The Cell Cycle & Mitosis
Model the cell cycle step by step: interphase, prophase, metaphase, anaphase, telophase, and cytokinesis — and see why S phase plus anaphase yield two identical diploid cells.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on The Cell Cycle & Mitosis, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every cell in your body traces back to a single fertilized egg that divided, and divided again, billions of times. Each of those divisions had to hand down a complete, correct copy of the genome — no missing chromosomes, no extra ones. The cell cycle is the ordered sequence that makes this possible, and mitosis is the part of the cycle where the copied chromosomes get sorted into two piles.
In this lesson you will build a working model of that cycle: the long preparation of interphase (G1, S, G2), the four stages of mitosis (prophase, metaphase, anaphase, telophase), and cytokinesis, which physically pinches one cell into two. The payoff is being able to explain, not just recite, why the two daughter cells are genetically identical to each other and to the parent cell — and that explanation always comes back to two events: DNA replication in S phase and the separation of sister chromatids in anaphase.
In this lesson you will build a working model of that cycle: the long preparation of interphase (G1, S, G2), the four stages of mitosis (prophase, metaphase, anaphase, telophase), and cytokinesis, which physically pinches one cell into two. The payoff is being able to explain, not just recite, why the two daughter cells are genetically identical to each other and to the parent cell — and that explanation always comes back to two events: DNA replication in S phase and the separation of sister chromatids in anaphase.
Interphase: Where the Copying Actually Happens
A common misconception is that interphase is a "resting" stage. It is the opposite — it occupies roughly 90 percent of the cycle in a typical dividing human cell, and it is when nearly all of the work gets done. Interphase has three parts.
In G1 (first gap), the cell grows, builds proteins, and makes more organelles. Each chromosome is present as a single, unreplicated DNA molecule. A human cell in G1 has 46 chromosomes and 46 chromatids.
In S phase (synthesis), the cell replicates its DNA. Every chromosome is copied semiconservatively, producing two identical sister chromatids held together at a region called the centromere. Notice what does not change: the chromosome number is still 46, because a two-chromatid structure is still one chromosome. What doubles is the amount of DNA and the number of chromatids (now 92).
In G2 (second gap), the cell keeps growing, duplicates its centrosomes, and checks the replicated DNA for damage before committing to division.
Where students go wrong: they say "the chromosomes double in S phase." Say instead "the DNA is replicated, so each chromosome now consists of two identical sister chromatids." That single sentence carries the whole logic of mitosis — if the copies were not made in S phase, anaphase would have nothing identical to separate, and the daughter cells could not be genetically identical.
Also note that chromosomes in interphase are long, thin, and diffuse (chromatin). You cannot see individual chromosomes in an interphase nucleus under a light microscope; condensation happens as mitosis begins.
In G1 (first gap), the cell grows, builds proteins, and makes more organelles. Each chromosome is present as a single, unreplicated DNA molecule. A human cell in G1 has 46 chromosomes and 46 chromatids.
In S phase (synthesis), the cell replicates its DNA. Every chromosome is copied semiconservatively, producing two identical sister chromatids held together at a region called the centromere. Notice what does not change: the chromosome number is still 46, because a two-chromatid structure is still one chromosome. What doubles is the amount of DNA and the number of chromatids (now 92).
In G2 (second gap), the cell keeps growing, duplicates its centrosomes, and checks the replicated DNA for damage before committing to division.
Where students go wrong: they say "the chromosomes double in S phase." Say instead "the DNA is replicated, so each chromosome now consists of two identical sister chromatids." That single sentence carries the whole logic of mitosis — if the copies were not made in S phase, anaphase would have nothing identical to separate, and the daughter cells could not be genetically identical.
Also note that chromosomes in interphase are long, thin, and diffuse (chromatin). You cannot see individual chromosomes in an interphase nucleus under a light microscope; condensation happens as mitosis begins.
The Four Stages of Mitosis
Mitosis divides the nucleus — it sorts the replicated chromosomes into two identical sets. Many students remember the order with PMAT.
Anaphase is the moment that guarantees genetic identity. Because sister chromatids were made by copying the same DNA molecule in S phase, and because every chromosome is attached to both poles at metaphase, one copy of every chromosome goes left and one goes right. Nothing is left over and nothing is duplicated.
