The Cell Cycle & Mitosis · Grade 7 Science
Chapter: Genes, Cell Division & HeredityThe Cell Cycle & Mitosis
Learning goals
- I can explain the cell cycle & mitosis.
- I can use evidence, a model, and stated assumptions to support a biological explanation.
Before this unit
Use the supplied vocabulary, distinguish an observation from an explanation, and identify any units or model assumptions. Revisit a connected earlier unit if needed; new terms are explained here.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
You replace the lining of your gut roughly every few days and produce something like two million red blood cells every second. Every one of those new cells needs a complete and accurate copy of 46 chromosomes. Getting two metres of DNA copied and then divided evenly between two cells, millions of times a second, without losing or duplicating anything, is a serious engineering problem — and the cell cycle is the solution.
The cycle has two parts and the order is the whole trick. First a long growth phase, during which the cell builds up materials, carries out its normal work, and at one point copies all of its DNA. That copying is replication, and it produces for each chromosome two identical copies joined together, called sister chromatids. The cell now holds twice the DNA it started with, still organised as the same number of chromosomes — each one simply consists of two joined copies rather than one.
Then mitosis separates them. The joined copies line up, the pairs are pulled apart, and one copy of every chromosome goes to each end of the cell. The cell then divides, and each daughter cell receives a complete set identical to the original. Copy first, then separate: that is why both daughters end up with the full number rather than half of it, and why the chromosome number stays constant generation after generation of cells. The temporary doubling during replication is not a permanent doubling of the set.
Build your understanding
Where this fits
You now know what is inherited and how it is organised into chromosomes. Before inheritance between generations, there is a more immediate problem: every time a cell divides, both daughters need a complete set. This unit is how that is achieved, and it is the groundwork for meiosis in the next unit.
Use three flexible 40–45-minute sessions: core background and Parts 1–2 with their checks; Parts 3–4 and the worked application; then remaining practice, a labeled model, revision, and the exit check. Checks completed during reading can count toward the practice set. Read one short part at a time. Investigations and chapter review/quiz have separate sessions, and flexible weeks allow more time.
Part 1 · Copy first, then separate
The sequence is not arbitrary. If a cell divided its chromosomes without copying them first, each daughter would get half a set and neither could function. By replicating first, the cell creates a situation where separating the copies leaves each side with everything.
During replication the two strands of the DNA molecule come apart and each acts as a template for building its partner, so the two resulting molecules are each half old and half new. That mechanism is what makes accurate copying possible, and it also means errors can be detected by comparison against the template — which is why replication is far more accurate than simple copying would be.
Afterwards each chromosome exists as two identical sister chromatids joined at a point. This is the stage that causes most confusion when chromosomes are counted, because a chromosome made of two chromatids is still one chromosome. The count has not changed; the amount of DNA has.
Pause and explain
Why must DNA be copied before the chromosomes are separated, rather than after?
Check your reasoning after answering
Because separating first would give each daughter cell half a set, and neither could function or divide again. Copying first means every chromosome exists as two identical joined copies, so pulling the copies apart leaves a complete set on each side. Copy then separate is what keeps the chromosome number constant through generations of cells.
Part 2 · Counting chromosomes with a stated convention
Chromosome counting trips people because the thing being counted changes shape. The convention is this: count the number of separate structures attached at their own centromere. Two sister chromatids joined together count as one chromosome; once they separate, they are two.
Work a case. A cell starts with six chromosomes. Replication copies the DNA, so there are now six chromosomes each made of two chromatids — still six, with twice the DNA. Mitosis separates the chromatids, sending one of each pair to opposite ends, so each end receives six. The cell divides and each daughter has six chromosomes, the same as the original.
Saying the convention out loud is part of a correct answer, because the number depends on it. "Six chromosomes, each consisting of two sister chromatids after replication" is unambiguous; "twelve" is wrong under the standard convention and would need explaining even if someone intended it to mean twelve chromatids.
Pause and explain
A cell with six chromosomes replicates its DNA and then divides by mitosis. How many chromosomes does each daughter cell have?
