How the Cell Cycle Works: Phases, Checkpoints, Cyclins and CDKs
Level: first steps for a new student, then GCSE foundations, A-level and introductory university extension. This lesson is about the cell cycle as a whole — the full sequence of phases and the controls that run them. The stages of mitosis themselves are taught in full in the lesson on Cell Division, and the older lesson on Mitosis: A Symphony of Cellular Division tells the same stage-by-stage story. This lesson does not repeat those stages. It answers a different question: how does a cell know when to grow, when to copy its DNA, when to check itself, and when to divide?
Here is the whole lesson in one sentence: a cell grows, it copies its DNA, it checks that the copy is right, and only then does it split into two daughter cells — and each daughter starts the same sequence again. Every step below explains what happens, how it happens, and why.
What is the cell cycle?
The cell cycle is the sequence of events in a cell's life, from one division to the next. A cell is the smallest unit of life. The cell membrane is the thin skin around it. Inside the cell sits the DNA. DNA is the instruction book that tells the cell what to do. The DNA Structure lesson explains how that instruction book is built.
Every cell in your body came from one cell dividing. Your skin cells divide to replace skin you lose. Your gut cells divide to replace the lining your food wears away. Your bone marrow cells divide to make new blood cells. All of that dividing follows the same cycle.
Think of the cycle as a route with four stops. The stops have short names: G1, S, G2 and M. The letter G stands for "gap". The letter S stands for "synthesis", which means making something new. The letter M stands for "mitosis", which is the division of the nucleus. The route goes G1, then S, then G2, then M — and then it starts again in the two new daughter cells.
Two words to keep straight from the start. Growth is a cell getting bigger and making more of itself. Division is one cell becoming two cells. A cell that only grew would become one giant cell. A cell that only divided would get smaller and smaller. The cycle does both, in the right order.
The cell cycle in one picture — G1, S, G2, M
Look at the wheel below before you read the details. It shows the whole cycle in one turn.
Here is what each stop means, in one line each. G1: the cell grows and gets ready. S: the cell copies its DNA. G2: the cell grows more and gets ready to divide. M: the cell divides into two. The first three stops together are called interphase. Interphase is the part of the cycle between divisions. It takes most of the time. M phase is the short part at the end.
There is also a side road called G0. A cell in G0 has stepped out of the cycle. It is resting, not dividing. You will meet G0 again below.
What happens in G1 phase?
G1 is the first gap. It is the longest phase. In a typical 24-hour cycle it takes about 11 hours. Four things happen, in order.
Step 1. The cell grows. What happens: the new daughter cell gets bigger. How: it takes in nutrients through its membrane and builds more cytoplasm — the jelly-like filling of the cell. Why: a cell must reach a sensible size before it divides, or each division would make smaller and smaller cells.
Step 2. The cell makes proteins. What happens: the cell builds the proteins it will need for the next steps. How: it reads its DNA and makes new protein molecules. The Protein Structure lesson explains what proteins are and why their shape matters. Why: copying DNA and dividing need tools — enzymes and building blocks — and G1 is when the cell makes them. The Types of RNA lesson explains the messengers that carry the instructions from DNA to the protein-making machinery.
Step 3. The cell makes more organelles. What happens: the cell copies its small working parts, like mitochondria, the parts that release energy. How: the organelles grow and split in two. Why: each daughter cell will need a full set of working parts. One daughter cannot take them all.
Step 4. The cell decides. What happens: the cell checks whether it is ready to copy its DNA. How: it runs the G1 checkpoint, which you will meet below. Why: copying DNA is expensive and risky. A cell that is too small, hungry or damaged should wait — or leave the cycle into G0 — rather than copy a broken instruction book.
What happens in S phase?
S stands for synthesis. In S phase the cell copies its DNA. Every chromosome is copied once. One chromosome becomes two identical copies, joined together. The two copies are called sister chromatids. A chromatid is one of the two identical halves of a copied chromosome. The Nucleic Acids lesson explains the building blocks DNA is made from.
Copying happens in four steps. Step 1. The DNA unwinds. What happens: the two strands of the DNA double helix separate, like a zip opening. How: enzymes break the weak bonds between the strands. Why: each strand must be free to act as a template — a pattern to copy from.
