How One Cell Becomes Two Without Losing a Single Chromosome
Learn the phases of the cell cycle, the stages and significance of mitosis and meiosis, and the key differences between the two kinds of cell division.
How does a single cell copy itself?
Every time a cut on your finger heals, cells near the wound grow, copy their DNA and split in two. Reproductive cells divide differently, halving the chromosome number so that a fertilised egg ends up with the right amount. Both depend on a tightly ordered cell cycle.
This lesson covers the stages of the cell cycle, mitosis, meiosis, and how the two divisions differ.
This lesson covers the stages of the cell cycle, mitosis, meiosis, and how the two divisions differ.
What are the stages of the cell cycle?
The cell cycle has a long interphase, made of the G1, S and G2 phases in which the cell grows and copies its DNA, followed by a short M phase in which first the nucleus and then the cytoplasm divide.
Interphase:
- G1 phase — the cell grows and is metabolically active, but its DNA is not yet copied
- S phase — DNA replicates, so the amount of DNA doubles while the chromosome number stays the same; in animal cells the centriole also duplicates
- G2 phase — proteins needed for division are made, and growth continues
M phase:
- Karyokinesis — division of the nucleus
- Cytokinesis — division of the cytoplasm
Quiescent stage. Some cells, such as many heart and nerve cells, leave G1 and enter an inactive G0 stage, where they stay alive but stop dividing.
An everyday example. Human cells growing in a laboratory dish take roughly a day to complete one cycle, and only about an hour of that is M phase.
The substance. DNA doubles in S phase, but the chromosome number does not — a cell with 2n chromosomes and 2C DNA in G1 still has 2n chromosomes, but 4C DNA, after S phase.
Interphase:
- G1 phase — the cell grows and is metabolically active, but its DNA is not yet copied
- S phase — DNA replicates, so the amount of DNA doubles while the chromosome number stays the same; in animal cells the centriole also duplicates
- G2 phase — proteins needed for division are made, and growth continues
M phase:
- Karyokinesis — division of the nucleus
- Cytokinesis — division of the cytoplasm
Quiescent stage. Some cells, such as many heart and nerve cells, leave G1 and enter an inactive G0 stage, where they stay alive but stop dividing.
An everyday example. Human cells growing in a laboratory dish take roughly a day to complete one cycle, and only about an hour of that is M phase.
The substance. DNA doubles in S phase, but the chromosome number does not — a cell with 2n chromosomes and 2C DNA in G1 still has 2n chromosomes, but 4C DNA, after S phase.
What happens in each stage of mitosis, and why is mitosis important?
Mitosis divides one nucleus into two identical nuclei through prophase, metaphase, anaphase and telophase, followed by cytokinesis, keeping the chromosome number unchanged.
Stages:
- Prophase — chromatin condenses into chromosomes, each of two sister chromatids; centrioles move towards opposite poles; the nucleolus, Golgi and ER disappear and the nuclear envelope breaks down
- Metaphase — chromosomes line up at the equator on the metaphase plate, with spindle fibres attached to their kinetochores
- Anaphase — centromeres split and sister chromatids move to opposite poles
- Telophase — chromosomes reach the poles and decondense; the nuclear envelope, nucleolus, Golgi and ER form again
- Cytokinesis — a furrow pinches an animal cell in two, while a plant cell builds a cell plate from the centre outwards
Significance:
- Growth of multicellular organisms
- Repair and replacement of cells, such as skin and the lining of the gut
- A constant chromosome number in body cells
- Asexual reproduction in many organisms
An everyday example. An onion root tip squashed and stained on a microscope slide shows mitosis at every stage, because root tips grow by rapid division.
The substance. Metaphase is the best stage to count and study chromosomes — they are most condensed and neatly aligned.
Stages:
- Prophase — chromatin condenses into chromosomes, each of two sister chromatids; centrioles move towards opposite poles; the nucleolus, Golgi and ER disappear and the nuclear envelope breaks down
- Metaphase — chromosomes line up at the equator on the metaphase plate, with spindle fibres attached to their kinetochores
- Anaphase — centromeres split and sister chromatids move to opposite poles
- Telophase — chromosomes reach the poles and decondense; the nuclear envelope, nucleolus, Golgi and ER form again
- Cytokinesis — a furrow pinches an animal cell in two, while a plant cell builds a cell plate from the centre outwards
Significance:
- Growth of multicellular organisms
- Repair and replacement of cells, such as skin and the lining of the gut
- A constant chromosome number in body cells
- Asexual reproduction in many organisms
An everyday example. An onion root tip squashed and stained on a microscope slide shows mitosis at every stage, because root tips grow by rapid division.
