Cell Division: Mitosis and Meiosis Explained for Exams
Complete notes on cell division for competitive exams: the cell cycle, the stages of mitosis, prophase I of meiosis, crossing over and the significance of each.
By GK24 Editorial Team· Published · 4 min read
Every cell in your body came from another cell that divided. Cell division is the process by which a parent cell gives rise to daughter cells, and it is how an organism grows from a single fertilised egg, replaces worn-out skin and blood cells, heals a wound and produces the gametes needed for the next generation. Two kinds of division matter for examinations. Mitosis keeps the chromosome number the same and runs the ordinary business of growth and repair. Meiosis halves the chromosome number and makes the sex cells. Questions on this chapter are almost always about the sequence of the stages, the one event that identifies each stage, and the numbers, so learn both in the same order every time.
The cell cycle
A dividing cell passes through a repeating sequence called the cell cycle, which has two broad parts. Interphase, sometimes misleadingly called the resting stage, occupies more than ninety-five per cent of the time and is when the cell grows and copies its DNA. It is divided into the G1 phase, in which the cell grows and makes proteins and RNA but the DNA is not copied; the S phase or synthesis phase, in which DNA replication takes place so that the amount of DNA doubles while the chromosome number stays the same; and the G2 phase, in which the cell prepares the proteins and organelles needed for division. Cells that stop dividing, such as mature nerve cells, leave the cycle and enter a quiescent stage called G0. The second part is the M phase, the division itself, which consists of karyokinesis, the division of the nucleus, followed by cytokinesis, the division of the cytoplasm. In a human cell growing in culture the whole cycle takes roughly twenty-four hours, of which the M phase takes only about an hour.
Mitosis, the equational division
Mitosis takes place in somatic or body cells. One parent cell produces two daughter cells, each with exactly the same number of chromosomes as the parent, which is why it is called the equational division. The term was coined by Walther Flemming in 1882. It has four stages.
| Stage | What happens |
|---|---|
| Prophase | Chromatin condenses into visible chromosomes, each with two sister chromatids joined at the centromere; the spindle begins to form and the centrioles move to opposite poles; by its end the nuclear envelope and the nucleolus disappear. It is the longest stage of mitosis. |
| Metaphase | Chromosomes line up at the equator of the cell on the metaphase plate and spindle fibres attach to the kinetochores at the centromeres. This is the best stage for studying the shape and size of chromosomes. |
| Anaphase | Each centromere splits, the sister chromatids separate and are pulled as daughter chromosomes to opposite poles. This is the shortest stage. |
| Telophase | The chromosomes at each pole uncoil back into chromatin, the nuclear envelope reforms and the nucleolus reappears, so two nuclei now exist in one cell. |
Cytokinesis then divides the cytoplasm. In an animal cell the membrane pinches inward to form a cleavage furrow; in a plant cell, whose rigid wall cannot be pinched, a cell plate forms in the middle and grows outward to the existing wall. Plant cells also lack centrioles, so their spindle is described as anastral.
Meiosis, the reductional division
Meiosis occurs only in the reproductive cells of organisms that reproduce sexually. The name was given by Farmer and Moore in 1905. One cycle of DNA replication is followed by two successive divisions, so a single diploid cell produces four haploid cells, each with half the parent's chromosome number. In human beings a cell with forty-six chromosomes produces gametes with twenty-three.
Meiosis I is the reduction division and its prophase is the longest and most complex phase in the whole of cell biology. It has five sub-stages, best remembered in order: leptotene, in which the chromosomes become visible as thin threads; zygotene, in which homologous chromosomes pair side by side, a process called synapsis, held together by the synaptonemal complex; pachytene, in which the paired chromosomes, now called bivalents or tetrads, exchange segments in crossing over; diplotene, in which the homologues begin to separate and the X-shaped points of exchange, the chiasmata, become visible; and diakinesis, in which the chiasmata move to the ends in terminalisation and the nuclear envelope breaks down. In metaphase I the bivalents line up at the equator; in anaphase I the whole homologous chromosomes move to opposite poles and, importantly, the centromeres do not split; telophase I then follows.
Meiosis II is an equational division like mitosis, with no further DNA replication. Here the centromeres do split and the sister chromatids separate, so the two cells formed in the first division become four.