Step 2. Each strand is read. What happens: the exposed bases on each strand are read in order. How: the bases pair up by a fixed rule — A with T, and C with G. Why: the rule means each old strand can only build one correct new strand. That is how the copy stays faithful.
Step 3. New strands are built. What happens: new nucleotides — the small units of DNA — are joined onto each old strand. How: the enzyme DNA polymerase does the joining, one nucleotide at a time. Why: the cell needs two complete double helices where there was one, so each daughter cell gets a full instruction book.
Step 4. The copies stay joined. What happens: the two identical chromatids stay attached at a point called the centromere. How: special proteins hold them together. Why: they must stay together until mitosis, when they are pulled apart — one to each daughter cell. If they separated now, the cell could lose track of them.
Here is the number that students get wrong most often. After S phase, the amount of DNA has doubled — from 2n to 4n. The letter n means one full set of DNA. But the number of chromosomes has NOT changed. A human cell still has 46 chromosomes. Each chromosome now has two chromatids instead of one. So the DNA amount doubles, and the chromosome number stays the same. Learn that difference now, because exams test it again and again.
One more job happens in S phase. The centrosome is duplicated. The centrosome is the small structure that organises the spindle. It makes a second copy of itself during S phase, so that by G2 the cell has two centrosomes ready to move to opposite ends.
What happens in G2 phase?
G2 is the second gap. In a typical 24-hour cycle it takes about 4 hours. Three things happen.
Step 1. The cell grows more. What happens: the cell keeps getting bigger and makes the proteins it needs for mitosis. How: the same protein-making machinery as in G1, now building spindle proteins and the enzymes that will break down the nuclear envelope. Why: mitosis needs its own tools, and they must be ready before the division starts.
Step 2. The two centrosomes move apart. What happens: the two centrosomes — the small structures that organise the spindle, copied during S phase — separate and move to opposite ends of the cell. How: motor proteins push them apart along microtubules. Why: the spindle needs two ends, one at each side of the cell, and each end needs a centrosome to anchor it. Without two centrosomes at opposite poles there is no spindle, and without a spindle the chromosomes cannot be pulled apart.
Step 3. The cell checks its DNA. What happens: the cell runs the G2 checkpoint. How: it checks that all the DNA was copied and that none of it is damaged. Why: a cell with half-copied or broken DNA must not be allowed into mitosis. If it divided now, the daughters would inherit the damage.
What happens in M phase?
M phase is the shortest phase — about 1 hour in a typical 24-hour cycle. It has two parts. Mitosis divides the nucleus into two identical nuclei. Cytokinesis then splits the whole cell into two daughter cells. Cytokinesis is the physical splitting of the cytoplasm. It is not a phase of the cycle. It is the last step of M phase.
M phase runs in four steps. Step 1. The chromosomes line up. What happens: the copied chromosomes — each one now two sister chromatids — line up in the middle of the cell. How: spindle fibres attach to each chromatid and pull gently from both sides. Why: lining up in the middle is the only way to be sure each daughter gets one copy of every chromosome.
Step 2. The chromatids are pulled apart. What happens: the sister chromatids separate and move to opposite ends of the cell. How: the spindle fibres shorten and drag them apart. Why: each end of the cell must end up with one full set of DNA.
Step 3. Two nuclei form. What happens: a new nuclear envelope forms around each set of chromosomes. How: the envelope rebuilds from the pieces it broke into. Why: each daughter needs its own nucleus to hold its DNA.
Step 4. The cell splits. What happens: the cytoplasm pinches in two — in animal cells a ring of protein squeezes the middle, in plant cells a new wall grows across. How: the pinching or wall-building runs from the edges to the centre. Why: without this step there would be one cell with two nuclei, not two cells. The full stage-by-stage story of mitosis — prophase, metaphase, anaphase and telophase — is in the Cell Division lesson, which is where to go next if you want every stage in detail.
What is G0 — the resting phase?
G0 is not a step in the cycle. It is the exit from it. A cell in G0 has stopped dividing. It still works — it still makes proteins, still uses energy — but it does not copy its DNA and does not divide.
Which cells rest in G0? Nerve cells and heart muscle cells spend most of their lives in G0. That is why damage to the brain or the heart heals so poorly: the cells cannot simply divide to replace the lost ones. Skin cells, gut lining cells and bone marrow cells are the opposite: they divide often, because they are constantly lost and need replacing.