The substance. Metaphase is the best stage to count and study chromosomes — they are most condensed and neatly aligned.
What happens in the stages of meiosis, and why is meiosis important?
Meiosis is two divisions after a single round of DNA replication — meiosis I separates homologous chromosomes and halves the chromosome number, and meiosis II separates sister chromatids — producing four haploid cells.
Prophase I, the longest stage, has five sub-stages:
- Leptotene — chromosomes become visible
- Zygotene — homologous chromosomes pair by synapsis, forming bivalents held by the synaptonemal complex
- Pachytene — crossing over exchanges segments between non-sister chromatids
- Diplotene — homologues begin to separate but stay joined at X-shaped chiasmata
- Diakinesis — chiasmata terminalise, the nucleolus disappears and the nuclear envelope breaks down
The rest of meiosis I. Bivalents line up at the equator in metaphase I, homologues move to opposite poles in anaphase I, and two haploid cells form in telophase I.
Meiosis II. It resembles mitosis: sister chromatids separate in anaphase II, giving four haploid cells.
Significance:
- Keeps the chromosome number constant from one generation to the next in sexually reproducing organisms
- Crossing over and independent assortment create genetic variation
An everyday example. Brothers and sisters with the same parents look different because each egg and sperm carries a unique mix of chromosomes shuffled during meiosis.
The substance. The chromosome number halves in meiosis I, not meiosis II — which is why meiosis I is called the reductional division.
Prophase I, the longest stage, has five sub-stages:
- Leptotene — chromosomes become visible
- Zygotene — homologous chromosomes pair by synapsis, forming bivalents held by the synaptonemal complex
- Pachytene — crossing over exchanges segments between non-sister chromatids
- Diplotene — homologues begin to separate but stay joined at X-shaped chiasmata
- Diakinesis — chiasmata terminalise, the nucleolus disappears and the nuclear envelope breaks down
The rest of meiosis I. Bivalents line up at the equator in metaphase I, homologues move to opposite poles in anaphase I, and two haploid cells form in telophase I.
Meiosis II. It resembles mitosis: sister chromatids separate in anaphase II, giving four haploid cells.
Significance:
- Keeps the chromosome number constant from one generation to the next in sexually reproducing organisms
- Crossing over and independent assortment create genetic variation
An everyday example. Brothers and sisters with the same parents look different because each egg and sperm carries a unique mix of chromosomes shuffled during meiosis.
The substance. The chromosome number halves in meiosis I, not meiosis II — which is why meiosis I is called the reductional division.
How is mitosis different from meiosis?
Mitosis produces two genetically identical cells with the parent's chromosome number from one division, while meiosis produces four genetically varied haploid cells from two divisions.
- Where it happens — body cells versus reproductive cells in the gonads or spore mother cells
- Number of divisions — one versus two
- Daughter cells — two versus four
- Chromosome number — unchanged versus halved
- Synapsis and crossing over — absent versus present in prophase I
- Genetic makeup — identical to the parent versus varied
- What separates in anaphase — sister chromatids versus homologous chromosomes in anaphase I
- Role — growth and repair versus gamete formation and variation
An everyday example. A banana plant sending up new suckers makes identical offspring through mitosis, while mustard seeds from cross-pollinated flowers carry the variety that meiosis provides.
The substance. Haploid cells can divide by mitosis too — in many plants a haploid spore grows into a haploid gametophyte by mitosis, so mitosis is not limited to diploid cells.
- Where it happens — body cells versus reproductive cells in the gonads or spore mother cells
- Number of divisions — one versus two
- Daughter cells — two versus four
- Chromosome number — unchanged versus halved
- Synapsis and crossing over — absent versus present in prophase I
- Genetic makeup — identical to the parent versus varied
- What separates in anaphase — sister chromatids versus homologous chromosomes in anaphase I
- Role — growth and repair versus gamete formation and variation
An everyday example. A banana plant sending up new suckers makes identical offspring through mitosis, while mustard seeds from cross-pollinated flowers carry the variety that meiosis provides.
