells make. The embryonic stem cells grown in labs come from these cells.

Hint
Toti: everything, placenta included. Pluri: many. Multi: several.

Worked solution
1. Until about day four each cell can still make both body and placenta: totipotent.
2. In the blastocyst the cells split up: the outer ones will make placenta, and the inner ones the body.
3. A cell that can make any body cell but not placenta is pluripotent. After the reveal the Scene shows $2$ ⁠ on the scale where $3$ ⁠ is totipotent and $1$ ⁠ multipotent.
Step 5 of 5
Open this step
Stem cells in adults
Practice · Choose
Question
Adult bone marrow holds stem cells that make billions of new blood cells every day: red, white and platelets. But they do not make nerve or skin cells. What kind of stem cells are they?

Correct answer
- Correct Multipotent
Setup, choices and feedback
The Scene at this step
Time since fertilisation
28 days
Cut open
Cut open, by fate
Choices
Shown in a random order.
- Correct Multipotent
- Wrong Pluripotent
  Misconception: Thinks every stem cell can become any kind of cell.
  Feedback from the catalogue
  Stem cells differ in what they can still become. The fertilised egg and the cells of the first days are totipotent: both the body and the placenta can come from them. The cells of the inner cell mass in the blastocyst are pluripotent: any cell type of the body, but not placenta. And adult stem cells, such as those in bone marrow, are multipotent: only a few related kinds, for instance the kinds of blood cell.
- Wrong Totipotent
  Misconception: Thinks every stem cell can become any kind of cell.
  Feedback from the catalogue
  Stem cells differ in what they can still become. The fertilised egg and the cells of the first days are totipotent: both the body and the placenta can come from them. The cells of the inner cell mass in the blastocyst are pluripotent: any cell type of the body, but not placenta. And adult stem cells, such as those in bone marrow, are multipotent: only a few related kinds, for instance the kinds of blood cell.
Feedback for a right answer
Multipotent: one family of cells, the blood cells. By the end of the first month the embryo’s cells have narrowed their options like this too, which is what the Scene shows on day $28$ ⁠.

Hint
How many kinds of cell can they make: everything, many, or one family?

Worked solution
1. Totipotent: body and placenta. Only the fertilised egg and the first cells.
2. Pluripotent: any body cell. The inner cell mass.
3. Multipotent: a few related kinds. Bone-marrow stem cells make only blood cells, so they are multipotent.
That is why a bone-marrow transplant can cure blood diseases, but cannot rebuild a damaged nerve.

Lesson 5 of 5 · About 25 minutes

When division runs wild

Cancer as cell division that has lost its controls one after another: brakes and accelerators, checkpoints that stop damaged DNA, a lump that recruits blood vessels, and the step that turns a benign tumour malignant.

Open the lesson

The 6 steps and their answers

Step 1 of 6
Open this step
A lining that renews
Practice · Choose
Question
Every cell has two copies of each gene. Why can one mutation in an accelerator gene already push the cell to divide, while a brake gene usually needs both copies broken?

Correct answer
- Correct An accelerator stuck on sends the signal to divide even when the other copy is fine, while one working copy of a brake is enough to stop
Setup, choices and feedback
The Scene at this step
Mutations
0
Show the checkpoints
Off
Choices
Shown in a random order.
- Correct An accelerator stuck on sends the signal to divide even when the other copy is fine, while one working copy of a brake is enough to stop
- Wrong Brake genes resist mutation better
  Feedback
  Mutations hit all genes about equally. The difference is what one broken copy does.
- Wrong One mutation is enough in a brake too: any mutation makes the cell cancerous
  Misconception: Thinks a single mutation turns a cell into a cancer cell.
  Feedback from the catalogue
  One mutation is almost never enough. A cell has several separate controls: brakes, accelerators, checkpoints, even the need for blood vessels. Each mutation breaks one of them at most, so a cancer usually needs several mutations in the same lineage, piling up over years or decades. That is why it is mostly a disease of older age.
- Wrong Accelerator genes have only one copy in the cell
  Feedback
  They have two copies too, one from each parent, like every gene on a chromosome that is not a sex chromosome.
Feedback for a right answer
A broken accelerator is like a stuck pedal: one is enough to make the car go, even if the other pedal is fine. A broken brake is like one of two brakes: the other still stops. So an oncogene acts in a single copy, and a tumour suppressor fails only when the second copy breaks too.

Worked solution
1. An accelerator passes on “divide”. A copy stuck on sends that signal all the time, whatever the other copy does: one copy is enough.
2. A brake passes on “stop”. The working copy still makes a protein that stops: the cell is held.
3. Only when the second copy breaks too is there no brake at all.
4. That is why someone who inherits one broken copy of a brake, such as BRCA1 or APC, starts one step ahead.
Step 2 of 6
Open this step
Checkpoints
Practice · Predict, then look
Question
In the polyp, two mutations, many cells whose DNA was damaged are amber: held at a checkpoint. The third mutation breaks the p53 checkpoint. Predict what you will see when Mutations goes up to $3$ ⁠.