G0 is not always a one-way door. Some cells can return. Liver cells sit in G0, but when part of the liver is damaged they re-enter the cycle and divide to repair it. That is one reason the liver can regrow after injury. Most nerve cells, by contrast, never come back.
Do not confuse G0 with G1. G1 is a working phase inside the cycle — the cell is growing and getting ready to copy its DNA. G0 is outside the cycle — the cell has stepped out and is not preparing to divide at all.
How long does the cell cycle take?
There is no single answer. Different cells run at different speeds. But one typical example is used in textbooks again and again: a cell that takes 24 hours to complete one cycle. The numbers below are practice figures — a teaching example, not measurements from any real cell.
| Phase | Hours | Share of the cycle | What happens |
|---|---|---|---|
| G1 | 11 | about 46% | Growth and preparation |
| S | 8 | about 33% | DNA copied |
| G2 | 4 | about 17% | More growth, final checks |
| M | 1 | about 4% | Division into two cells |
Worked example 1: how long is interphase?
The figures below are practice figures, invented for this lesson — not real data from any experiment. A cell has a 24-hour cycle: G1 takes 11 hours, S takes 8 hours, G2 takes 4 hours, M takes 1 hour. Step one: add the interphase phases — G1 + S + G2 = 11 + 8 + 4 = 23 hours. Step two: work out the share — 23 ÷ 24 is about 0.96, or about 96%. Step three: check the rest — M phase is 1 ÷ 24, about 0.04, or about 4%. So almost the whole cycle is interphase. Division is a short event at the end of a long preparation. That is why, when you look at cells under a microscope, most of them are in interphase. The Microscopy and Cell Size lesson explains how to look at cells and what you can see.
Worked example 2: how fast do the numbers grow?
The figures below are practice figures, invented for this lesson — not real data from any experiment. One cell divides every 24 hours. Step one: after 1 day there are 2 cells. Step two: after 2 days there are 4 cells. Step three: after 3 days there are 2 × 2 × 2 = 8 cells. Step four: after 5 days there are 2 × 2 × 2 × 2 × 2 = 32 cells. The number doubles every cycle. It grows by doubling, not by adding. That is why uncontrolled division is so dangerous: after 10 days the same starting cell would have made 1,024 cells, and after 20 days over a million. The arithmetic is simple — and that is exactly the point.
Real cells do not all run on 24 hours. Some gut cells divide every day. Some liver cells divide only when they are needed. Embryo cells can divide in under an hour. And some cells, like nerve cells, never divide at all. The cycle length depends on the cell type, the signals it receives, and how much food is available. The Membrane Transport lesson explains how nutrients get into the cell in the first place.
The checkpoints — the cell cycle's three safety gates
A checkpoint is a point in the cycle where the cell stops and checks itself before it is allowed to go on. Think of a gate on a road. The gate stays shut until the guard is satisfied. Then it opens and the cell moves on. If the check fails, the gate stays shut. The cell waits, repairs the problem, or leaves the cycle into G0.
There are three main checkpoints. Each one asks a different question.
Checkpoint 1: the G1 checkpoint. Where it sits: at the end of G1, before S phase. What it checks: is the cell big enough, does it have the nutrients it needs, and is its DNA undamaged? How: the cell reads signals from its environment — growth factors, which are chemical messages telling cells to divide — and checks its DNA for damage. Why it matters: this is the main decision point. A cell that passes goes on to copy its DNA. A cell that fails waits in G1 — or leaves the cycle into G0. Copying damaged DNA would pass the damage to both daughters.
Checkpoint 2: the G2 checkpoint. Where it sits: at the end of G2, before M phase. What it checks: was all the DNA copied, and is it undamaged? How: the cell checks that every chromosome has been fully copied and that no breaks are left unrepaired. Why it matters: a cell with half-copied or broken DNA must not be allowed into mitosis. If it divided now, the daughters would inherit broken instruction books.
