6.3 Comparison of Mitosis and Meiosis

Cell division is crucial for life. Mitosis and meiosis are two types of cell division with distinct purposes and outcomes. While mitosis produces identical cells for growth and repair, meiosis creates diverse gametes for reproduction.

Understanding the differences between mitosis and meiosis is key to grasping how organisms grow, heal, and reproduce. These processes shape genetic inheritance and diversity, influencing evolution and adaptation in living things.

Cell Division Outcomes

Chromosome Number Changes

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• Mitosis maintains the original chromosome number in the daughter cells, resulting in diploid cells (2n)
• Meiosis reduces the chromosome number by half, producing haploid cells (n)
  • This is necessary for sexual reproduction to maintain the species' chromosome number across generations
  • Haploid gametes (sperm and egg cells) fuse during fertilization to restore the diploid chromosome number (2n)

Genetic Diversity Differences

• Mitosis produces genetically identical daughter cells to the parent cell (clones)
  • Ensures genetic stability and consistent function in somatic cells (body cells)
• Meiosis generates genetic diversity among the resulting haploid cells
  • Crossing over during prophase I shuffles genetic material between homologous chromosomes
  • Independent assortment during metaphase I randomly distributes maternal and paternal chromosomes
  • Random fertilization of gametes further increases genetic variation in offspring

Daughter Cell Characteristics

• Mitosis produces two genetically identical diploid daughter cells
  • Each daughter cell is a clone of the parent cell (barring mutations)
  • Daughter cells have the same number and type of chromosomes as the parent cell
• Meiosis produces four genetically distinct haploid daughter cells
  • Each daughter cell contains half the number of chromosomes as the parent cell
  • Daughter cells are not genetically identical to each other or the parent cell due to genetic recombination

Cell Division Process

Number of Cell Divisions

• Mitosis involves a single cell division
  • Interphase (G1, S, G2) followed by mitotic phase (PMAT) results in two daughter cells
• Meiosis consists of two successive cell divisions (meiosis I and meiosis II)
  • Interphase (G1, S, G2) followed by meiosis I (PMAT) and meiosis II (PMAT) produces four daughter cells

Crossing Over Events

• Crossing over occurs during prophase I of meiosis
  • Homologous chromosomes pair up and form synapses
  • Non-sister chromatids exchange genetic material at chiasmata, creating new allele combinations
• Crossing over does not occur during mitosis
  • Sister chromatids remain intact and do not exchange genetic material

Homologous Chromosome Pairing

• Homologous chromosomes pair up during prophase I of meiosis (synapsis)
  • One maternal and one paternal chromosome of the same type align closely
  • Pairing allows for crossing over and proper segregation of homologs
• Homologous pairing does not occur in mitosis
  • Chromosomes align independently at the metaphase plate
  • Sister chromatids separate during anaphase, but homologs do not pair

Cell Division Function

Growth and Repair through Mitosis

• Mitosis is used for growth, development, and repair of tissues
  • Generates new cells to increase tissue size during growth (embryonic development)
  • Replaces damaged or lost cells to maintain tissue function (wound healing, skin regeneration)
• Mitosis maintains genetic stability by producing identical daughter cells
  • Ensures consistent cellular function within tissues and organs

Reproduction through Meiosis

• Meiosis is essential for sexual reproduction
  • Produces haploid gametes (sperm and egg cells) for fertilization
  • Enables the formation of genetically diverse offspring
• Meiosis introduces genetic variation through crossing over and random assortment
  • Contributes to adaptability and evolution of species
  • Allows for new combinations of traits in each generation