31. Meiosis. Origin of germ cells.

Fundamental Difference Between Meiosis and Mitosis

• Location of Division Meiosis occurs exclusively in germ cells, which are the precursors of gametes—sperm in males and eggs in females. This process is essential for sexual reproduction. In contrast, mitosis takes place in somatic (body) cells, enabling growth, repair, and asexual reproduction in multicellular organisms.
• Number of Cell Divisions Meiosis is a two-step division process. The original diploid cell undergoes two successive divisions—Meiosis I and Meiosis II—resulting in four daughter cells. Mitosis, however, involves only one division and produces two genetically identical daughter cells.
• Chromosome Number in Parent and Daughter Cells The parent cell in meiosis is always diploid (containing two sets of chromosomes). Meiosis reduces this number by half, producing haploid cells (with one set of chromosomes). Mitosis, on the other hand, preserves the chromosome number; daughter cells retain the same ploidy as the mother cell—either diploid or haploid, depending on the starting cell.
• DNA Content and Genetic Variation In meiosis, only the first division (Meiosis I) is preceded by an S-phase, during which DNA is replicated. Mitosis also includes an S-phase before division. Importantly, meiosis introduces genetic variation through mechanisms like homologous recombination and independent assortment. Mitosis does not change the genetic content—daughter cells are exact copies of the parent.
• Chromosome Behavior and Homolog Pairing During prophase I of meiosis, homologous chromosomes (one from each parent) find each other and align in a process known as synapsis. This step is absent in mitosis, where homologous chromosomes do not pair.
• Crossing-Over Unique to meiosis, crossing-over occurs during prophase I. Segments of DNA are exchanged between non-sister chromatids of homologous chromosomes, creating recombinant chromosomes. This exchange increases genetic diversity in offspring. Mitosis lacks any form of crossing-over.
• Centromere Dynamics In meiosis, centromeres remain intact during the first division (Meiosis I), and only separate during Meiosis II. In mitosis, centromeres split during a single anaphase.
• Genetic Identity of Daughter Cells The outcome of meiosis is genetically unique haploid cells, each containing a different set of DNA sequences due to crossing-over and random segregation. Mitosis, by contrast, produces genetically identical cells.
• DNA Distribution Each daughter cell resulting from meiosis receives only half of the DNA strands present in the original cell. In mitosis, each daughter cell inherits a full, identical set of DNA.