Checkpoint 3: the spindle checkpoint. Where it sits: during mitosis, in M phase. It is also called the M checkpoint. What it checks: is every chromosome attached to the spindle and lined up in the middle? How: the cell waits until every chromosome reports that it is attached to spindle fibres from both sides. Why it matters: if the cell pulled the chromatids apart too early, one daughter could get two copies of a chromosome and the other none. That mistake is called aneuploidy — the wrong number of chromosomes. Aneuploidy is a common cause of miscarriage. Down syndrome is the best-known example of aneuploidy in a living child: an extra copy of chromosome 21, usually from an error in meiosis rather than in mitosis — so it shows what the wrong number of chromosomes means, even though this checkpoint is not the usual cause.
Notice the pattern. Each checkpoint sits just before a step that cannot be undone. Before copying the DNA. Before dividing. Before pulling the chromosomes apart. The cell checks before it commits, because after it commits there is no way back.
How does a checkpoint decide? The p53 and p21 brake
A checkpoint is not a person making a decision. It is a chain of proteins. The most important chain is the p53 and p21 brake. It works at the G1 checkpoint when DNA is damaged. It runs in four steps.
Step 1. Damage is found. What happens: a break or error is found in the DNA. How: sensor proteins patrol the DNA and detect the damage. Why: the cell cannot repair what it has not noticed.
Step 2. p53 switches on. What happens: the protein p53 becomes active. How: the sensors add a phosphate tag to p53, which wakes it up. Why: p53 is the signal that tells the cell "stop — there is damage here". The Chemical Basis of Life lesson explains what a phosphate group is and how adding one can switch a protein on or off.
Step 3. p21 is made. What happens: p53 switches on the gene for another protein, p21. How: p53 binds to the DNA near the p21 gene and starts its reading. Why: p21 is the actual brake. p53 gives the order; p21 does the stopping.
Step 4. p21 blocks the cyclin-CDK pairs. What happens: p21 binds to the cyclin-CDK pairs and stops them working. How: it sits on the pair and blocks its active site. Why: no cyclin-CDK activity means no move into the next phase. The cycle pauses. The cell now has time to repair the damage. If the repair works, the brake comes off and the cycle continues. If the damage cannot be repaired, p53 can order the cell to destroy itself — a tidy self-destruction called apoptosis. Apoptosis is programmed cell death: the cell breaks itself down cleanly, without harming its neighbours. It is better for one damaged cell to die than for it to divide and pass the damage on.
Scientists call p53 the "guardian of the genome". The genome is the full set of DNA in a cell. When p53 is broken — and it is broken in many cancers — the brake fails. Damaged cells keep dividing, mistakes pile up, and a tumour can form. You will meet that link again below.
Cyclins and CDKs — the switches that drive the cell cycle
Checkpoints are the brakes. Cyclins and CDKs are the engine. Together they decide when the cycle moves on.
A cyclin is a protein whose amount rises and falls through the cycle. It is made, it does its job, and then it is destroyed. Its rise is the signal that starts the next phase. Its fall is what lets the cycle move on. A CDK is a cyclin-dependent kinase. A kinase is an enzyme that adds phosphate groups to other proteins. CDK stands for "cyclin-dependent" because on its own the CDK is idle. It only works when its cyclin partner is attached to it.
Think of it as a key and an engine. The CDK is the engine. The cyclin is the key. Without the key, the engine does not start. When the key turns, the engine runs and adds phosphate tags to target proteins — and those tags switch the target proteins on, which drives the cell into the next phase. When the cyclin is destroyed, the key is gone, and the engine stops.
Four pairs do most of the work. Learn them as a table.
| Cyclin-CDK pair | When it works | What it does |
|---|---|---|
| Cyclin D with CDK4 or CDK6 | G1 | Starts the growth phase and prepares the cell for the G1 checkpoint decision |
| Cyclin E with CDK2 | G1 into S | Pushes the cell into S phase and starts DNA copying |
| Cyclin A with CDK2 | S and G2 | Keeps DNA copying going and prepares the cell for mitosis |
| Cyclin B with CDK1 | G2 into M | Drives the cell into mitosis: breaks down the nuclear envelope and builds the spindle |
How does one switch actually work? It runs in four steps. Step 1. The cyclin is made. What happens: the cell builds the cyclin for the next phase. How: it reads the cyclin gene and makes the protein. Why: the cyclin is the signal — no cyclin, no signal, no next phase.
