Cell Cycle and Cell Division
Understanding how cells grow, replicate their DNA, and divide — the fundamental process underlying all life and growth
In this chapter
All organisms, even the largest, start their life from a single cell. You may wonder how a single cell then goes on to form such large organisms. Growth and reproduction are characteristics of cells, indeed of all living organisms. All cells reproduce by dividing into two, with each parental cell giving rise to two daughter cells each time they divide. These newly formed daughter cells can themselves grow and divide, giving rise to a new cell population that is formed by the growth and division of a single parental cell and its progeny.
In other words, such cycles of growth and division allow a single cell to form a structure consisting of millions of cells.
10.1 Cell Cycle
Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes — cell division, DNA replication, and cell growth — have to take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes.
The cell cycle is the sequence of events by which a cell duplicates its genome, synthesises the other constituents of the cell and eventually divides into two daughter cells. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control.
Phases of the Cell Cycle
A typical eukaryotic cell cycle is illustrated by human cells in culture. These cells divide once in approximately every 24 hours. However, this duration of cell cycle can vary from organism to organism and also from cell type to cell type. Yeast, for example, can progress through the cell cycle in only about 90 minutes.
The cell cycle is divided into two basic phases:
- Interphase — the phase between two successive M phases, lasting more than 95% of the cell cycle duration
- M Phase (Mitosis phase) — the phase when actual cell division or mitosis occurs, lasting only about an hour in a 24-hour human cell cycle
In the 24-hour average duration of the cell cycle of a human cell, cell division proper lasts for only about an hour. The interphase lasts more than 95% of the duration of the cell cycle — it is the time during which the cell is preparing for division by undergoing both cell growth and DNA replication in an orderly manner.
The M Phase starts with the nuclear division, corresponding to the separation of daughter chromosomes (karyokinesis) and usually ends with division of cytoplasm (cytokinesis). The interphase, though called the "resting phase," is the time during which the cell is metabolically active and continuously grows.
The interphase is divided into three further phases:
Some cells in adult animals do not appear to exhibit division (e.g., heart cells) and many other cells divide only occasionally, as needed to replace cells that have been lost because of injury or cell death. These cells that do not divide further exit G₁ phase to enter an inactive stage called the quiescent stage (G₀) of the cell cycle. Cells in this stage remain metabolically active but no longer proliferate unless called on to do so depending on the requirement of the organism.
In animals, mitotic cell division is only seen in the diploid somatic cells. However, there are a few exceptions — for example, male honey bees where haploid cells divide by mitosis. Against this, plants can show mitotic divisions in both haploid and diploid cells.
10.2 M Phase (Mitosis)
This is the most dramatic period of the cell cycle, involving a major reorganisation of virtually all components of the cell. Since the number of chromosomes in the parent and progeny cells is the same, it is also called equational division. Though for convenience mitosis has been divided into four stages of nuclear division (karyokinesis), it is very essential to understand that cell division is a progressive process and very clear-cut lines cannot be drawn between various stages.
Karyokinesis involves the following four stages:
- Prophase
- Metaphase
- Anaphase
- Telophase
Prophase
Prophase is the first stage of karyokinesis of mitosis, following the S and G₂ phases of interphase. In the S and G₂ phases, the new DNA molecules formed are not distinct but intertwined. Prophase is marked by the initiation of condensation of chromosomal material. The chromosomal material becomes untangled during the process of chromatin condensation. The centrosome, which had undergone duplication during S phase of interphase, now begins to move towards opposite poles of the cell.
The completion of prophase can be marked by the following characteristic events:
- Chromosomal material condenses to form compact mitotic chromosomes. Chromosomes are seen to be composed of two chromatids attached together at the centromere.
- Centrosome which had undergone duplication during interphase, begins to move towards opposite poles of the cell. Each centrosome radiates out microtubules called asters. The two asters together with spindle fibres form the mitotic apparatus.
Cells at the end of prophase, when viewed under the microscope, do not show golgi complexes, endoplasmic reticulum, nucleolus and the nuclear envelope.
