6.Cell Division – mitosis, meiosis; Cell cycle and its regulation

Cell Division – mitosis, meiosis; Cell cycle and its regulation. Apoptosis

Mitosis:

• Definition:
  • Mitosis is a fundamental process in which a parent cell divides, resulting in two daughter cells. Each daughter cell receives an identical set of chromosomes, ensuring genetic continuity.
  • The period between mitotic divisions is called interphase, during which DNA replication occurs, and the nucleus appears as it does in most histological preparations.
• Phases of Mitosis:

1.               Prophase:

• 1. • Replicated chromatin condenses into rod-shaped structures known as chromosomes. Each chromosome consists of duplicate sister chromatids.
     • Outside the nucleus, centrosomes (containing centrioles) separate and migrate to opposite poles of the cell.
     • The mitotic spindle microtubules appear between the centrosomes, and the nucleolus disappears.
     • Late in prophase, the nuclear envelope breaks down.1.               Metaphase:
       • 1. • Condensed chromosomes attach to microtubules of the mitotic spindle at the kinetochore.
            • The kinetochore organizes the chromosomes in the equatorial plate (center of the cell), allowing for proper alignment.
            • Later, sister chromatids are pulled apart toward opposite poles of the mitotic spindle. 2.               Anaphase:
            • Sister chromatids separate from each other and are gradually pulled toward opposite spindle poles by kinesin motors along the microtubules. 3.               Telophase:
            • The two sets of chromosomes reach the spindle poles and begin reverting to their non-condensed state.
            • Microtubules of the spindle depolymerize, and the nuclear envelope starts reassembling around each set of daughter chromosomes.
            • During cytokinesis, a contractile ring containing actin filaments develops in the peripheral cytoplasm, dividing the cytoplasm and its organelles into two daughter cells, each with one nucleus.
              
              Meiosis:
       • Definition:
         • Meiosis is a specialized cell division process that occurs during the formation of sperm and egg cells (gametes).
         • The resulting cells are haploid (n, 23 chromosomes), containing only one chromosome from each homologous pair.
         • Union of haploid egg and sperm cells during fertilization forms a new diploid zygote, which can develop into a new individual.
       • Prophase I:
         • During an elongated prophase, chromatin condenses, and homologous chromosomes physically come together in synapsis, forming tetrads.
         • Genetic material mixes from both maternal and paternal genes.
         • Prophase I is extended during male gametogenesis in humans, while oocytes arrest in this phase from fetal ovary formation through reproductive maturity.
       • Metaphase I, Anaphase I, and Telophase I:
         • Chromosomes continue to regular metaphase, anaphase, and telophase.
         • Anaphase I involves the separation of homologous chromosomes that came together during synapsis.
         • Each separated chromosome still contains two chromatids held together at the centromere.
       • Second Division:
         • Each of the two new cells divides again, more rapidly and without DNA replication.
         • Chromatids now separate at the centromere and are pulled to opposite poles as individual chromosomes.
       • Summary:
         • Mitosis produces two diploid cells, maintaining genetic continuity.
         • Meiosis involves two divisions and produces four haploid cells.
         • During meiotic crossing over, new combinations of gene alleles arise, making each haploid cell genetically unique. The intricate dance of cell division ensures the continuity of life.