Step 2. The cyclin attaches to its CDK. What happens: the cyclin and the CDK join together into a pair. How: they fit together like two pieces of a puzzle. Why: the CDK on its own is idle. Only the pair can work.
Step 3. The pair adds phosphate tags. What happens: the active pair adds phosphate groups to target proteins. How: the CDK takes a phosphate group from ATP — the cell's energy currency — and sticks it onto the target. Why: the phosphate tag changes the target protein's shape, which switches it on or off. Many tags together switch the whole cell into the next phase.
Step 4. The cyclin is destroyed. What happens: the cyclin is broken down by the cell's clean-up machinery. How: it is tagged for destruction and chopped into pieces. Why: if the cyclin stayed, the phase would never end. Destruction is what lets the cycle move on. This is why the curves in Figure 4 fall as well as rise.
What happens when the cell cycle loses control?
Checkpoints are the brakes. Cyclins and CDKs are the engine. A healthy cell needs both. When the brakes fail, the engine runs without stopping — and the cell divides again and again, even when its DNA is damaged.
It happens in steps. Step 1. A brake breaks. What happens: a gene that controls the cycle is damaged by a mutation. How: a mutation is a change in the DNA sequence — caused by radiation, by chemicals, or by a copying mistake that was never repaired. The Inheritance and Genetic Variation lesson explains how mutations arise and are passed on. Why it matters: the most common brake to break is p53, the guardian of the genome.
Step 2. Damaged cells keep dividing. What happens: the cell divides even though its DNA is broken. How: without p53, p21 is never made, the cyclin-CDK pairs are never blocked, and the G1 checkpoint waves the cell through. Why it matters: every division copies the damage into two more cells.
Step 3. Mistakes pile up. What happens: each round of division adds new mutations. How: damaged DNA is harder to copy faithfully, so errors accumulate. Why it matters: sooner or later a mutation hits another control gene, and the cell divides faster still.
Step 4. A tumour can form. What happens: the growing mass of dividing cells becomes a tumour. How: the cells ignore the signals that tell normal cells to stop, and they keep doubling — remember worked example 2, where one cell became 32 in five days. Why it matters: this is the link between the cell cycle and cancer. It is why checkpoints matter, and it is why so much cancer research is about the cell cycle. Many cancer drugs work by blocking cyclin-CDK pairs or by forcing damaged cells into apoptosis.
Keep the claim measured. Not every broken checkpoint causes cancer. The body has more than one brake, and most damaged cells are caught by another checkpoint or destroyed by apoptosis. Cancer needs several controls to fail at once. But every cancer involves the cell cycle losing control — and that is the point to carry into the exam.
Cell cycle vs mitosis vs cell division — do not mix them up
These words are used loosely in everyday speech. In an exam they are not interchangeable. Learn them as pairs.
| Word | What it means | What it is NOT |
|---|---|---|
| Cell cycle | The whole sequence: G1, S, G2 and M, from one division to the next | Not mitosis — mitosis is only the M phase |
| Mitosis | The division of the nucleus into two identical nuclei | Not the whole cycle, and not the splitting of the cell itself |
| Cytokinesis | The splitting of the cytoplasm into two cells | Not a phase of the cycle — it is the last step of M phase |
| Cell division | The everyday name for mitosis plus cytokinesis together | Not the whole cycle |
| G0 | A resting state outside the cycle | Not G1 — G1 is a working phase inside the cycle |
| Cyclin | The protein that rises and falls and switches the next phase on | Not the enzyme — the CDK is the enzyme |
| CDK | The enzyme that adds phosphate tags, but only when a cyclin is attached | Not the signal — the cyclin is the signal |
| Checkpoint | A decision point where the cell stops and checks before moving on | Not a phase — a checkpoint sits between phases |
| Growth | The cell getting bigger and making more of itself | Not division — division is one cell becoming two |
Mistakes that sound right
"Interphase is when the cell rests." No. Interphase is when the cell does almost all of its work: it grows, it copies its DNA, it makes proteins, it checks itself. Only M phase is division. Resting is G0, not interphase.
"The cell divides in interphase." No. The cell divides in M phase. Interphase is the preparation for division.
"After S phase there are 92 chromosomes." No. A human cell still has 46 chromosomes after S phase. Each chromosome now has two chromatids. The DNA amount doubles. The chromosome number does not.