The substance. Haploid cells can divide by mitosis too — in many plants a haploid spore grows into a haploid gametophyte by mitosis, so mitosis is not limited to diploid cells.
Exam tip
What earns full marks on cell division diagrams?
Draw each chromosome with two chromatids joined at the centromere before anaphase and as single chromatids after it, and label the spindle fibres and poles.
- Cell cycle: G1, S, G2 and M, with G0 for non-dividing cells
- Mitosis: prophase, metaphase, anaphase, telophase, then cytokinesis
- Prophase I: leptotene, zygotene, pachytene, diplotene, diakinesis
The trap. Writing that crossing over happens in zygotene. Synapsis happens in zygotene; crossing over happens in pachytene.
- Cell cycle: G1, S, G2 and M, with G0 for non-dividing cells
- Mitosis: prophase, metaphase, anaphase, telophase, then cytokinesis
- Prophase I: leptotene, zygotene, pachytene, diplotene, diakinesis
The trap. Writing that crossing over happens in zygotene. Synapsis happens in zygotene; crossing over happens in pachytene.
Did you know
Why do cancer cells keep dividing when normal cells stop?
Normal cells pass through checkpoints in the cell cycle, where proteins check that the DNA is intact and the cell is ready before it moves on. If something is wrong, the cycle pauses or the cell destroys itself.
In cancer cells, mutations switch off these brakes. The cells ignore the signals that tell them to stop and keep cycling, piling up into a tumour.
Many anti-cancer drugs attack dividing cells, for example by blocking the spindle — which is why fast-dividing tissues such as hair roots are also affected during treatment.
In cancer cells, mutations switch off these brakes. The cells ignore the signals that tell them to stop and keep cycling, piling up into a tumour.
Many anti-cancer drugs attack dividing cells, for example by blocking the spindle — which is why fast-dividing tissues such as hair roots are also affected during treatment.
Exam relevance
How does NEET test the cell cycle, mitosis and meiosis?
Cell Cycle and Cell Division is a recurring NEET chapter, and its questions are precise about stages.
What gets asked. Events of each mitotic stage, the five sub-stages of prophase I, changes in chromosome number and DNA content through the cycle, and the differences between mitosis and meiosis.
Question types. Mostly statement-based and match-the-column questions, plus diagram-based questions that show a dividing cell and ask for its stage.
Why it matters later. Meiosis underlies Principles of Inheritance and Variation, where crossing over and independent assortment explain Mendel's laws and linkage.
The trap that costs marks. Mixing up chromosome number and DNA content — after S phase a 2n cell has 4C DNA but still 2n chromosomes.
What gets asked. Events of each mitotic stage, the five sub-stages of prophase I, changes in chromosome number and DNA content through the cycle, and the differences between mitosis and meiosis.
Question types. Mostly statement-based and match-the-column questions, plus diagram-based questions that show a dividing cell and ask for its stage.
Why it matters later. Meiosis underlies Principles of Inheritance and Variation, where crossing over and independent assortment explain Mendel's laws and linkage.
The trap that costs marks. Mixing up chromosome number and DNA content — after S phase a 2n cell has 4C DNA but still 2n chromosomes.
Key takeaways
What must you be able to do from this lesson?
- Cell cycle: interphase with G1, S and G2, then M phase; non-dividing cells rest in G0
- Mitosis: prophase, metaphase, anaphase and telophase, keeping the chromosome number for growth and repair
- Meiosis: two divisions, synapsis and crossing over in prophase I, and four haploid cells
- Differences: one versus two divisions, identical versus varied cells, and a constant versus halved chromosome number
A cell has 16 chromosomes in G1. How many chromosomes and how many chromatids does it have at metaphase of mitosis?
- Mitosis: prophase, metaphase, anaphase and telophase, keeping the chromosome number for growth and repair
- Meiosis: two divisions, synapsis and crossing over in prophase I, and four haploid cells
- Differences: one versus two divisions, identical versus varied cells, and a constant versus halved chromosome number
A cell has 16 chromosomes in G1. How many chromosomes and how many chromatids does it have at metaphase of mitosis?