Check your reasoning after answering
Six. Replication doubles the amount of DNA, giving six chromosomes each made of two joined sister chromatids — still six under the standard convention. Mitosis separates those chromatids and sends one of each to each end, so each daughter cell receives a full set of six, identical to the original cell's.
Part 3 · What mitosis is for
Mitosis produces genetically identical cells, which is exactly what growth and repair require. A growing organism needs more cells of the same kinds; a healing wound needs replacements for the cells that were damaged; and tissues that wear out, like your gut lining and your skin, need continuous replacement. In each case a faithful copy is what is wanted.
It is also how many organisms reproduce. Asexual reproduction from the last chapter runs on mitosis: a strawberry runner, a budding hydra, a rooted cutting all produce offspring by the same copying division, which is why their offspring are genetic copies.
Notice what mitosis does not do. It generates no variation at all — that is its purpose. Variation in an asexual lineage comes only from mutation, and the reshuffling that sexual reproduction depends on happens in a different kind of division, which is the next unit.
Pause and explain
Why is it an advantage that mitosis produces identical cells?
Check your reasoning after answering
Because growth and repair need more cells of the same kind. A healing wound needs replacements that do the job the damaged cells did, and a growing organism needs more of the tissues it already has. Faithful copying is the requirement, and it is also why asexual reproduction, which runs on mitosis, produces genetic copies of the parent.
Part 4 · Regulation, and the limits of a diagram
The cycle is controlled. A cell does not divide whenever it can; it passes through checkpoints at which conditions are assessed — whether the DNA was copied completely, whether damage was detected, whether the cell is large enough and has the materials. If something is wrong, the cycle halts until it is resolved or the cell is destroyed.
That regulation is as important as the mechanism. Uncontrolled division is the common feature of cancers, which is why the control system is studied so intensively. This course explains the mechanism; anything concerning a real person's health belongs to a qualified professional, which is the same boundary the defence unit drew.
Finally, the diagrams are models and simplify heavily. They show a handful of large chromosomes rather than 46; they freeze a continuous process into discrete stages; they draw the cell as a tidy oval; and they give no sense of the timing, which varies enormously between cell types. The stages are a useful vocabulary for describing a process that does not actually stop.
Pause and explain
Name two ways a textbook mitosis diagram differs from what happens in a real cell.
Check your reasoning after answering
It shows a few large chromosomes rather than the real number — 46 in a human cell — and it freezes a continuous process into separate still stages that a real cell passes through without pausing. It also omits the checkpoints that regulate whether division proceeds at all, and gives no sense of timing, which varies widely between cell types.
Apply the whole unit · Count a normal division model
Illustrative cell, not a measured specimen: it starts with six chromosomes. Its DNA is copied, the corresponding copies separate normally, and the cell forms two daughter cells.
Your task
State the chromosome number in each daughter cell, and distinguish the copying of DNA from a permanent doubling of the chromosome set.
What your response should show
- Explain the relevant biological process accurately.
- Use the supplied evidence and show calculation steps with units where needed.
- Connect a labeled model or mechanism with the claim.
- State an assumption, evidence limitation, or useful next observation.
Response guidance — after your attempt
Start with the count: six chromosomes before replication.
Apply replication: the DNA is copied, so each chromosome now consists of two identical sister chromatids joined together — still six chromosomes, with twice the DNA.
Apply separation: mitosis pulls the sister chromatids apart, sending one copy of each chromosome to each end of the cell.
State the result: each daughter cell receives six chromosomes, the same number the original had.
Each daughter cell has six chromosomes, the same as the cell that produced them. The sequence explains why. The cell begins with six. Replication copies the DNA so that each chromosome consists of two identical sister chromatids joined at a centromere: under the standard convention, structures attached at their own centromere are counted, so there are still six chromosomes, now carrying twice the DNA. Mitosis then separates the sister chromatids and sends one copy of each chromosome to each end of the cell, so each end has six complete chromosomes, and the cell divides. The distinction the task asks for is between the amount of DNA and the number of chromosomes in a set. Replication doubles the DNA temporarily, and that doubling is undone by the separation — the whole point of copying first is that both daughters end up with a complete set rather than half of one. A permanent doubling of the chromosome set would mean the daughters had twelve, which would be a different event entirely and would change what the cells were. Stating the counting convention is part of a correct answer, because "twelve" would be the result of counting chromatids rather than chromosomes at the replicated stage. It is also worth noting that this account describes a normal division; the cycle has checkpoints that can halt it, and a diagram of the process freezes something continuous into stages.