A frequent error is confusing metaphase alignment with the paired alignment seen in meiosis. In mitosis, individual chromosomes line up in one row; homologous chromosomes are not paired with each other and do not exchange segments. Mitosis is a copying process, not a shuffling process.
| Stage | Key events | What you see under a microscope |
|---|---|---|
| Prophase | Chromatin condenses into visible chromosomes, each with two sister chromatids; nucleolus disappears; centrosomes move apart and the spindle forms; late in prophase (prometaphase) the nuclear envelope breaks down and spindle microtubules attach to kinetochores at centromeres | Thick, distinct chromosomes, no clear nuclear boundary |
| Metaphase | Spindle fibers align all chromosomes single-file along the metaphase plate, the imaginary midline of the cell; each chromosome is attached to microtubules from both poles | A neat line of chromosomes across the cell's equator |
| Anaphase | Cohesin holding sister chromatids is cleaved; the chromatids separate and are pulled to opposite poles, so each former chromatid is now an independent chromosome | Two V-shaped clusters of chromosomes moving apart |
| Telophase | Chromosomes arrive at the poles and decondense; two new nuclear envelopes form; nucleoli reappear; the spindle breaks down | Two nuclei in one elongated cell |
A frequent error is confusing metaphase alignment with the paired alignment seen in meiosis. In mitosis, individual chromosomes line up in one row; homologous chromosomes are not paired with each other and do not exchange segments. Mitosis is a copying process, not a shuffling process.
Cytokinesis and the Diploid Result
Mitosis divides the nucleus; cytokinesis divides the cytoplasm. It usually overlaps the end of anaphase and telophase, but it is a separate process, and the mechanism differs by cell type.
In animal cells, a contractile ring of actin and myosin filaments just under the membrane tightens like a drawstring, creating a cleavage furrow that deepens until the cell pinches in two. In plant cells, the rigid cell wall prevents pinching, so vesicles from the Golgi gather at the midline and fuse to form a cell plate, which grows outward and becomes the new cell wall separating the two cells.
When cytokinesis finishes, each daughter cell has one nucleus containing a full set of chromosomes — the same number the parent had in G1. If the parent was diploid (), both daughters are diploid. For a human cell, goes to two cells of 46 chromosomes each. Cytoplasm and organelles are divided between the daughters, though not always in exactly equal amounts.
Tie the whole model together in one causal chain: S phase makes an identical copy of every chromosome; metaphase attaches each chromosome to both poles; anaphase separates the identical copies; cytokinesis puts one complete set in each new cell. Change any link and the outcome breaks. If S phase were skipped, the daughter cells would each get half a genome. If a chromosome failed to attach properly, one daughter could receive an extra chromosome and the other none — the kind of error that later lessons on regulation and checkpoints address directly.
In animal cells, a contractile ring of actin and myosin filaments just under the membrane tightens like a drawstring, creating a cleavage furrow that deepens until the cell pinches in two. In plant cells, the rigid cell wall prevents pinching, so vesicles from the Golgi gather at the midline and fuse to form a cell plate, which grows outward and becomes the new cell wall separating the two cells.
When cytokinesis finishes, each daughter cell has one nucleus containing a full set of chromosomes — the same number the parent had in G1. If the parent was diploid (), both daughters are diploid. For a human cell, goes to two cells of 46 chromosomes each. Cytoplasm and organelles are divided between the daughters, though not always in exactly equal amounts.
Tie the whole model together in one causal chain: S phase makes an identical copy of every chromosome; metaphase attaches each chromosome to both poles; anaphase separates the identical copies; cytokinesis puts one complete set in each new cell. Change any link and the outcome breaks. If S phase were skipped, the daughter cells would each get half a genome. If a chromosome failed to attach properly, one daughter could receive an extra chromosome and the other none — the kind of error that later lessons on regulation and checkpoints address directly.
Counting Chromosomes, Chromatids, and DNA
Most mistakes in this topic are bookkeeping mistakes, so practice tracking three separate quantities through the cycle for a cell with .
Read the table carefully at the anaphase row. Once sister chromatids separate, each one is counted as a full chromosome, so the cell momentarily contains 92 chromosomes. That is not a mistake in the table — it is the definition of a chromosome: one continuous DNA molecule with its own centromere.
Two more distinctions worth keeping straight. Sister chromatids are identical copies of one chromosome, joined at the centromere and produced in S phase. Homologous chromosomes are a matched pair — one from each parent — carrying the same genes but possibly different alleles; they are not identical and they are not attached. Mitosis separates sister chromatids only.
