The Cell Cycle and Cell Division
FoundationThe cell cycle covers interphase and the division phase: interphase copies DNA and builds proteins, nuclear division runs through prophase, metaphase, anaphase and telophase, and the chromosomes are shared equally between two daughter cells; meiosis instead makes gametes with half the chromosome number.
- The cell cycle consists of interphase (G1, S, G2) and the division phase; interphase takes up most of the time.
- DNA replication finishes in the S phase, giving every chromosome two identical sister chromatids.
- Mitosis in four stages: prophase condenses, metaphase lines up, anaphase separates, telophase rebuilds.
文體小知識：科學知識說明文——按細胞週期的次序講解間期、有絲分裂四期與胞質分裂，再比較有絲分裂與減數分裂的異同。
§Text Passage
1The whole sequence from the end of one cell division to the end of the next is the cell cycle, made up of the lengthy interphase and the brief division phase. Division cannot happen at any moment: the cell must first complete two big tasks in interphase — copying its DNA and synthesising enough protein — or the daughter cells will lack complete genetic material.
2Interphase runs in three stages: in G1 the cell grows rapidly, making proteins and duplicating organelles; in S it replicates its DNA, so every chromosome now has an identical copy — the two copies are sister chromatids joined at the centromere; in G2 the cell keeps growing and makes final preparations for division.
3Nuclear division (mitosis) runs through four stages: in prophase the chromatin condenses into clearly visible chromosomes, the nuclear membrane breaks down and the spindle forms; in metaphase all the chromosomes line up on the equator of the cell; in anaphase the sister chromatids separate and move to opposite poles; in telophase nuclear membranes reform and two nuclei appear.
4After nuclear division comes cytokinesis: in an animal cell the membrane folds inwards at the middle to form a cleavage furrow that splits the cell in two, while in a plant cell a cell plate forms at the equator and grows outward into a new cell wall. The two daughter cells carry exactly the same genetic material as the parent.
5Mitosis multiplies body cells for growth and repair; meiosis happens only in the reproductive organs, and its two successive divisions produce four gametes with half the chromosome number, along with genetic variation. After fertilisation the full number is restored, keeping the chromosome number stable between generations.
1Summary 重點摘要
The cell cycle covers interphase and the division phase: interphase copies DNA and builds proteins, nuclear division runs through prophase, metaphase, anaphase and telophase, and the chromosomes are shared equally between two daughter cells; meiosis instead makes gametes with half the chromosome number.
2Vocabulary cards Vocabulary
The whole sequence from the end of one division to the end of the next, covering interphase and the division phase.
The longest stage of the cell cycle (G1, S and G2), in which the cell grows and replicates its DNA.
A DNA-and-protein structure carrying genetic information; it condenses into a visible form during division.
One of the two identical copies of a replicated chromosome, the pair joined at the centromere.
The fibrous structure formed between the poles of a dividing cell that moves the chromosomes.
Nuclear division that distributes replicated chromosomes equally to two daughter cells, used for growth and repair.
The division of the cytoplasm after nuclear division, forming two daughter cells.
The structure formed at the equator of a dividing plant cell that develops into the new cell wall.
Two successive divisions producing four cells with half the chromosome number, used to make gametes.
The sex cells (sperm or egg) produced by meiosis, carrying half the chromosome number of body cells.
3Step by step Steps
- Step 1: The cell cycle at a glanceThe span from the end of one division to the end of the next is the cell cycle, made up of two great stages: interphase and the division phase. Interphase takes up most of the cycle, during which the cell grows and prepares to divide; the division phase consists of nuclear division (mitosis) and cytokinesis. For example, in constantly renewed tissue such as the skin epidermis, basal cells keep running through the cycle to replace shed surface cells, while nerve cells stay in interphase for life and never divide again.
- Step 2: Interphase — preparing to divideInterphase runs through G1, S and G2: in G1 the cell grows fast, synthesising proteins and duplicating organelles; in S the DNA is replicated, every chromosome making an identical copy, and the two copies — sister chromatids — stay joined at the centromere; in G2 the cell continues making what division requires. For example, no chromosomes are visible under the microscope during interphase: only loose chromatin fills the nucleus, because the DNA is in thread form, easy to copy and to read.
