Cell Growth and Division

• All organisms start life as a single cell.
• Growth and reproduction are characteristics 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.
• Newly formed daughter cells can grow and divide, forming a new cell population through the growth and division of a single parental cell and its progeny.
• Cycles of growth and division allow a single cell to form a structure consisting of millions of cells.
• Cell division is an important process in all living organisms.
• During cell division, DNA replication and cell growth occur.
• Cell division, DNA replication, and cell growth must be coordinated.
• Coordination ensures correct division and the formation of progeny cells containing intact genomes.
• The cell cycle involves a cell duplicating its genome, synthesizing other constituents, and eventually dividing into two daughter cells.
• Cell growth (cytoplasmic increase) is continuous, but DNA synthesis occurs only during one specific stage.
• Replicated chromosomes (DNA) are distributed to daughter nuclei through a series of events controlled genetically.

Phases of Cell Cycle

• A typical eukaryotic cell cycle, exemplified by human cells in culture, takes approximately 24 hours for one division.
• The duration of the cell cycle varies among organisms and cell types; yeast can complete a cycle in about 90 minutes.
• The cell cycle is divided into two basic phases: Interphase and M Phase (Mitosis phase).
• The M Phase involves actual cell division or mitosis.
• Interphase is the phase between two successive M phases.
• Cell division lasts only about one hour in a 24-hour human cell cycle.
• Interphase lasts for more than 95% of the cell cycle.
• The M Phase begins with nuclear division (karyokinesis) and ends with cytoplasmic division (cytokinesis).
• Interphase is when the cell prepares for division through cell growth and DNA replication.
• Interphase is divided into three phases: G₁ phase (Gap 1), S phase (Synthesis), and G₂ phase (Gap 2).
• G₁ phase is the interval between mitosis and the initiation of DNA replication, during which the cell grows but does not replicate its DNA.
• S phase is for DNA synthesis or replication, where the DNA amount per cell doubles from 2C to 4C, but the chromosome number remains the same.
• In animal cells during the S phase, DNA replication begins in the nucleus, and the centriole duplicates in the cytoplasm.
• During the G₂ phase, proteins are synthesized for mitosis preparation as cell growth continues.
• Some adult animal cells do not divide (e.g., heart cells), while others divide occasionally to replace lost cells.
• Cells that exit G₁ phase enter an inactive stage called the quiescent stage (G₀) and remain metabolically active but do not proliferate unless stimulated.
• Mitotic cell division in animals is seen in diploid somatic cells, with exceptions like male honey bees.
• Plants can show mitotic divisions in both haploid and diploid cells.

M Phase

• The M Phase is the most dramatic period, involving a major reorganization of cell components.
• The number of chromosomes remains the same in parent and progeny cells, and is also known as equational division.
• Mitosis is divided into four karyokinesis stages: Prophase, Metaphase, Anaphase, and Telophase, it should be understood that cell division is a progressive process without clear lines between stages.

Prophase

• Prophase, the first stage of karyokinesis, follows the S and G₂ phases of interphase.
• The new DNA molecules are intertwined, but prophase is marked by the initiation of condensation of chromosomal material.
• Chromosomal material becomes untangled during chromatin condensation.
• The centrosome, which had duplicated during the S phase, moves towards opposite poles of the cell.
• Completion of prophase is marked by the following characteristic events:
  • Chromosomal material condenses to form compact mitotic chromosomes composed of two chromatids attached at the centromere.
  • The centrosome, having duplicated during interphase, moves to opposite poles, radiating microtubules called asters that form the mitotic apparatus with spindle fibers.
• At the end of prophase, cells viewed under a microscope do not show the Golgi complex, endoplasmic reticulum, nucleolus, and nuclear envelope.

Metaphase

• The disintegration of the nuclear envelope marks the start of metaphase.
• Chromosomes spread through the cytoplasm; condensation is completed, making them observable under a microscope.
• The morphology of chromosomes is easily studied at metaphase.
• The metaphase chromosome has two sister chromatids held together by the centromere.
• Disc-shaped structures on the centromeres, called kinetochores, serve as spindle fiber attachment sites.
• Metaphase is characterized by all chromosomes lying at the equator, with one chromatid connected to the spindle fibers from one pole and the sister chromatid connected to the fibers from the opposite pole.
• The plane of alignment of the chromosomes at metaphase is referred to as the metaphase plate.
• Key features of metaphase
  • Spindle fibers attach to kinetochores of chromosomes.
  • Chromosomes are moved to the spindle equator by spindle fibers for both poles, where they get aligned along metaphase plate.

Anaphase

• At the start of anaphase, each chromosome at the metaphase plate splits, and the two daughter chromatids, now daughter chromosomes, migrate to opposite poles.
• The centromere of each chromosome is directed towards the pole, leading the chromosome with the arms trailing behind.
• Anaphase is characterized by centromeres splitting and chromatids separating and moving to opposite poles.

