Mitosis and Meiosis Overview

Cell Reproduction and Differentiation

• In unicellular organisms, cell division is equivalent to reproduction
• In multicellular organisms, cell division is a part of development, with one cell division leading to growth and development

Cell Cycle

• The complete cell cycle in human cells takes 18-24 hours
• Interphase: a long growth period between cell divisions, consisting of:
  • G1 (first gap): 5-6 hours, primary period of cell growth, organelles are produced, and the volume of the cytoplasm increases
  • S (synthesis): 10-12 hours, DNA replication, transcription, and translation occur
  • G2 (second gap): 4-6 hours, preparation for cell division, chromatin condenses into chromosomes

Regulation of Cell Reproduction

• Cell reproduction is regulated through internal surveillance and control mechanisms
• Checkpoints are present at G1, G2, and M phases, where "go ahead" signals are required for the cycle to progress
• Outside influences, such as hormones, growth factors, and the presence of other cells, can modify the cell cycle

Cell Divisions

• Mitosis: generates new diploid cells (2N), with two sets of chromosomes from parent cells, used for asexual reproduction
• Meiosis: generates haploid cells (1N), with one set of chromosomes per cell, used for producing gametes or spores

Mitosis

• Prophase: chromatin condenses into visible chromosomes, sister chromatids are joined at the centromere, and nucleoli disappear
• Prometaphase: nuclear envelope fragments, and more condensed chromosomes form, with kinetochores attached to microtubules
• Metaphase: duplicate chromosomes form a single line at the equator, with centrosomes at opposite poles
• Anaphase: sister chromatids separate, and daughter chromosomes are pulled towards poles
• Telophase: reverse of prophase, with daughter nuclei forming, and nuclear membrane reforming

Meiosis

• Meiosis I: separation of homologous chromosomes, resulting in 2 haploid cells
• Meiosis II: separation of sister chromatids, resulting in 4 haploid cells
• Meiosis I consists of:
  • Prophase I: homologous chromosomes pair, crossing over occurs, and synapsis forms
  • Metaphase I: pairs of homologous chromosomes at the metaphase plate
  • Anaphase I: homologs separate, and sister chromatids remain attached at the centromere
  • Telophase I: homologs separate, and sister chromatids remain attached at the centromere, with cytokinesis occurring simultaneously
• Meiosis II consists of:
  • Prophase II: separation of sister chromatids
  • Metaphase II: sister chromatids separate, with kinetochores attached to microtubules
  • Anaphase II: sister chromatids separate, and daughter chromosomes are pulled towards poles
  • Telophase II: reverse of prophase, with daughter nuclei forming, and nuclear membrane reforming

The Human Life Cycle

• Overview of early embryonic stage:
  • Developmental stages: zygote, blastula, gastrula, embryogenesis, and organogenesis
  • Early differentiation: formation of three primary germ layers (ectoderm, endoderm, and mesoderm)
  • Later differentiation: formation of specific tissues and organs from the germ layers
  • Environmental factors influence cell differentiation
  • Application: cloning, cancer cells, and reproductive systems