Metaphase
The complete disintegration of the nuclear envelope marks the start of the second phase of mitosis, hence the chromosomes are spread through the cytoplasm of the cell. By this stage, condensation of chromosomes is completed and they can be observed clearly under the microscope. This then, is the stage at which morphology of chromosomes is most easily studied.
At this stage, a metaphase chromosome is made up of two sister chromatids, which are held together by the centromere. Small disc-shaped structures at the surface of the centromeres are called kinetochores. These structures serve as the sites of attachment of spindle fibres to the chromosomes that are moved into position at the centre of the cell.
Hence, the metaphase is characterised by all the chromosomes coming to lie at the equator with one chromatid of each chromosome connected by its kinetochore to spindle fibres from one pole and its sister chromatid connected by its kinetochore to spindle fibres from the opposite pole. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
• Spindle fibres attach to kinetochores of chromosomes.
• Chromosomes are moved to spindle equator and get aligned along the metaphase plate through spindle fibres to both poles.
Anaphase
At the onset of anaphase, each chromosome arranged at the metaphase plate is split simultaneously and the two daughter chromatids, now referred to as daughter chromosomes of the future daughter nuclei, begin their migration towards the two opposite poles. As each chromosome moves away from the equatorial plate, the centromere of each chromosome remains directed towards the pole and hence at the leading edge, with the arms of the chromosome trailing behind.
• Centromeres split and chromatids separate.
• Chromatids move to opposite poles.
Telophase
At the beginning of the final stage of karyokinesis, i.e., telophase, the chromosomes that have reached their respective poles decondense and lose their individuality. The individual chromosomes can no longer be seen and each set of chromatin material tends to collect at each of the two poles.
• Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
• Nuclear envelope develops around the chromosome clusters at each pole forming two daughter nuclei.
• Nucleolus, golgi complex and ER reform.
Cytokinesis
Mitosis accomplishes not only the segregation of duplicated chromosomes into daughter nuclei (karyokinesis), but the cell itself is divided into two daughter cells by the separation of cytoplasm called cytokinesis at the end of which cell division gets completed.
Animal Cells
Cytokinesis is achieved by the appearance of a furrow in the plasma membrane. The furrow gradually deepens and ultimately joins in the centre dividing the cell cytoplasm into two.
Plant Cells
Plant cells are enclosed by a relatively inextensible cell wall. Wall formation starts in the centre of the cell and grows outward to meet the existing lateral walls. The formation of the new cell wall begins with the formation of a simple precursor, called the cell-plate that represents the middle lamella between the walls of two adjacent cells.
At the time of cytoplasmic division, organelles like mitochondria and plastids get distributed between the two daughter cells. In some organisms karyokinesis is not followed by cytokinesis as a result of which multinucleate condition arises leading to the formation of syncytium (e.g., liquid endosperm in coconut).
10.3 Significance of Mitosis
Mitosis or the equational division is usually restricted to the diploid cells only. However, in some lower plants and in some social insects, haploid cells also divide by mitosis.
Mitosis usually results in the production of diploid daughter cells with identical genetic complement. The growth of multicellular organisms is due to mitosis. Cell growth results in disturbing the ratio between the nucleus and the cytoplasm. It therefore becomes essential for the cell to divide to restore the nucleo-cytoplasmic ratio.
A very significant contribution of mitosis is cell repair. The cells of the upper layer of the epidermis, cells of the lining of the gut, and blood cells are being constantly replaced. Mitotic divisions in the meristematic tissues — the apical and the lateral cambium — result in a continuous growth of plants throughout their life.
Growth
Mitosis produces genetically identical diploid daughter cells, enabling the growth of multicellular organisms from a single-celled zygote.
Cell Repair
Damaged or lost cells in the epidermis, gut lining, and blood are constantly replaced through mitotic divisions.
Nucleo-Cytoplasmic Ratio
As cells grow, the nucleus-to-cytoplasm ratio decreases. Mitosis restores this ratio by dividing the cell.