Scientific references for this unit
Parent / teacher teaching notes
Ask for a because-statement and a labeled relationship, not just a memorized word. Work through one calculation step at a time, permit oral or diagram responses, and use flexible sessions for difficult reading. Compare the learner’s actual evidence with the supplied model; do not reward copying or agreement with a personal preference. No private family, genetic, or medical records are required.
The key idea
Cells grow, use resources, copy genetic material, and sometimes divide. In a typical eukaryotic cell cycle, DNA is copied before the nucleus divides through mitosis. Division of the cell contents produces daughter cells. Mitosis helps growth, repair, and some asexual processes; it is not the gamete-producing process called meiosis.
Worked examples
Example 1 · Track DNA and chromosome number separately
- Before replication: four chromosomes, each a single DNA molecule — call the DNA amount one unit.
- After replication: still four chromosomes, because each now consists of two joined sister chromatids counted as one — DNA amount two units.
- In each daughter: four chromosomes, each a single molecule again after separation — DNA amount one unit.
Chromosome number and DNA amount are two different quantities. Track them in separate columns.
Example 2 · Diagnose a division error
- Check the total: 5 + 7 = 12, which is the six replicated chromosomes' worth of material, so nothing was lost or gained overall.
- Identify the failure: the separation sent both copies of one chromosome to the same end instead of one to each.
- Name the consequence: both daughters have an incomplete or unbalanced set even though the total is correct.
A correct total does not mean a correct division. Check both daughters, not the sum.
Avoid these mistakes
- Mistake: replication permanently doubles the chromosome set. It doubles the DNA temporarily; separation then gives each daughter a complete single set, which is why the number stays constant.
- Mistake: a chromosome made of two chromatids counts as two. Under the standard convention it is one chromosome until the chromatids separate — six replicated chromosomes are six, not twelve.
- Mistake: mitosis produces variation. It produces identical cells by design, which is what growth and repair need; variation in an asexual lineage comes only from mutation.
- Mistake: the diagram is the process. Diagrams show a few large chromosomes instead of 46 and freeze a continuous process into still stages, omitting the checkpoints that control it.
Check your understanding
Why is copying needed before the modeled division?
Discuss the answer
It provides corresponding genetic information for the daughter nuclei rather than merely splitting the original information into incomplete portions.
Does DNA replication mean each daughter must have eight chromosomes in a four-chromosome model?
Discuss the answer
No. Proper separation gives each daughter the original four-chromosome set; copying DNA is not permanent doubling of each daughter’s set.
Words to know
- cell cycle
- The sequence a cell passes through between divisions: a long growth phase including DNA replication, then division, with checkpoints controlling progress.
- replication
- Copying the DNA, with each strand acting as a template for its partner, so that each chromosome becomes two identical joined copies.
- sister chromatids
- The two identical copies of a chromosome joined after replication; counted as one chromosome until they separate.
- mitosis
- The division that separates sister chromatids so each daughter cell receives a complete identical set; it produces no variation.
- daughter cell
- Either of the two cells produced, each with the same chromosome number as the original.
Extend your scientific explanation
Compare a second supplied example or the linked educational reference. Identify a connection, a model limit, and an additional observation that would test the explanation.
In the real world
Use a clear question, reliable observations, and a biological mechanism to evaluate a claim. State what remains uncertain before applying a classroom model to a different setting.
Exit check
Can a stable cell count coexist with division?
Check your explanation after answering
Yes. Division can replace cells that are lost, giving no net change in total count.
Explain your reasoning without notes. If a key idea is missing, revisit the lesson and try again.