Cell Cycle and Cell Division Notes

10.1 Cell Cycle

  • Growth and reproduction are characteristics of cells; all organisms start life from a single cell and form larger structures through growth and division.
  • Cell division is a central process in all living organisms; during division, DNA replication and cell growth occur in a coordinated manner to ensure correct division and intact genomes in progeny.
  • The sequence of events by which a cell duplicates its genome, synthesises cellular constituents, and divides into two daughter cells is termed the cell cycle.
  • Although cell growth (cytoplasmic increase) is continuous, DNA synthesis occurs only during one specific stage in the cycle.
  • Replicated chromosomes are distributed to daughter nuclei by a complex series of events under genetic control during cell division.
  • Cell cycle and cell division are the foundation for development, growth, and tissue renewal in organisms.

10.1.1 Phases of Cell Cycle

  • A typical eukaryotic cell cycle (e.g., human cells in culture) divides roughly every 24 hours, but duration varies across organisms and cell types.
    • Example: Yeast can progress through the cycle in about 90 minutes.
    • In humans: cycle duration is ≈ 24 h; M phase lasts about an hour; interphase occupies >95% of the cycle.
    • Expressed numerically: T{cycle} \,\approx\, 24\text{ h}; \quad T{M} \approx 1\text{ h}; \quad T{interphase} \gtrsim 0.95\,T{cycle}
  • The cell cycle consists of two basic phases:
    • Interphase: the phase between two successive M phases, where cell growth and DNA replication occur.
    • M Phase (Mitosis): the phase when actual cell division (mitosis) occurs, followed by cytokinesis.
  • Interphase is the preparation phase and lasts for most of the cycle; M Phase is the division phase.
  • The interphase is subdivided into three phases:
    • G1 phase (Gap 1): interval between mitosis and initiation of DNA replication; metabolic activity and growth occur; no DNA replication.
    • S phase (Synthesis): DNA replication occurs; amount of DNA per cell doubles from 2C to 4C; chromosome number does not increase (assuming diploid, 2n).
    • G2 phase (Gap 2): proteins synthesized for mitosis; cell continues to grow.
  • In animal cells, during S phase, DNA replication begins in the nucleus and centrioles duplicate in the cytoplasm.
  • If the initial DNA content is denoted as 2C, then after S phase it increases to 4C (DNA content doubles) while the chromosome number remains the same (2n).
  • G1, S, and G2 together constitute Interphase (the resting phase in name, but biologically active).
  • M Phase (mitosis) includes nuclear division (karyokinesis) followed by cytoplasmic division (cytokinesis).
  • The interphase interval is a time for cell growth and DNA replication in a highly orderly manner.
  • Not all cells divide continuously:
    • Some adult cells do not divide and enter a quiescent stage called G0; they remain metabolically active but do not proliferate unless stimulated.
    • In animals, mitotic division is typically restricted to diploid somatic cells; there are exceptions where haploid cells can divide by mitosis (e.g., male honey bees).
    • In plants, mitotic divisions can occur in both haploid and diploid cells; meristematic tissues are responsible for continuous growth.
  • Meristematic tissues in higher plants include apical meristems and lateral meristems (e.g., cambium) that drive ongoing growth; plants can show persistent mitosis in meristematic tissues.
  • In some organisms, mitosis is uncoupled from cytokinesis, leading to multinucleate cells (syncytium), as seen in coconut liquid endosperm.
  • Questions and prompts linked to this section:
    • Why do plants show meristematic growth throughout life?
    • Do all animal cells divide all the time? Where are tissues with persistent division activity located?
    • What are the haploid and diploid contexts for mitosis in different organisms?

10.2 M Phase

  • M Phase is the most dramatic period of the cell cycle, involving major reorganization of cellular components; it is also called equational division because the chromosome number is conserved between parent and progeny.

  • Mitosis (karyokinesis) is traditionally divided into four stages, though the process is continuous:

    • Prophase
    • Metaphase
    • Anaphase
    • Telophase

  • Cytokinesis follows mitosis and completes cell division by separating the cytoplasm into two daughter cells.