Plant Growth
Mitotic divisions in meristematic tissues (apical and lateral cambium) result in continuous growth throughout a plant's life.
10.4 Meiosis
The production of offspring by sexual reproduction includes the fusion of two gametes, each with a complete haploid set of chromosomes. Gametes are formed from specialised diploid cells. This specialised kind of cell division that reduces the chromosome number by half results in the production of haploid daughter cells. This kind of division is called meiosis.
Meiosis ensures the production of haploid phase in the life cycle of sexually reproducing organisms whereas fertilisation restores the diploid phase. We come across meiosis during gametogenesis in plants and animals. This leads to the formation of haploid gametes.
• Meiosis involves two sequential cycles of nuclear and cell division called meiosis I and meiosis II but only a single cycle of DNA replication.
• Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids at the S phase.
• Meiosis involves pairing of homologous chromosomes and recombination between non-sister chromatids of homologous chromosomes.
• Four haploid cells are formed at the end of meiosis II.
Meiotic events can be grouped under the following phases:
| Meiosis I | Meiosis II |
|---|---|
| Prophase I | Prophase II |
| Metaphase I | Metaphase II |
| Anaphase I | Anaphase II |
| Telophase I | Telophase II |
Meiosis I
Prophase I of the first meiotic division is typically longer and more complex when compared to prophase of mitosis. It has been further subdivided into the following five phases based on chromosomal behaviour: Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.
Leptotene
Chromosomes become gradually visible under the light microscope. The compaction of chromosomes continues throughout leptotene.
Zygotene
Chromosomes start pairing together — this process of association is called synapsis. Paired chromosomes are called homologous chromosomes. Synapsis is accompanied by the formation of a complex structure called the synaptonemal complex. The complex formed by a pair of synapsed homologous chromosomes is called a bivalent or a tetrad.
Pachytene
The four chromatids of each bivalent becomes distinct and clearly appear as tetrads. This stage is characterised by the appearance of recombination nodules — the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. It is an enzyme-mediated process; the enzyme involved is called recombinase. Crossing over leads to recombination of genetic material. Recombination is completed by the end of pachytene, leaving the chromosomes linked at the sites of crossing over.
Diplotene
Recognised by the dissolution of the synaptonemal complex and the tendency of the recombined homologous chromosomes of the bivalents to separate from each other except at the sites of crossovers. These X-shaped structures are called chiasmata. In oocytes of some vertebrates, diplotene can last for months or years.
Diakinesis
The final stage of meiotic prophase I is diakinesis. This is marked by terminalisation of chiasmata. During this phase the chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. By the end of diakinesis, the nucleolus disappears and the nuclear envelope also breaks down.
Metaphase I
The bivalent chromosomes align on the metaphase plate. The orientation of the homologous chromosome pairs is random — this random alignment contributes to genetic variation.
Anaphase I
The homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
Telophase I
The nuclear membrane and nucleolus reappear, cytokinesis follows and this is called a dyad of cells. Although in many cases the chromosomes do undergo some dispersion, they do not reach the extremely extended state of the interphase nucleus.
The stage between the two meiotic divisions is called interkinesis and is generally short-lived. There is no replication of DNA during interkinesis. Interkinesis is followed by prophase II, a much simpler prophase than prophase I.
Meiosis II
Prophase II
Meiosis II is initiated immediately after cytokinesis, usually before the chromosomes have fully elongated. In contrast to meiosis I, meiosis II resembles a normal mitosis. The nuclear membrane disappears by the end of prophase II. The chromosomes again become compact.
Metaphase II
At this stage, the chromosomes align at the equator and the microtubules from opposite poles of the spindle get attached to the kinetochores of sister chromatids.
Anaphase II
Begins with the simultaneous splitting of the centromere of each chromosome (which was holding the sister chromatids together), allowing them to move toward opposite poles of the cell by shortening of microtubules attached to kinetochores.