Why each division matters
- Mitosis keeps the chromosome number constant, so every body cell carries the same genetic information, and it provides for growth, repair of tissues, replacement of dead cells and asexual reproduction.
- Meiosis halves the chromosome number so that fertilisation restores it instead of doubling it generation after generation.
- Meiosis is the main source of genetic variation, through crossing over in pachytene and through the independent assortment of maternal and paternal chromosomes in metaphase I.
- Errors in these processes matter: an uncontrolled mitosis underlies tumour growth, and a failure of chromosomes to separate, called non-disjunction, causes conditions such as Down's syndrome.
Exam Point of View
Papers set three kinds of question from this chapter. The first asks for the stage in which a named event happens, so remember the one event that identifies each: condensation in prophase, alignment at the equator in metaphase, splitting of the centromere in anaphase and reformation of the nuclear envelope in telophase. The second asks for the numbers: two diploid cells from mitosis, four haploid cells from meiosis, 46 chromosomes in a human body cell and 23 in a gamete, and more than 95 per cent of the cycle spent in interphase. The third asks the sub-stages of prophase I in order, with synapsis in zygotene and crossing over in pachytene the two most frequently tested. The traps are calling interphase a resting stage, forgetting that anaphase I does not split the centromere, and attributing the cleavage furrow to plant cells.
Important Facts
| DNA replication | S phase of interphase |
|---|---|
| Longest part of the cell cycle | Interphase, more than 95 per cent of the duration |
| Quiescent stage | G0, entered by cells that stop dividing |
| Mitosis occurs in | Somatic or body cells; gives two diploid cells |
| Longest stage of mitosis | Prophase; the shortest is anaphase |
| Best stage to study chromosomes | Metaphase |
| Cytokinesis in animals and plants | Cleavage furrow and cell plate respectively |
| Meiosis occurs in | Reproductive cells; gives four haploid cells |
| Sub-stages of prophase I | Leptotene, zygotene, pachytene, diplotene, diakinesis |
| Synapsis | Zygotene, through the synaptonemal complex |
| Crossing over | Pachytene, at recombination nodules |
| Term mitosis | Coined by Walther Flemming in 1882 |
| Term meiosis | Given by Farmer and Moore in 1905 |
| Human chromosome number | 46 in a body cell, 23 in a gamete |
Practice MCQs on this topic
In which phase of the cell cycle does DNA replication take place?
- A.G1 phase
- B.S phase
- C.G2 phase
- D.M phase
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During this phase the DNA of the cell is copied so that the amount of DNA in the nucleus doubles, while the number of chromosomes remains unchanged because each chromosome now simply consists of two identical sister chromatids joined at the centromere.
Read the full explanation →Mitosis is called the equational division because
- A.it produces four daughter cells
- B.the chromosome number of the daughter cells is the same as that of the parent cell
- C.it halves the chromosome number
- D.it occurs only in reproductive cells
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In mitosis each centromere splits in anaphase and one chromatid of every chromosome goes to each pole, so the two daughter cells receive exactly the same number of chromosomes as the parent cell had, and exactly the same genetic information.
Read the full explanation →At which stage of mitosis do the chromosomes line up at the equator of the cell?
- A.Prophase
- B.Metaphase
- C.Anaphase
- D.Telophase
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In this stage the condensed chromosomes, each consisting of two sister chromatids, are moved by the spindle fibres until their centromeres lie in one plane across the middle of the cell, known as the metaphase plate or equatorial plate.
Read the full explanation →The splitting of the centromere and the movement of sister chromatids to opposite poles occurs in
- A.prophase
- B.metaphase
- C.anaphase
- D.telophase
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Once the chromosomes are aligned on the metaphase plate and every kinetochore is attached to a spindle fibre, the centromere holding the two sister chromatids together splits.
Read the full explanation →Cytokinesis in a plant cell differs from that in an animal cell because
- A.a cleavage furrow forms in the plant cell
- B.a cell plate forms in the plant cell
- C.the plant cell does not divide its cytoplasm
- D.the plant cell divides its nucleus twice
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An animal cell divides its cytoplasm by a cleavage furrow, in which the plasma membrane is pulled inward around the middle until the cell is pinched into two.