  • Nuclear division (karyokinesis) is followed by cytoplasmic division; together they complete cell division.

  • Prophase: first stage after S and G2; chromosomal material condenses into visible chromosomes; centrosomes move to opposite poles and form the mitotic spindle with asters; chromosomes condense into compact mitotic chromosomes composed of two sister chromatids held at the centromere.

  • Metaphase: nuclear envelope disappears; chromosomes align at the metaphase plate (equator) with kinetochores attached to spindle fibers from opposite poles; each chromosome has two sister chromatids connected at the centromere and attached to microtubules from opposite poles.

  • Anaphase: centromeres split; sister chromatids separate and are pulled toward opposite poles; chromatids become daughter chromosomes; chromosomes move toward poles with centromeres leading.

  • Telophase: chromosomes arrive at poles, de-condense, and become indistinct; nuclear envelope re-forms around each group of chromosomes to produce two nuclei; nucleolus, Golgi, and ER re-form.

  • Cytokinesis: division of the cytoplasm completes cell division.

    • In animal cells: a contractile actomyosin ring forms a cleavage furrow that deepens to split the cell.
    • In plant cells: due to a rigid cell wall, cytokinesis occurs via vesicle-derived cell plate formation in the center that grows outward to fuse with preexisting walls, forming two daughter cells.
    • In some organisms, karyokinesis may not be followed by cytokinesis, leading to multinucleate cells (syncytia).

  • Example question from the text for practice: A single onion root tip cell has 16 chromosomes. Determine the chromosome count at G1, after S, and after M; and the DNA content at G1, after S, and at G2 if the M phase content is 2C. Answer:

    • G1: 16 chromosomes; DNA content 2C
    • After S: 16 chromosomes; DNA content 4C
    • After M phase (end of mitosis and cytokinesis): two daughter cells each with 16 chromosomes and 2C DNA content
    • If the content after M phase is 2C in the daughter cells, then:
    • G1 of daughter cell: 2C
    • After S: 4C
    • G2: 4C

10.3 Significance of Mitosis

  • Mitosis is typically restricted to diploid cells, but some lower plants and certain haplodiploid organisms can show mitosis in haploid cells.
  • Main outcomes of mitosis:
    • Produces diploid daughter cells with identical genetic content (in standard somatic cell divisions).
    • Drives growth of multicellular organisms by increasing cell number.
    • Restores the nucleo-cytoplasmic ratio disrupted by cell growth; division helps maintain a balanced nucleus-to-cytoplasm ratio.
    • Facilitates cell repair through replacement of damaged cells (e.g., epidermal cells, gut lining, blood cells).
    • In plants, mitosis in meristematic tissues (apical and lateral cambium) underpins perennial growth.
  • Additional notes:
    • Meristematic tissues enable continuous growth in plants throughout life.
    • In some organisms, cytokinesis may be uncoupled from karyokinesis, creating multinucleate cells (syncytia).

10.4 Meiosis

  • Meiosis is the specialized type of cell division that reduces chromosome number by half to produce haploid gametes for sexual reproduction.
  • Meiosis ensures the conservation of chromosome number across generations in sexually reproducing organisms when gametes fuse (fertilization) to restore the diploid chromosome number.
  • Key features of meiosis:
    • Involves two sequential nuclear/cell divisions: Meiosis I and Meiosis II, with only one round of DNA replication (i.e., one S phase).
    • Meiosis I is initiated after parental chromosomes replicate to produce identical sister chromatids (S phase).
    • Homologous chromosomes pair and recombine (crossing over) between non-sister chromatids.
    • Four haploid cells are formed at the end of Meiosis II.
  • Meiosis is divided into two phases:
    • Meiosis I
    • Meiosis II
  • Meiosis events can be grouped into the following phases:
    • Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I
    • Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II

