Chapter 10: Cell Cycle and Cell Division

Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication of its DNA (DNA replication) to produce two daughter cells. The cell cycle is divided into two basic phases:

• Interphase
• M Phase (Mitosis or Meiosis)

Interphase

Interphase is the resting phase where the cell prepares for division by undergoing growth and DNA replication. It lasts more than 95% of the duration of the cell cycle. Interphase is further divided into three sub-phases:

• G1 phase (Gap 1): Interval between mitosis and initiation of DNA replication. The cell is metabolically active and continuously grows.
• S phase (Synthesis): Period during which DNA synthesis or replication takes place. Amount of DNA per cell doubles, but the chromosome number remains the same.
• G2 phase (Gap 2): Proteins are synthesized in preparation for mitosis while cell growth continues.

Quiescent Stage (G0)

Cells that do not divide further exit the G1 phase to enter an inactive stage called the quiescent stage (G0). Cells in this stage remain metabolically active but do not proliferate unless called on to do so depending on the requirement of the organism.

Definition: The cell cycle is the ordered sequence of events in which a cell grows and divides to create two new daughter cells.

Mitosis

Mitosis is the type of cell division where chromosomes replicate and are equally distributed into two daughter cells. It is also known as equational division because the chromosome number in the daughter cells is equal to that of the parent cell.

Significance of Mitosis

• Produces diploid daughter cells with identical genetic complement.
• Usually results in the growth of multicellular organisms.
• Cell repair and replacement of old or damaged cells.
• Maintains the surface-area-to-volume ratio in growing cells.
• Essential for asexual reproduction in unicellular organisms.

Phases of Mitosis

Mitosis is divided into four main stages of nuclear division (karyokinesis), followed by cytokinesis.

1. Prophase

Prophase is the first stage of mitosis following the S and G2 phases of interphase. During this phase, chromatin fibers condense into distinct chromosomes, and the mitotic apparatus (spindle fibers) begins to form. Nucleolus and nuclear envelope disappear towards the end of prophase.

2. Metaphase

The complete disappearance of the nuclear envelope marks the start of metaphase. Chromosomes are spread through the cytoplasm. At this stage, chromosomes are condensed and can be observed clearly under the microscope. Chromosomes come to lie at the equator, with the centromere attached to the spindle fibers. The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.

3. Anaphase

At the onset of anaphase, each chromosome arranged at the metaphase plate splits 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 the chromosomes move away from the equatorial plate, the centromere of each chromosome remains directed towards the pole.

4. Telophase

The final stage of karyokinesis. Chromosomes reach the opposite poles and their identity is lost as discrete elements. Nuclear envelope develops around the chromosome clusters at each pole. Nucleolus, Golgi complex, and ER reform.

Cytokinesis

Mitosis accomplishes not only the segregation of duplicated chromosomes into nuclei (karyokinesis) but the cell itself is divided into two daughter cells by the separation of cytoplasm, which is called cytokinesis. In animal cells, this is achieved by the appearance of a furrow in the plasma membrane. In plant cells, wall formation starts in the center of the cell and grows outward to meet the existing lateral walls.

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 specialized diploid cells. This specialized 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.

Significance of Meiosis

• Conserves the specific chromosome number of each species across generations.
• Increases genetic variability in the population of organisms from one generation to the next due to crossing over.
• Important for evolution and adaptation.

Phases of Meiosis

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 S phase of interphase.

Meiosis I

Meiosis I is heterotypic or reductional division. It is divided into four phases:

• Prophase I: Typically longer and more complex compared to prophase of mitosis. It is subdivided into 5 phases based on chromosomal behavior: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
• Metaphase I: The bivalent chromosomes align on the equatorial plate. Spindle fibers attach to the homologous pairs.
• Anaphase I: Homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
• Telophase I: The nuclear membrane and nucleolus reappear, cytokinesis follows, and this is often followed by interkinesis.

Prophase I Sub-stages

• Leptotene: Chromosomes become gradually visible under the light microscope.
• Zygotene: Chromosomes start pairing together and this process of association is called synapsis. Such paired chromosomes are called homologous chromosomes. Electron micrographs of this stage indicate that chromosome synapsis is accompanied by the formation of a complex structure called synaptonemal complex.
• Pachytene: Bivalent chromosomes now clearly appear as tetrads. This stage is characterized 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.
• Diplotene: The beginning of diplotene is recognized 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.
• Diakinesis: Marked by terminalisation of chiasmata. The chromosomes are fully condensed and the meiotic spindle is assembled to prepare the homologous chromosomes for separation. Nucleolus nucleates and the nuclear envelope breaks down.

Meiosis II

Meiosis II resembles mitosis. It is equational division. During Anaphase II, the sister chromatids separate, unlike Anaphase I where homologous chromosomes separate.

Law/Definition: Crossing over is the exchange of genetic segments between non-sister chromatids of homologous chromosomes during the pachytene stage of prophase I, leading to genetic recombination.

Exam-Oriented Notes

• Key Trap: DNA replication occurs only once, prior to Meiosis I, and not between Meiosis I and Meiosis II.
• Terminology Alert: Chromatids separate in Anaphase of mitosis and Anaphase II of meiosis, whereas homologous chromosomes separate in Anaphase I.
• Ploidy Check: The daughter cells at the end of meiosis are always haploid (n), whereas mitosis maintains the parental ploidy level (2n to 2n).