Correct answer
- Correct The amber cells vanish, and red cells divide with their damaged DNA
Setup, choices and feedback
The Scene at this step
Mutations
2
Show the checkpoints
Onfixed
Choices
- Correct The amber cells vanish, and red cells divide with their damaged DNA
- Wrong More amber cells: more damage, more halts
  Feedback
  There is more damage, but what halts a damaged cell is p53, and at this stage it is broken.
- Wrong All the damaged cells destroy themselves
  Feedback
  Self-destruction is set off by the same p53. Without it, apoptosis stops too.
Once the prediction is locked in, the Scene changes
Mutations
3
The Scene then shows: 2 · 0 the tissue; looking inside: 1 the damage checkpoint still holds, 2 it is broken/nodes/tissue/outputs/checks
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
The amber cells and the brown fragments vanish, and red cells divide with their damage instead. Without the checkpoint every new error passes to the offspring, and the lump becomes unstable: its cells vary in size, the nuclei are large and dark, and new mutations pile up faster.

Worked solution
1. At the checkpoint, p53 halts a cell with damaged DNA and, if the damage is not repaired, sets off apoptosis.
2. The third mutation breaks p53: both ways are closed.
3. Damaged cells go on dividing: red instead of amber and brown.
4. From here each further mutation comes faster, which is why TP53 is broken in about half of all tumours.
Step 3 of 6
Open this step
The lump
Practice · Calculate
Question
A lump of about a billion cells, $10^9$ ⁠, started from one cell whose offspring all kept dividing. How many doublings of the number of cells does it take to get from one cell to a billion?

Correct answer
29.9 ± 0.6
Setup, choices and feedback
The Scene at this step
Mutations
4fixed
Show the checkpoints
Off
Accepted within
± 0.6
Computed from the Scene at this step’s setting
Doublings from one cell to the clone’s typical size, log2 of its cells (0 with no clone)/nodes/tissue/outputs/doublings
Wrong answers it recognises
- 1000000000 ± 10%
  Misconception: Gives the number of cells as the number of doublings it takes to make them.
  Feedback from the catalogue
  Each doubling multiplies the number of cells by $2$ ⁠, so after $n$ ⁠ doublings there are $2^n$ ⁠ cells, not $n$ ⁠. To find how many doublings it takes, ask how many times you must multiply by $2$ ⁠: $2^{10} \approx 1000$ ⁠, so every $10$ ⁠ doublings add a factor of a thousand.
Feedback for a right answer
About $30$ ⁠ doublings: $2^{30} \approx 1.07 \times 10^9$ ⁠. It is a surprise: a lump a centimetre across, the smallest you can sometimes feel or see on a scan, is already three-quarters of the way. Just $10$ ⁠ more doublings make $10^{12}$ ⁠ cells, about a kilogram.

Hint
$2^{10} = 1024$ ⁠, nearly a thousand. How many times must you multiply by a thousand to reach a billion?

Worked solution
1. After $n$ ⁠ doublings there are $2^n$ ⁠ cells. Look for $n$ ⁠ with $2^n = 10^9$ ⁠.
2. $2^{10} \approx 10^3$ ⁠, so $2^{30} = (2^{10})^3 \approx (10^3)^3 = 10^9$ ⁠.
3. $n \approx 30$ ⁠ (exactly, $\log_2 10^9 \approx 29.9$ ⁠).
If you answered a billion, you gave the number of cells, not the number of doublings.
Step 4 of 6
Open this step
Benign or malignant
Practice · Predict, then look
Question
The lump, four mutations, is already a centimetre across with its own blood vessels, and it is still benign. Predict what the fifth mutation changes, the one that makes it malignant.

Correct answer
- Correct Cells break through the basement membrane and push into the tissue below
Setup, choices and feedback
The Scene at this step
Mutations
4
Show the checkpoints
Offfixed
Choices
- Correct Cells break through the basement membrane and push into the tissue below
- Wrong The lump grows much bigger
  Misconception: Thinks any tumour, or any big one, is a cancer.
  Feedback from the catalogue
  A tumour is any lump of cells that divide too much. It is benign as long as its cells stay on their own side of the basement membrane, however big it grows. It is malignant, a cancer, only when cells break through the membrane into the tissue below. Size does not decide it; breaking through does.
- Wrong The cells divide faster
  Feedback
  Fast division is already there in the polyp and the lump, and both are benign. What sets malignant apart is invasion.
Once the prediction is locked in, the Scene changes
Mutations
5
The Scene then shows: 1 · 1 once cells have broken through the basement membrane, else 0/nodes/tissue/outputs/invasive
A prediction is never marked as a failure: the learner sees the outcome and the explanation either way.
Feedback for a right answer
Breaking through. At $5$ ⁠ mutations the cells cut the basement membrane and push into the connective tissue in strands and single cells, while the lump itself has hardly changed. A tumour that invades like this is malignant. That is why doctors examine a thin slice of a tumour under a microscope: only the section shows whether the membrane is breached.