10.4.1 Meiosis I

  • Prophase I: typically longer and more complex than mitotic prophase; subdivided into five stages based on chromosomal behavior:
    • Leptotene: chromosomes become progressively visible; condensation begins.
    • Zygotene: pairing of homologous chromosomes (synapsis) begins; synaptonemal complex forms; homologous chromosomes become tightly paired.
    • Pachytene: four chromatids (a tetrad/bivalent) are visible; crossing over occurs at recombination nodules between non-sister chromatids of homologous chromosomes; recombination nodules mark sites of crossing over; recombinase catalyzes crossing over; recombination leads to genetic material exchange and genetic variation.
    • Diplotene: synaptonemal complex dissolves; homologous chromosomes separate except at chiasmata where crossovers occurred; chiasmata are X-shaped structures.
    • Diakinesis: terminalization of chiasmata; chromosomes fully condense; the meiotic spindle begins to assemble; nucleolus disappears and the nuclear envelope breaks down; this marks the transition to metaphase I.
  • Metaphase I: homologous chromosomes (bivalents) align on the equatorial plate; microtubules attach to kinetochores of homologous chromosomes; bivalents line up with homologs oriented toward opposite poles; the arrangement is such that sister chromatids are not co-oriented to the same pole as in mitosis; bivalents are held at the metaphase plate by kinetochore microtubules.

10.4.2 Meiosis II

  • Prophase II: initiation immediately after cytokinesis of Meiosis I; nuclear envelope breaks down; chromosomes condense again.
  • Metaphase II: chromosomes align at the equator; kinetochores attach sister chromatids to spindle fibers from opposite poles.
  • Anaphase II: centromeres split; sister chromatids separate and move toward opposite poles.
  • Telophase II: chromosomes arrive at poles; nuclear envelopes reform; nucleolus reappears; cytokinesis follows, resulting in four haploid cells.

10.5 Significance of Meiosis

  • Meiosis conserves the species-specific chromosome number across generations by producing haploid gametes, enabling sexual reproduction when gametes fuse.
  • It increases genetic variability in populations, driving evolution via recombination and independent assortment.

SUMMARY

  • According to the cell theory, cells arise from preexisting cells via cell division.
  • A sexually reproducing organism starts life from a single cell (zygote); cell division continues throughout life.
  • The cell cycle consists of Interphase (G1, S, G2) and Mitosis (M phase), followed by cytokinesis.
  • Interphase preparation includes growth and DNA replication; M phase ensures equal distribution of chromosomes and cytoplasm.
  • In mitosis, chromosome number is conserved (equational division); in meiosis, chromosome number is halved (reduction division), with two rounds of division and one round of DNA replication.
  • Meiosis I includes homologous chromosome pairing, crossing over, and reduction of chromosome number; Meiosis II resembles mitosis and further separates sister chromatids to yield four haploid products.
  • The cell cycle exhibits variations among organisms; some cells enter G0 (quiescent state), and certain cells can divide in haploid contexts (e.g., haploid insects) or in haploid plant tissues.
  • Plant growth relies on meristematic tissue (apical, lateral cambium) that maintains mitosis for lifelong growth; animal tissues exhibit mitosis primarily in diploid somatic cells but exceptions exist.

EXERCISES (from the text)

  • 1. What is the average cell cycle span for a mammalian cell?
  • 2. Distinguish cytokinesis from karyokinesis.
  • 3. Describe the events taking place during interphase.
  • 4. What is Go (quiescent phase) of cell cycle?
  • 5. Why is mitosis called equational division?
  • 6. 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.
  • 7. Describe the following: (a) synapsis (b) bivalent (c) chiasmata; Draw a diagram to illustrate your answer.
  • 8. How does cytokinesis in plant cells differ from that in animal cells?
  • 9. Find examples where the four daughter cells from meiosis are equal in size and where they are found unequal in size.
    1. Distinguish anaphase of mitosis from anaphase I of meiosis.
    1. List the main differences between mitosis and meiosis.
    1. What is the significance of meiosis?
    1. 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.
    1. Can there be mitosis without DNA replication in ‘S’ phase?
    1. Can there be DNA replication without cell division?
    1. 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