"Mitosis is the cell cycle." No. Mitosis is one short phase of the cycle — about 1 hour out of 24. The cell cycle is the whole sequence.
"Cytokinesis is a phase of the cell cycle." No. Cytokinesis is the splitting of the cell at the end of M phase. The phases are G1, S, G2 and M.
"Checkpoints make the cell divide faster." No. Checkpoints slow the cycle down. They stop the cell until it is ready. A cell with no checkpoints divides faster — and that is exactly the problem.
How to answer an exam question on the cell cycle
Most cell cycle questions ask for a sequence or a cause. The pattern that earns marks is: name the phase, say what happens in it, and say why it matters. Never give a list of names with no explanation. Never write "the cell grows" without saying what it grows for.
Example question: "Explain why the cell cycle has checkpoints." Model paragraph: "Checkpoints stop the cell before a step that cannot be undone. The G1 checkpoint checks that the cell is big enough, fed, and its DNA is undamaged, so damaged DNA is not copied. The G2 checkpoint checks that DNA copying is complete and undamaged, so broken DNA is not passed to the daughters. The spindle checkpoint checks that every chromosome is attached, so the daughters get the right number. Without checkpoints, damaged cells would divide, mistakes would pile up, and a tumour could form. The p53 and p21 brake is one way a checkpoint stops the cycle: p53 finds the damage, switches on p21, and p21 blocks the cyclin-CDK pairs until repair is done."
Notice the structure: three checkpoints, each with what it checks and why, then the consequence, then the mechanism. That is the whole answer in six sentences.
Frequently asked questions
Is the cell cycle the same as mitosis?
No. Mitosis is one short phase of the cell cycle — the M phase, about 1 hour of a 24-hour cycle. The cell cycle is the whole sequence: G1, S, G2 and M.
Why is most of the cycle interphase?
Because preparation takes longer than division. Growing, copying all the DNA, making the proteins for mitosis and checking everything takes about 23 hours. The division itself takes about 1 hour.
Does the chromosome number change during the cycle?
No. A human cell has 46 chromosomes in G1, in S, in G2 and in M. What changes is the DNA amount: 2n in G1, 4n after S phase, and back to 2n in each daughter cell.
Can a cell leave G0 and start dividing again?
Some can. Liver cells sit in G0 but return to the cycle when the liver is damaged. Most nerve cells never return.
What is the difference between a cyclin and a CDK?
The cyclin is the signal: it rises and falls, and its rise starts the next phase. The CDK is the engine: it adds phosphate tags to target proteins, but it only works when its cyclin partner is attached.
Why do checkpoints matter for cancer?
Checkpoints are the brakes. When they fail — for example when p53 is broken — damaged cells keep dividing, mistakes pile up, and a tumour can form. That is why so much cancer research is about the cell cycle.
Continue learning
- Cell Division — mitosis and meiosis in full: every stage, and how the two kinds of division differ.
- Mitosis: A Symphony of Cellular Division — the stage-by-stage story of mitosis in the older lesson.
- Introduction to DNA Structure — the double helix, the base-pairing rule, and why copying works.
- Introduction to Nucleic Acids — the building blocks of DNA and RNA.
- Types of RNA — the messengers that carry instructions from DNA to the protein-making machinery.
- Protein Structure — what proteins are, why their shape matters, and how they are made.
- Inheritance and Genetic Variation — how mutations arise, how they are passed on, and why they matter.
Sources and further reading
These notes were written from the sources below and from the site's own linked lessons. The timing and cell-count figures in the worked examples are invented for teaching and are labelled as such.
- OpenStax, Biology 2e, Chapter 10: Cell Reproduction — the cell cycle, checkpoints, cyclins and CDKs, and the control of cell division.
- Khan Academy, Biology: The cell cycle and mitosis — phases of the cell cycle, checkpoints, and the regulation of the cycle.
- Biology LibreTexts, Cell Biology: The Cell Cycle — G1, S, G2 and M phases, checkpoints, and cyclin-CDK regulation.
- Alberts et al., Molecular Biology of the Cell, 6th edition (2014) — the cell cycle control system, p53 and p21, and the spindle checkpoint.
- National Cancer Institute, Cell Cycle — how checkpoints and cyclin-CDK pairs control division, and what happens when control fails.
