How a Scraped Knee Heals, One Cell Division at a Time
Follow a cell through G1, S, G2 and M phases and the resting G0 stage, track chromosomes through prophase, metaphase, anaphase and telophase, compare cytokinesis in animal and plant cells, and see why mitosis matters for growth and repair.
How does one cell become two without losing any instructions?
Every cell in your body came from a single fertilised egg, copied again and again.
The ordered sequence of growth, DNA copying and splitting is the cell cycle, and the division that shares chromosomes equally is mitosis.
This part covers the phases of the cell cycle, the stages of mitosis, cytokinesis, and why mitosis matters.
The ordered sequence of growth, DNA copying and splitting is the cell cycle, and the division that shares chromosomes equally is mitosis.
This part covers the phases of the cell cycle, the stages of mitosis, cytokinesis, and why mitosis matters.
What happens in the G1, S, G2 and M phases, and what is the G0 stage?
The cell cycle has an interphase — G1 for growth, S for DNA replication and G2 for preparing to divide — followed by the M phase of actual division, while cells that stop dividing leave G1 and enter a resting G0 stage.
Timing. A human cell in culture divides about once every hours, and the M phase takes only about an hour of that.
The phases:
- G1 (gap 1) — the cell is metabolically active and grows continuously, but does not copy its DNA
- S (synthesis) — DNA is replicated; the amount of DNA per cell doubles from 2C to 4C, but the chromosome number stays 2n; in animal cells the centriole duplicates in the cytoplasm
- G2 (gap 2) — proteins needed for mitosis are made while growth continues
- M (mitosis) — the chromosomes are shared out and the cell divides
G0, the quiescent stage. Cells that do not divide further exit G1 into G0. They stay metabolically active but no longer proliferate unless called on to do so.
An everyday example. Before sharing a notebook with a friend, you photocopy all of it first — just as a cell copies all its DNA in S phase before dividing.
The substance. Interphase is the busiest part of the cycle, not a pause between divisions.
Timing. A human cell in culture divides about once every hours, and the M phase takes only about an hour of that.
The phases:
- G1 (gap 1) — the cell is metabolically active and grows continuously, but does not copy its DNA
- S (synthesis) — DNA is replicated; the amount of DNA per cell doubles from 2C to 4C, but the chromosome number stays 2n; in animal cells the centriole duplicates in the cytoplasm
- G2 (gap 2) — proteins needed for mitosis are made while growth continues
- M (mitosis) — the chromosomes are shared out and the cell divides
G0, the quiescent stage. Cells that do not divide further exit G1 into G0. They stay metabolically active but no longer proliferate unless called on to do so.
An everyday example. Before sharing a notebook with a friend, you photocopy all of it first — just as a cell copies all its DNA in S phase before dividing.
The substance. Interphase is the busiest part of the cycle, not a pause between divisions.
What happens to the chromosomes in prophase, metaphase, anaphase and telophase?
In prophase chromosomes condense and the spindle begins to form, in metaphase they line up at the cell's equator attached to spindle fibres, in anaphase sister chromatids separate to opposite poles, and in telophase new nuclei form around each set.
Prophase:
- Chromatin condenses into visible chromosomes, each of two sister chromatids joined at the centromere
- In animal cells, the centrioles move towards opposite poles and the spindle begins to form
- By its end, the Golgi complex, ER, nucleolus and nuclear envelope are no longer visible
Metaphase:
- The nuclear envelope has completely broken down
- Chromosomes are most condensed, making this the best stage to study their shape
- Spindle fibres attach to kinetochores, disc-shaped structures at the centromere
- Chromosomes line up at the metaphase plate
Anaphase:
- Each centromere splits, and the sister chromatids — now daughter chromosomes — move to opposite poles
- The centromere leads while the arms trail behind
Telophase:
- Chromosomes reach the poles and decondense, losing their individual identity
- The nuclear envelope, nucleolus, Golgi complex and ER reappear
An everyday example. Squashed onion root tips on a school lab slide show cells caught at every stage, because a growing root tip keeps dividing.
The substance. Mitosis is an equational division — each daughter nucleus receives exactly the same chromosome set as the parent.