- Step 3: Prophase and metaphaseIn prophase the loose chromatin gradually condenses into chromosomes clearly visible under the light microscope, the nuclear membrane and nucleolus disappear, and the spindle forms between the two poles. In metaphase, spindle fibres manoeuvre every chromosome onto the equator of the cell — the most recognisable scene in mitosis. For example, if a micrograph shows chromosomes neatly lined up in a single row across the middle, the cell is in metaphase.
- Step 4: Anaphase and telophaseIn anaphase the two sister chromatids of every chromosome separate at the centromere and are pulled to opposite poles, guaranteeing that the two sets of chromosomes are identical. In telophase the chromosomes, now at the poles, loosen back into chromatin; nuclear membranes and nucleoli reform, and two identical nuclei appear. For example, anaphase micrographs typically show two groups of chromosomes being dragged towards the poles — the very evidence that separation is under way.
- Step 5: CytokinesisOnce nuclear division is complete, the cytoplasm and organelles are shared out and the cell splits in two. In an animal cell the membrane folds inwards at the middle to form a cleavage furrow that keeps narrowing until two daughter cells separate; in a plant cell vesicles gather at the equator into a cell plate that grows outward and becomes the new cell wall. For example, because plant cells have walls they cannot fold inward like animal cells, so they separate the two new cells from the inside out with a cell plate.
- Step 6: Why mitosis mattersMitosis distributes the replicated chromosomes precisely, so both daughter cells carry exactly the parent's genetic material and the chromosome number is unchanged. It multiplies body cells for growth, renewal and repair. For example, after a cut, cells at the wound edge multiply rapidly by mitosis to fill the gap with identical new cells; the blood-forming cells of the bone marrow also use mitosis to keep supplying red blood cells and white blood cells.
- Step 7: MeiosisMeiosis happens in the reproductive organs: the cell divides twice in succession, producing four daughter cells, each with half the parent's chromosome number; these develop into gametes — sperm and eggs. When two gametes fuse at fertilisation the full number is restored, keeping the chromosome number stable from generation to generation. During meiosis, chromosomes also exchange segments, producing genetic variation. For example, human body cells carry 46 chromosomes, sperm and eggs each carry 23, and fertilisation restores the count to 46.
4Diagrams Diagrams
5Worked example Example
Micrographs A, B, C and D show four cells from an onion root tip, each at a different stage of division: A has condensed chromosomes and no nuclear membrane; B has its chromosomes lined up across the middle of the cell; C shows two groups of chromosomes moving to opposite poles; D shows two new nuclei with a cell plate between them. Identify the stage in each, and explain why chromosomes must be replicated first and then shared out equally.
- A: chromosomes condensed, nuclear membrane gone — the features of prophase.
- B: chromosomes arranged on the equator — metaphase; C: sister chromatids separated and moving to the poles — anaphase.
- D: two new nuclei formed with a cell plate between them — telophase plus cytokinesis.
- In the S phase of interphase every chromosome has already been copied into two identical sister chromatids; anaphase then sends one copy to each pole, so each daughter cell receives a complete, identical set of chromosomes.
- Answer: A prophase, B metaphase, C anaphase, D telophase (with cytokinesis). Replicating the chromosomes first and then sharing them out equally guarantees that both daughter cells carry the same chromosome number and genetic information as the parent cell — the key to how mitosis maintains genetic stability.
6Common mistakes Mistakes
7Quick check Quick check
Q1Name the three stages of interphase and what happens in each.
Q2What is the key feature of metaphase in mitosis?
Q3How does the nuclear membrane differ between prophase and telophase?
Q4Why does wound repair rely on mitosis rather than meiosis?
Q5Give two key differences in outcome between meiosis and mitosis.
8Recap Recap
- The cell cycle consists of interphase (G1, S, G2) and the division phase; interphase takes up most of the time.
- DNA replication finishes in the S phase, giving every chromosome two identical sister chromatids.
- Mitosis in four stages: prophase condenses, metaphase lines up, anaphase separates, telophase rebuilds.
- Cytokinesis uses an infolding membrane in animal cells and a cell plate in plant cells; the result is two genetically identical daughter cells.
- Mitosis serves growth and repair; meiosis makes gametes with half the chromosome number, preparing for sexual reproduction.