Telophase

• At the beginning of telophase, the chromosomes reaching their respective poles decondense and lose their individuality.
• Telophase has the following key events:
  • Chromosomes cluster at opposite spindle poles and their identity is lost as discrete elements.
  • The nuclear envelope develops around chromosome clusters at each pole, forming two daughter nuclei.
  • Nucleolus, golgi complex and ER reform.

Cytokinesis

• Mitosis segregates duplicated chromosomes into daughter nuclei (karyokinesis), and the cell divides into two daughter cells through cytokinesis.
• In animal cells, cytokinesis is achieved by a furrow in the plasma membrane that deepens and divides the cytoplasm.
• Plant cells undergo cytokinesis via a different mechanism due to their inextensible cell wall.
• In plant cells, wall formation begins with a cell-plate, that becomes the middle lamella between adjacent cells' walls.
• Organelles like mitochondria and plastids are distributed between the two daughter cells during cytoplasmic division.
• In some organisms, karyokinesis is not followed by cytokinesis, leading to a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).

Significance of Mitosis

• Mitosis is typically restricted to diploid cells, but haploid cells also divide by mitosis in some lower plants and social insects.
• Mitosis produces diploid daughter cells with identical genetic complements.
• The growth of multicellular organisms happens due to mitosis.
• Cell growth can disturb the nucleo-cytoplasmic ratio and division restores the ratio.
• Mitosis contributes to cell repair.
• Mitotic divisions in meristematic tissues result in continuous growth of plants.

Meiosis

• Sexual reproduction involves the fusion of two gametes, each with a complete haploid set of chromosomes.
• Gametes are formed from diploid cells through a specialized cell division called meiosis.
• Meiosis reduces the chromosome number by half, resulting in haploid daughter cells.
• Meiosis allows the production of the haploid phase in sexually reproducing organisms where diploid phase will be restored by fertilization.
• Meiosis occurs during gametogenesis in plants and animals, leading to the formation of haploid gametes.
• Key features of meiosis:
  • It involves two sequential cycles of nuclear and cell division (meiosis I and meiosis II) with only a single cycle of DNA replication.
  • Meiosis I is initiated after the parental chromosomes have replicated to produce identical sister chromatids during the S phase.
  • It involves pairing of homologous chromosomes and recombination between non-sister chromatids.
  • At the end of meiosis II, four haploid cells are formed.
• Meiotic events are grouped under the following phases:
  • Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I
  • Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II

Meiosis I

• Prophase I is more complex compared to prophase of mitosis and is divided into five phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
  • Leptotene: Chromosomes become gradually visible under the microscope; compaction continues.
  • Zygotene: Chromosomes pair together (synapsis), forming homologous chromosomes.
  • Synaptonemal complex is formed.
  • A pair of synapsed homologous chromosomes forms a bivalent or a tetrad.
  • Pachytene: Each bivalent chromosomes consists of four chromatids and appears as tetrads.
  • Recombination nodules appear, forming sites where crossing over occurs between non-sister chromatids.
  • Crossing over, an enzyme-mediated process involving recombinase, leads to genetic material recombination.
  • Recombination between homologous chromosomes is completed by the end of Pachytene, allowing the chromosomes to be linked at the crossing over sites
  • Diplotene: The synaptonemal complex dissolves, and recombined homologous chromosomes start to separate, except at crossover sites (chiasmata).
  • Diakinesis: Terminalization of chiasmata occurs, chromosomes are fully condensed, and the meiotic spindle assembles.
  • The nucleolus disappears; the nuclear envelope breaks down; represents the transition to metaphase.
  • Metaphase I: Bivalent chromosomes align on the equatorial plate.
  • Microtubules from opposite poles of the spindle attach to the kinetochore.
  • Anaphase I: Homologous chromosomes separate, while sister chromatids remain associated at their centromeres.
  • Telophase I: The nuclear membrane and nucleolus reappear, followed by cytokinesis, resulting in a dyad of cells.

Meiosis II

• Phase between the two meiotic divisions is called interkinesis, is generally short lived and involves no replication of DNA.
• Prophase II is initiated immediately after cytokinesis II, is similar to mitosis and is contrasted by the absence of fully elongated chromosomes in meiosis I.
• Metaphase II - Chromosomes align at the equator and the microtubules attach to the kinetochores of sister chromatids.
• Anaphase II - Simultaneous splitting of the centromere of each chromosome that was holding together the sister chromatids happens and they move to opposite poles.
• Telophase II - Meiosis ends where two groups of chromosomes are enclosed by a nuclear envelope; cytoplasm divides forming tetrad of four haploid daughter cells.

Significance of Meiosis

• Meiosis allows specific chromosome numbers to be conserved across generations in sexually reproducing organisms.
• It increases the genetic variability in populations, which is vital for evolution.