Prophase:
- Chromatin condenses into visible chromosomes, each of two sister chromatids joined at the centromere
- In animal cells, the centrioles move towards opposite poles and the spindle begins to form
- By its end, the Golgi complex, ER, nucleolus and nuclear envelope are no longer visible
Metaphase:
- The nuclear envelope has completely broken down
- Chromosomes are most condensed, making this the best stage to study their shape
- Spindle fibres attach to kinetochores, disc-shaped structures at the centromere
- Chromosomes line up at the metaphase plate
Anaphase:
- Each centromere splits, and the sister chromatids — now daughter chromosomes — move to opposite poles
- The centromere leads while the arms trail behind
Telophase:
- Chromosomes reach the poles and decondense, losing their individual identity
- The nuclear envelope, nucleolus, Golgi complex and ER reappear
An everyday example. Squashed onion root tips on a school lab slide show cells caught at every stage, because a growing root tip keeps dividing.
The substance. Mitosis is an equational division — each daughter nucleus receives exactly the same chromosome set as the parent.
How does cytokinesis differ in animal and plant cells, and what is a syncytium?
Cytokinesis divides the cytoplasm after the nucleus has divided: animal cells pinch in by a furrow in the plasma membrane, plant cells build a cell plate from the centre outwards, and if the nucleus divides without the cytoplasm dividing, a multinucleate syncytium forms.
Two separate events. Division of the nucleus is karyokinesis; splitting the rest of the cell is cytokinesis.
In animal cells:
- A furrow appears in the plasma membrane
- It deepens gradually until it meets in the centre, cutting the cytoplasm in two
In plant cells:
- The rigid cell wall cannot pinch in, so a new wall forms from the centre outwards
- It begins as a simple precursor, the cell plate, which becomes the middle lamella between the walls of the two new cells
Syncytium. When karyokinesis is not followed by cytokinesis, one cell ends up with many nuclei — a syncytium, as in the liquid endosperm of coconut.
An everyday example. Pinching a long roll of dough in the middle until it splits into two balls is like an animal cell's furrow.
The substance. Direction is the key contrast — the animal furrow moves inwards from the edge, the plant cell plate grows outwards from the centre.
Two separate events. Division of the nucleus is karyokinesis; splitting the rest of the cell is cytokinesis.
In animal cells:
- A furrow appears in the plasma membrane
- It deepens gradually until it meets in the centre, cutting the cytoplasm in two
In plant cells:
- The rigid cell wall cannot pinch in, so a new wall forms from the centre outwards
- It begins as a simple precursor, the cell plate, which becomes the middle lamella between the walls of the two new cells
Syncytium. When karyokinesis is not followed by cytokinesis, one cell ends up with many nuclei — a syncytium, as in the liquid endosperm of coconut.
An everyday example. Pinching a long roll of dough in the middle until it splits into two balls is like an animal cell's furrow.
The substance. Direction is the key contrast — the animal furrow moves inwards from the edge, the plant cell plate grows outwards from the centre.
Why is mitosis essential for growth, repair and keeping cells in balance?
Mitosis produces genetically identical diploid daughter cells, which lets multicellular organisms grow, replace worn-out or damaged cells and restore the balance between nucleus and cytoplasm, and lets plants keep growing throughout life from meristems.
Its roles:
- Growth — a single-celled zygote becomes a body of countless cells, all with the same genetic set
- Nucleo-cytoplasmic ratio — as a cell grows, its cytoplasm increases while it still has one nucleus; division restores the ratio
- Repair and replacement — cells of the upper layer of the epidermis, the lining of the gut and blood cells are constantly replaced
- Plant growth — mitosis in the apical and lateral meristems, such as the cambium, lets plants grow in length and girth
An everyday example. A scraped knee heals over a few days as skin cells around the wound divide by mitosis to fill the gap.
The substance. Mitosis keeps genetic information constant — variation among offspring has to come from meiosis.
Its roles:
- Growth — a single-celled zygote becomes a body of countless cells, all with the same genetic set
- Nucleo-cytoplasmic ratio — as a cell grows, its cytoplasm increases while it still has one nucleus; division restores the ratio
- Repair and replacement — cells of the upper layer of the epidermis, the lining of the gut and blood cells are constantly replaced
- Plant growth — mitosis in the apical and lateral meristems, such as the cambium, lets plants grow in length and girth
An everyday example. A scraped knee heals over a few days as skin cells around the wound divide by mitosis to fill the gap.
The substance. Mitosis keeps genetic information constant — variation among offspring has to come from meiosis.
Exam tip
What earns full marks on the cell cycle and mitosis?
Draw each stage of mitosis with just four chromosomes — clear, labelled diagrams score better than long descriptions.