- PreachBio, Cell Division — the stages of mitosis in full, for the M-phase section.
- PreachBio, DNA Structure — the double helix and base-pairing rules, for the S-phase section.
20 Explained MCQs — answers hidden until opened
Open each question to reveal its answer and explanation. The interactive quiz below uses a different set of questions.
1. What is the cell cycle?
A. The list of stages inside mitosis only B. The sequence of events from one cell division to the next C. The process of making energy in a cell D. The path food takes through a cell
Answer: B. The cell cycle is the whole story of a cell's life: it grows, copies its DNA, checks itself, and then divides into two daughter cells — and each daughter starts the cycle again.
2. Which phases make up interphase?
A. G1 and M only B. Prophase, metaphase, anaphase and telophase C. S and M only D. G1, S and G2
Answer: D. Interphase is the part of the cycle between divisions: G1 (first gap), S (DNA copied) and G2 (second gap). M phase — mitosis and cytokinesis — is the short part at the end.
3. What happens during G1 phase?
A. The cell grows, makes proteins, and prepares for DNA copying B. The cell copies its DNA C. The chromosomes line up in the middle D. The cell splits into two
Answer: A. G1 is the growth phase. The cell gets bigger, makes the proteins it will need, and checks that it has enough nutrients before it is allowed to copy its DNA.
4. What happens during S phase?
A. The cell rests and does nothing B. The chromosomes are pulled apart C. The cell splits into two daughter cells D. The cell copies its DNA
Answer: D. S stands for synthesis — the synthesis of new DNA. Each chromosome is copied, so one chromosome becomes two identical sister chromatids joined together.
5. Practice figures: a cell has 6 units of DNA in G1. How much DNA does it have after S phase?
A. 6 units B. 24 units C. 3 units D. 12 units
Answer: D. DNA is doubled in S phase: 6 units become 12 units. The cell still has the same number of chromosomes — 46 in a human cell — but each chromosome now has two chromatids, so the DNA amount has doubled from 2n to 4n.
6. What happens during G2 phase?
A. The cell grows more, makes the proteins needed for mitosis, and checks its DNA B. The cell divides into four cells C. The cell copies its DNA a second time D. The cell leaves the cycle for good
Answer: A. G2 is the second gap: more growth, the proteins for mitosis are made, and the cell checks that its DNA was copied fully and without damage before it is allowed to divide.
7. What is M phase?
A. The phase where the cell copies its DNA B. The resting phase of the cycle C. Mitosis and cytokinesis: the nucleus divides, then the whole cell splits in two D. The phase where the cell grows fastest
Answer: C. M is the shortest phase. Mitosis divides the nucleus into two identical nuclei, and cytokinesis then splits the cytoplasm, giving two daughter cells.
8. What is G0 phase?
A. The first stage of mitosis B. A resting state outside the cycle, where a cell has stopped dividing C. The phase where DNA is copied D. A checkpoint that stops damaged cells
Answer: B. G0 is not a step in the cycle — it is the exit from it. Nerve cells and heart muscle cells leave the cycle and sit in G0, mostly for the rest of the cell's life. Some cells, like liver cells, can return from G0 when needed.
9. What is a checkpoint in the cell cycle?
A. A protein that copies DNA B. The moment the cell splits in two C. A point where the cell stops and checks itself before it is allowed to go on D. A place where DNA is stored
Answer: C. A checkpoint is a decision point. The cell pauses, checks that everything is ready — big enough, fed, DNA undamaged — and only then is it allowed to move to the next phase. A failed check stops the cycle.
10. What does the G1 checkpoint check?
A. Whether the cell is big enough, has nutrients, and its DNA is undamaged B. Whether DNA copying is finished C. Whether the chromosomes are attached to the spindle D. Whether the cell has divided already
Answer: A. The G1 checkpoint is the main decision point: is the cell big enough, does it have the nutrients it needs, and is its DNA undamaged? If the answer is no, the cell waits in G1 — or leaves the cycle into G0.
11. What does the G2 checkpoint check?
A. Whether the cell has enough water B. Whether the chromosomes are lined up C. Whether the DNA was copied completely and without damage D. Whether the cell is big enough to start growing
Answer: C. The G2 checkpoint comes after S phase. It asks: is all the DNA copied, and is it damaged? A cell with broken or half-copied DNA is not allowed into mitosis.