Worked solution
1. Benign: the cells stay above the basement membrane, however big.
2. Malignant: the cells invade through it into the neighbouring tissue.
3. In the Scene, between $4$ ⁠ and $5$ ⁠, the clone stays at about a billion cells; what changes are the tongues of cells below the membrane.
4. Cancer is defined by invasion, not size.
Step 5 of 6
Open this step
Years in the making
Practice · Choose
Question
Why do most cancers appear in older people?

Correct answer
- Correct It takes several mutations in the same cell lineage, and each one takes time to arise and spread through the clone
Setup, choices and feedback
The Scene at this step
Mutations
1
Show the checkpoints
Off
Choices
Shown in a random order.
- Correct It takes several mutations in the same cell lineage, and each one takes time to arise and spread through the clone
- Wrong Because over the years the chance grows that one cell gets the one mutation that causes cancer
  Misconception: Thinks a single mutation turns a cell into a cancer cell.
  Feedback from the catalogue
  One mutation is almost never enough. A cell has several separate controls: brakes, accelerators, checkpoints, even the need for blood vessels. Each mutation breaks one of them at most, so a cancer usually needs several mutations in the same lineage, piling up over years or decades. That is why it is mostly a disease of older age.
- Wrong Because old cells divide faster
  Feedback
  The reverse: with age many cells divide more slowly. What grows with age is the number of mutations that have built up.
- Wrong Because cancer is an inherited disease that wakes up in old age
  Feedback
  Most mutations behind a cancer arise in body cells during life. An inherited broken copy only moves things one step ahead.
Feedback for a right answer
Cancer is a series of steps. Each mutation that breaks a control has to arise by chance, and then the clone of its offspring has to grow big enough for the next mutation to arise inside it. Five or six such steps usually take decades.

Worked solution
1. In the Scene each mutation breaks one control: from a crowded patch, to a polyp, to an unstable lump, to blood vessels, to invasion and spread.
2. Each step needs a new mutation in a cell that already carries the earlier ones.
3. Mutations build up slowly, mostly with each division, so each step takes years.
4. The longer you live, the more lineages have had time to go through all the steps.
Step 6 of 6
Open this step
Stopping division
Practice · Choose
Question
Many chemotherapy drugs jam the mitotic spindle and kill dividing cells. Why do they often cause hair loss and digestive trouble?

Correct answer
- Correct Hair follicles and the gut lining divide all the time too, so they are hit as well
Setup, choices and feedback
The Scene at this step
Mutations
6
Show the checkpoints
On
Choices
Shown in a random order.
- Correct Hair follicles and the gut lining divide all the time too, so they are hit as well
- Wrong The drugs are equally poisonous to every cell in the body
  Feedback
  If so, nerve and muscle cells, which hardly divide, would be hit just as hard. The damage gathers in tissues that divide.
- Wrong Because the cancer has spread to the hair and gut
  Feedback
  The side effects appear even when the cancer has not reached them. They come from the drug, not the tumour.
Feedback for a right answer
The drug does not recognise cancer; it recognises division. The gut lining renews every few days, and hair follicles and bone marrow divide all the time, so they are hit along with the tumour. That is why newer treatments target the particular protein a mutation left stuck on, or help the immune system recognise the tumour.

Worked solution
1. A drug that jams the spindle hits every cell that is in mitosis.
2. Cancer cells divide a lot, so they are hit a lot.
3. Healthy tissues have cells that divide just as fast: the gut lining (as at the start of the Lesson), hair follicles and bone marrow.
4. Hence hair loss, nausea and harm to blood cells are common side effects.

Across the unit · 15 misconceptions

Misconception catalogue

Each common mistake this unit recognises, in a few words. Open one for what the learner reads when an answer matches it, and where it is asked.

Index

The cell cycle

Why cells divide

Question

Correct answer

The Scene at this step

Choices

Hint

Worked solution

Counting chromosomes

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Copied in advance

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

One becomes two

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

The daughters

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Mitosis step by step

Prophase

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

Metaphase

Question

Correct answer

The Scene at this step

Choices

Once the prediction is locked in, the Scene changes

Hint

Worked solution

Anaphase

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Telophase

Question

Goal

The Scene at this step

One setting that reaches it

Hint

Worked solution

A human cell

Question

Correct answer

The Scene at this step

Wrong answers it recognises

Hint

Worked solution

Meiosis

Half a set

Question

Correct answer

The Scene at this step

Choices

Hint