Finally, remember that DNA content per cell doubles during S phase and is halved at cytokinesis, returning to the G1 level. If you sketch DNA amount against time, you get a slow rise through S, a flat plateau through G2 and mitosis, then a sudden drop when the cell divides.
| Point in cycle | Chromosomes in the cell | Chromatids per chromosome | Total chromatids |
|---|---|---|---|
| G1 | 46 | 1 | 46 |
| After S / G2 | 46 | 2 | 92 |
| Metaphase | 46 | 2 | 92 |
| Late anaphase | 92 (46 per pole) | 1 | 92 |
| Each daughter cell after cytokinesis | 46 | 1 | 46 |
Two more distinctions worth keeping straight. Sister chromatids are identical copies of one chromosome, joined at the centromere and produced in S phase. Homologous chromosomes are a matched pair — one from each parent — carrying the same genes but possibly different alleles; they are not identical and they are not attached. Mitosis separates sister chromatids only.
Finally, remember that DNA content per cell doubles during S phase and is halved at cytokinesis, returning to the G1 level. If you sketch DNA amount against time, you get a slow rise through S, a flat plateau through G2 and mitosis, then a sudden drop when the cell divides.
Key terms
- Cell cycle.
- The ordered sequence of growth and division a cell passes through: interphase (G1, S, G2) followed by mitosis and cytokinesis.
- S phase.
- The stage of interphase in which DNA is replicated, giving every chromosome two identical sister chromatids while the chromosome number stays the same.
- Sister chromatids.
- The two identical DNA copies of a single replicated chromosome, joined at the centromere until anaphase.
- Centromere.
- The constricted region of a replicated chromosome where sister chromatids are held together and where kinetochores assemble for spindle attachment.
- Spindle.
- The structure of microtubules built from the centrosomes that attaches to chromosomes and moves them during mitosis.
- Metaphase plate.
- The imaginary plane at the cell's equator along which chromosomes align during metaphase, each attached to microtubules from both poles.
- Cytokinesis.
- Division of the cytoplasm into two cells, by a cleavage furrow in animal cells or a cell plate in plant cells.
- Diploid ().
- Having two complete sets of chromosomes, one inherited from each parent; mitosis of a diploid cell produces two diploid cells.
Worked example
A root tip cell from a plant species has a diploid number of . Track the cell through one full cell cycle and answer: (a) how many chromosomes and chromatids are present in G1; (b) how many chromosomes and chromatids are present at metaphase; (c) how many chromosomes move to each pole during anaphase; (d) how many chromosomes are in each daughter cell after cytokinesis, and are the cells haploid or diploid?
Start in G1. The cell has not replicated its DNA, so each of the 12 chromosomes is a single DNA molecule. That is 12 chromosomes and 12 chromatids.
Now pass through S phase. Every chromosome is copied, so each one now consists of two identical sister chromatids joined at a centromere. The chromosome count does not change, because two attached sister chromatids still count as one chromosome. At metaphase, then, the cell has 12 chromosomes and chromatids, all lined up single-file on the metaphase plate with each chromosome attached to spindle fibers from both poles.
Anaphase: cohesin is cleaved and the sister chromatids separate. Each separated chromatid now counts as its own chromosome. Since each of the 12 chromosomes contributes exactly one chromatid to each side, 12 chromosomes travel to each pole (24 chromosomes in the cell at that instant).
Cytokinesis in a plant cell forms a cell plate down the middle, splitting the cell in two. Each daughter cell receives one pole's worth of chromosomes: 12 chromosomes, each with one chromatid.
So (a) 12 and 12; (b) 12 and 24; (c) 12 per pole; (d) 12 chromosomes per daughter cell, which is — both daughters are diploid, and both are genetically identical to the parent cell because every chromosome they carry is a faithful S-phase copy.
Now pass through S phase. Every chromosome is copied, so each one now consists of two identical sister chromatids joined at a centromere. The chromosome count does not change, because two attached sister chromatids still count as one chromosome. At metaphase, then, the cell has 12 chromosomes and chromatids, all lined up single-file on the metaphase plate with each chromosome attached to spindle fibers from both poles.
Anaphase: cohesin is cleaved and the sister chromatids separate. Each separated chromatid now counts as its own chromosome. Since each of the 12 chromosomes contributes exactly one chromatid to each side, 12 chromosomes travel to each pole (24 chromosomes in the cell at that instant).
Cytokinesis in a plant cell forms a cell plate down the middle, splitting the cell in two. Each daughter cell receives one pole's worth of chromosomes: 12 chromosomes, each with one chromatid.