Read the full explanation →How many daughter cells are produced at the end of meiosis in one parent cell, and of what kind?
- A.Two diploid cells
- B.Two haploid cells
- C.Four haploid cells
- D.Four diploid cells
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Meiosis begins with a single round of DNA replication and is then followed by two successive nuclear divisions. Meiosis I is the reduction division, in which homologous chromosomes separate and the two cells formed already carry half the parental chromosome number.
Read the full explanation →Crossing over between homologous chromosomes takes place in which sub-stage of prophase I?
- A.Leptotene
- B.Zygotene
- C.Pachytene
- D.Diakinesis
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By this sub-stage the homologous chromosomes have already paired in zygotene and are held together as bivalents or tetrads, each made of four chromatids.
Read the full explanation →The pairing of homologous chromosomes during prophase I is called
- A.synapsis
- B.terminalisation
- C.disjunction
- D.cytokinesis
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In the zygotene sub-stage of prophase I the two members of each pair of homologous chromosomes come to lie side by side along their whole length, matching point for point, and the structure that forms between them and holds them together is called the synaptonemal complex.
Read the full explanation →In anaphase I of meiosis, unlike in the anaphase of mitosis,
- A.the centromeres split and sister chromatids separate
- B.the centromeres do not split and whole homologous chromosomes move to opposite poles
- C.the chromosomes line up at the equator
- D.the nuclear envelope is formed
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This single difference is the reason meiosis I is called the reduction division. In anaphase I the two homologous chromosomes of each bivalent are pulled to opposite poles, but each of them still consists of two sister chromatids joined at an undivided centromere.
Read the full explanation →Which of the following is the longest phase of the cell cycle?
- A.Prophase
- B.Interphase
- C.Metaphase
- D.Telophase
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Interphase occupies more than ninety-five per cent of the duration of the cell cycle, because in it the cell must grow to a sufficient size, synthesise proteins and RNA, duplicate its entire DNA and prepare the structures needed for division.
Read the full explanation →The term mitosis was coined by
- A.Robert Hooke
- B.Walther Flemming
- C.Farmer and Moore
- D.Rudolf Virchow
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His detailed drawings of the stages are the basis of the account still given in textbooks.
Read the full explanation →Which of the following is the main reason why meiosis is important for sexually reproducing organisms?
- A.It repairs damaged tissues
- B.It keeps the chromosome number constant across generations and creates genetic variation
- C.It allows cells to grow larger
- D.It produces identical copies of the parent cell
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If gametes carried the full chromosome number, fertilisation would double it in every generation. Meiosis halves the number when gametes are formed, so that the union of two gametes restores the original diploid number and keeps it constant from one generation to the next.
Read the full explanation →Frequently Asked Questions
What is the main difference between mitosis and meiosis?
Mitosis occurs in body cells, involves one division and produces two daughter cells with the same chromosome number as the parent. Meiosis occurs in reproductive cells, involves two successive divisions after a single DNA replication and produces four cells with half the parent's chromosome number.
Why is interphase not a resting stage?
Because the cell is extremely active during it. In G1 it grows and makes RNA and proteins, in S it copies its entire DNA, and in G2 it builds the proteins and organelles needed for division. It only looks inactive because no visible movement of chromosomes is taking place.
In which stage is crossing over seen and why does it matter?
Crossing over occurs in pachytene, the third sub-stage of prophase I, when non-sister chromatids of paired homologous chromosomes exchange segments. It creates new combinations of genes and is the chief source of genetic variation among the offspring of sexually reproducing organisms.
Why do plant cells form a cell plate instead of a cleavage furrow?
Because a plant cell is enclosed in a rigid cellulose wall that cannot be pinched inward. Vesicles therefore collect at the centre of the cell and fuse into a cell plate, which grows outward until it joins the existing wall and separates the two daughter cells.
What happens if chromosomes fail to separate during cell division?
The failure is called non-disjunction. A daughter cell then receives one chromosome too many or too few, and when such a gamete takes part in fertilisation the resulting individual has an abnormal chromosome number, as in Down's syndrome.
Why does meiosis halve the chromosome number?
So that fertilisation can restore it. If each gamete carried the full diploid number, the number would double with every generation. Halving it in meiosis and doubling it at fertilisation keeps the chromosome number of a species constant.