12. What does the spindle checkpoint check?
A. Whether the cell has nutrients B. Whether the cell is big enough C. Whether the DNA has been copied D. Whether every chromosome is attached to the spindle and lined up
Answer: D. The spindle checkpoint — also called the M checkpoint — runs during mitosis. It holds the cell until every chromosome is attached to the spindle fibres and lined up in the middle. Only then may the sister chromatids be pulled apart.
13. What is a cyclin?
A. A checkpoint that stops the cycle B. A protein whose amount rises and falls through the cycle, switching the next phase on C. An enzyme that copies DNA D. A part of the spindle
Answer: B. A cyclin is a protein that is made, does its job, and is then destroyed. Its rise is the signal that starts the next phase — and its fall is what lets the cycle move on.
14. What is a CDK?
A. A type of DNA damage B. A phase of the cell cycle C. A protein that pulls chromosomes apart D. A cyclin-dependent kinase: an enzyme that adds phosphate groups to target proteins, but only works when a cyclin is attached to it
Answer: D. CDK stands for cyclin-dependent kinase. On its own it is idle. When its cyclin partner attaches, it switches on and adds phosphate groups to other proteins — and those phosphate tags are what drive the cell into the next phase.
15. Which cyclin-CDK pair pushes the cell from G1 into S phase?
A. Cyclin A with CDK2 B. Cyclin D with CDK4 or CDK6 C. Cyclin E with CDK2 D. Cyclin B with CDK1
Answer: C. Cyclin E with CDK2 is the pair that starts DNA copying. Cyclin D with CDK4 or CDK6 works earlier in G1; cyclin A with CDK2 works during S phase; cyclin B with CDK1 drives the cell into mitosis.
16. Which cyclin-CDK pair drives the cell into mitosis?
A. Cyclin D with CDK4 or CDK6 B. Cyclin B with CDK1 C. Cyclin E with CDK2 D. Cyclin A with CDK2
Answer: B. Cyclin B with CDK1 is the pair that drives the cell into mitosis. It switches on the proteins that break down the nuclear envelope and build the spindle.
17. What does the protein p53 do when DNA is damaged?
A. It switches the cyclins off forever B. It stops the cycle, giving the cell time to repair the damage — and if repair fails, it can order the cell to destroy itself C. It pulls the chromosomes apart early D. It copies the damaged DNA faster
Answer: B. p53 is the DNA-damage brake. When it finds damage, it switches on p21, which blocks the cyclin-CDK pairs and pauses the cycle while repair happens. If the damage cannot be repaired, p53 can push the cell into apoptosis — programmed self-destruction.
18. Practice figures: a typical cell cycle lasts 24 hours — G1 11 hours, S 8 hours, G2 4 hours, M 1 hour. How long is interphase?
A. 12 hours B. 1 hour C. 23 hours D. 24 hours
Answer: C. Interphase is G1 + S + G2: 11 + 8 + 4 = 23 hours. M phase is only 1 hour. So about 96% of the cycle — 23 out of 24 hours — is interphase, and only about 4% is division.
19. Practice figures: in the 24-hour cycle above, what fraction of the cycle is M phase?
A. About 17% B. About 46% C. About 4% D. About 33%
Answer: C. M phase is 1 hour out of 24: 1 ÷ 24 is about 0.04, or 4%. G1 is 11 ÷ 24, about 46%; S is 8 ÷ 24, about 33%; G2 is 4 ÷ 24, about 17%.
20. Which claim stays inside the evidence of the practice figures in this lesson?
A. In the practice figures, a cell with a 24-hour cycle spent 11 hours in G1 and 1 hour in M phase B. M phase is always the longest phase of the cycle C. All cells spend 46% of their cycle in G1 D. Every cell in the body divides every 24 hours
Answer: A. Only the claim that repeats the practice figures is safe: in the practice set the cycle was 24 hours, G1 was 11 hours and M phase was 1 hour. The others turn one worked example into a rule about all cells.
Separate Interactive Quiz — 20 different questions
These quiz questions are different from the MCQs above. Select an answer for each question. You will not see whether an answer is correct until you submit all 20 questions. Use Reset to try again.
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