Telophase II
Meiosis ends with telophase II, in which the two groups of chromosomes once again get enclosed by a nuclear envelope; cytokinesis follows resulting in the formation of a tetrad of cells, i.e., four haploid daughter cells.
Mitosis vs Meiosis — A Comparison
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | One division | Two successive divisions (I and II) |
| DNA replication | Once, before division | Once, before meiosis I only |
| Daughter cells formed | Two diploid cells | Four haploid cells |
| Chromosome number | Same as parent (2n → 2n) | Reduced by half (2n → n) |
| Crossing over | Does not occur | Occurs during pachytene of prophase I |
| Homologous pairing | Does not occur | Occurs during zygotene of prophase I |
| Significance | Growth, repair, asexual reproduction | Formation of gametes, genetic variation |
10.5 Significance of Meiosis
Meiosis is the mechanism by which conservation of specific chromosome number of each species is achieved across generations in sexually reproducing organisms, even though the process, per se, paradoxically, results in reduction of chromosome number by half.
It also increases the genetic variability in the population of organisms from one generation to the next. Variations are very important for the process of evolution.
• Chromosome number conservation: Meiosis reduces the chromosome number by half during gamete formation. When two gametes fuse during fertilisation, the original diploid chromosome number is restored — ensuring species continuity across generations.
• Genetic variation: Crossing over during pachytene and random alignment of homologous pairs during metaphase I introduce new combinations of alleles, increasing genetic diversity — raw material for evolution.
• According to cell theory, cells arise from pre-existing cells through cell division. Any sexually reproducing organism starts its life cycle from a single-celled zygote. Cell division does not stop with the formation of the mature organism but continues throughout its life cycle.
• The stages through which a cell passes from one division to the next is called the cell cycle. It is divided into Interphase (G₁, S, G₂) — a period of preparation for cell division — and Mitosis (M phase) — the actual period of cell division.
• G₁ phase is when the cell grows and carries out normal metabolism. S phase marks DNA replication and chromosome duplication. G₂ phase is the period of cytoplasmic growth.
• Mitosis is divided into four stages: prophase, metaphase, anaphase and telophase. Chromosome condensation occurs during prophase. During metaphase, chromosomes align at the equatorial plate. During anaphase, centromeres divide and chromatids move to opposite poles. Telophase sees nuclear envelope reformation. It is the equational division in which the chromosome number of the parent is conserved in the daughter cell.
• Meiosis occurs in diploid cells destined to form gametes. It is called reduction division since it reduces the chromosome number by half. Meiosis I involves pairing of homologous chromosomes, crossing over, and their separation. Meiosis II resembles mitosis. At the end of meiosis, four haploid cells are formed.
📝 Exercises
- What is the average cell cycle span for a mammalian cell?
- Distinguish cytokinesis from karyokinesis.
- Describe the events taking place during interphase.
- What is G₀ (quiescent phase) of cell cycle?
- Why is mitosis called equational division?
-
Name the stage of cell cycle at which one of the following events occur:
(i) Chromosomes are moved to spindle equator.
(ii) Centromere splits and chromatids separate.
(iii) Pairing between homologous chromosomes takes place.
(iv) Crossing over between homologous chromosomes takes place. - Describe the following: (a) synapsis (b) bivalent (c) chiasmata. Draw a diagram to illustrate your answer.
- How does cytokinesis in plant cells differ from that in animal cells?
- Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
- Distinguish anaphase of mitosis from anaphase I of meiosis.
- List the main differences between mitosis and meiosis.
- What is the significance of meiosis?
-
Discuss with your teacher about
(i) haploid insects and lower plants where cell division occurs, and
(ii) some haploid cells in higher plants where cell division does not occur. - Can there be mitosis without DNA replication in S phase?
- Can there be DNA replication without cell division?
-
Analyse the events during every stage of cell cycle and notice how the following two parameters change:
(i) number of chromosomes (N) per cell
(ii) amount of DNA content (C) per cell