- Interphase: G1 growth; S doubles DNA from 2C to 4C with chromosome number unchanged; G2 prepares for division
- G0: metabolically active but not dividing
- Prophase: condensation, spindle forms, envelope and nucleolus vanish
- Metaphase: kinetochores attached, chromosomes at the plate; best stage for chromosome shape
- Anaphase: centromeres split, chromatids move apart
- Telophase: decondensation, envelope and nucleolus return
- Cytokinesis: furrow in animals, cell plate in plants; syncytium in coconut endosperm
The trap. Saying the chromosome number doubles in S phase. DNA doubles; the chromosome number stays 2n.
- Interphase: G1 growth; S doubles DNA from 2C to 4C with chromosome number unchanged; G2 prepares for division
- G0: metabolically active but not dividing
- Prophase: condensation, spindle forms, envelope and nucleolus vanish
- Metaphase: kinetochores attached, chromosomes at the plate; best stage for chromosome shape
- Anaphase: centromeres split, chromatids move apart
- Telophase: decondensation, envelope and nucleolus return
- Cytokinesis: furrow in animals, cell plate in plants; syncytium in coconut endosperm
The trap. Saying the chromosome number doubles in S phase. DNA doubles; the chromosome number stays 2n.
Did you know
What happens when a cell ignores the signal to stop dividing?
Normal cells grown in a dish divide until they touch their neighbours, and then they stop — a property called contact inhibition.
Cancer cells lose this control. They pass through the cell cycle again and again, piling up into a mass of cells called a tumour.
Many anticancer drugs work by interfering with dividing cells, for example by disrupting the spindle. That is also why such treatment can affect other fast-dividing tissues, such as hair roots and the lining of the gut — a clear reminder that well-controlled mitosis is essential for health.
Cancer cells lose this control. They pass through the cell cycle again and again, piling up into a mass of cells called a tumour.
Many anticancer drugs work by interfering with dividing cells, for example by disrupting the spindle. That is also why such treatment can affect other fast-dividing tissues, such as hair roots and the lining of the gut — a clear reminder that well-controlled mitosis is essential for health.
Exam relevance
How are the cell cycle and mitosis tested in NEET?
Cell Cycle and Cell Division is part of the Cell Structure and Function unit of NEET Biology, and its stage-by-stage details are asked very precisely.
What gets asked. Events of G1, S and G2, DNA content versus chromosome number at each phase, the G0 stage, the order of events in prophase, metaphase, anaphase and telophase, the best stage to study chromosome morphology, cytokinesis in plant and animal cells, and syncytium examples.
Question types. Statement-based questions, sequence-ordering questions, match-the-column lists and diagram-identification questions.
The trap that costs marks. Calling G0 cells inactive — they are metabolically active but no longer divide.
What gets asked. Events of G1, S and G2, DNA content versus chromosome number at each phase, the G0 stage, the order of events in prophase, metaphase, anaphase and telophase, the best stage to study chromosome morphology, cytokinesis in plant and animal cells, and syncytium examples.
Question types. Statement-based questions, sequence-ordering questions, match-the-column lists and diagram-identification questions.
The trap that costs marks. Calling G0 cells inactive — they are metabolically active but no longer divide.
Key takeaways
What must you be able to do from this part?
- Cell cycle: G1 growth; S phase takes DNA from 2C to 4C with chromosome number 2n; G2 preparation; M phase division; G0 quiescent but active
- Mitosis: prophase condensation; metaphase plate and kinetochores; anaphase centromere split; telophase nuclei reform
- Cytokinesis: animal furrow inwards; plant cell plate outwards forming the middle lamella; syncytium when cytokinesis is skipped
- Significance: growth, nucleo-cytoplasmic ratio, replacement of epidermis, gut lining and blood cells; meristem growth in plants
A cell has chromosomes and 2C DNA in G1. State its chromosome number and DNA content in G2, at metaphase and in each daughter cell.
- Mitosis: prophase condensation; metaphase plate and kinetochores; anaphase centromere split; telophase nuclei reform
- Cytokinesis: animal furrow inwards; plant cell plate outwards forming the middle lamella; syncytium when cytokinesis is skipped
- Significance: growth, nucleo-cytoplasmic ratio, replacement of epidermis, gut lining and blood cells; meristem growth in plants
A cell has chromosomes and 2C DNA in G1. State its chromosome number and DNA content in G2, at metaphase and in each daughter cell.