So (a) 12 and 12; (b) 12 and 24; (c) 12 per pole; (d) 12 chromosomes per daughter cell, which is — both daughters are diploid, and both are genetically identical to the parent cell because every chromosome they carry is a faithful S-phase copy.
Practice questions
During which event are the two identical DNA copies of each chromosome physically pulled apart into separate sets?
- S phase, when DNA is replicated
- Metaphase, when chromosomes align on the metaphase plate
- Anaphase, when sister chromatids separate and move to opposite poles
- Cytokinesis, when the cleavage furrow pinches the cell in two
Answer: Anaphase, when sister chromatids separate and move to opposite poles
S phase creates the identical copies but leaves them attached at the centromere, so nothing is separated yet. Metaphase only positions the chromosomes and establishes attachment to both poles. Cytokinesis divides cytoplasm after the chromosome sets are already apart. Anaphase is the event where cohesin is cleaved and the sister chromatids move to opposite poles, which is what makes the two future nuclei genetically identical.
A student says, "A human cell has 46 chromosomes in G1, and after S phase it has 92 chromosomes." Identify the error and rewrite the statement correctly, explaining what quantity actually doubles.
Answer: The chromosome number stays at 46 after S phase; what doubles is the DNA content and the number of chromatids (from 46 to 92). Correct version: "A human cell has 46 chromosomes in G1; after S phase it still has 46 chromosomes, but each one now consists of two identical sister chromatids, for 92 chromatids total."
A chromosome is defined as one continuous DNA molecule with its own centromere. When replication produces two copies joined at a shared centromere, the structure is still counted as a single chromosome — a replicated one. The count only reaches 92 briefly in anaphase, after cohesin is cleaved and each chromatid becomes an independent chromosome with its own centromere region. Keeping chromosome number and chromatid number as separate columns in your notes prevents this mix-up.
A biologist observes a plant cell with two decondensing masses of chromosomes at opposite ends and a line of vesicles fusing across the middle of the cell. Name the stage or stages being observed and explain what structure the vesicles are building.
Answer: The cell is in telophase with cytokinesis underway; the vesicles are forming a cell plate that will become the new cell wall between the two daughter cells.
Decondensing chromosomes at both poles with new nuclear envelopes forming is telophase — the reverse of prophase condensation. Because plant cells have a rigid cell wall, they cannot form a cleavage furrow the way animal cells do. Instead, Golgi-derived vesicles gather at the midline, fuse into a cell plate, and deposit wall material that grows outward until it joins the existing wall, completing the separation of the two cells.
FAQ
- Why are the two daughter cells produced by mitosis genetically identical?
- Because of two linked events. In S phase, each chromosome is replicated into two identical sister chromatids, so the cell holds two exact copies of every chromosome. Then at metaphase each chromosome attaches to spindle fibers from both poles, and in anaphase the sister chromatids separate so that one copy of every chromosome goes to each pole. Each daughter cell therefore receives a complete, matching set. Unlike meiosis, mitosis involves no pairing of homologous chromosomes and no crossing over, so no reshuffling occurs.
- How is interphase different from mitosis?
- Interphase is the growth and preparation portion of the cycle — the cell enlarges, makes proteins and organelles in G1, replicates its DNA in S, and finishes preparing in G2. Mitosis is the much shorter nuclear division portion, made of prophase, metaphase, anaphase, and telophase. Chromosomes are visible as distinct condensed structures only during mitosis; during interphase the DNA is in the extended chromatin form. Interphase is not a rest period, and it takes up most of the cycle.
- Is cytokinesis part of mitosis?
- No. Mitosis refers specifically to division of the nucleus and its chromosomes. Cytokinesis is the separate division of the cytoplasm that follows, and it usually begins during anaphase and finishes after telophase. They are counted as two distinct processes, which is why the phrase "mitosis and cytokinesis" appears together — an animal cell uses a contractile ring and cleavage furrow, while a plant cell builds a cell plate.
- What is the difference between sister chromatids and homologous chromosomes?
- Sister chromatids are two identical copies of a single chromosome, produced by replication in S phase and physically joined at the centromere. Homologous chromosomes are a matched pair of different chromosomes — one inherited from each parent — that carry the same genes but may carry different alleles, and they are not attached to each other. Mitosis separates sister chromatids only; homologous pairs are separated in meiosis, which is covered later in this unit.
Learn this with a teacher, not a page
The Crimsora tutor teaches The Cell Cycle & Mitosis live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.