🔬General Biology I Unit 10 – Cell Reproduction

Key Concepts

• Cell reproduction enables growth, development, and repair in multicellular organisms
• Two main types of cell division: mitosis (somatic cells) and meiosis (reproductive cells)
• Cell cycle consists of interphase (G1, S, G2) and mitotic phase (mitosis and cytokinesis)
  • Interphase prepares the cell for division by duplicating organelles and DNA
  • Mitotic phase separates duplicated genetic material and divides the cytoplasm
• Mitosis produces genetically identical daughter cells while meiosis generates genetic diversity
• Checkpoints and regulatory proteins (cyclin and cyclin-dependent kinases) control cell cycle progression
• Errors in cell division can lead to genetic disorders, developmental abnormalities, and cancer
• Understanding cell reproduction has applications in medicine, agriculture, and biotechnology

Cell Cycle Phases

• Interphase is the longest phase of the cell cycle and is divided into three sub-phases
  • G1 (Gap 1): cell grows, synthesizes proteins, and prepares for DNA replication
  • S (Synthesis): DNA replication occurs, ensuring each daughter cell receives a complete set of genetic material
  • G2 (Gap 2): cell continues to grow and prepares for mitosis by synthesizing proteins and organelles
• Mitotic phase consists of mitosis (nuclear division) and cytokinesis (cytoplasmic division)
  • Prophase: chromatin condenses into chromosomes, nuclear envelope breaks down, and spindle fibers form
  • Metaphase: chromosomes align at the cell's equator, attached to spindle fibers at the centromere
  • Anaphase: sister chromatids separate and move towards opposite poles of the cell
  • Telophase: chromosomes decondense, nuclear envelopes reform, and cytokinesis begins
• Cytokinesis differs in animal (cleavage furrow) and plant cells (cell plate formation)

Mitosis vs. Meiosis

• Mitosis produces two genetically identical daughter cells with the same number of chromosomes as the parent cell (diploid)
  • Occurs in somatic cells for growth, repair, and replacement of damaged or worn-out cells
  • Maintains genetic stability by precisely duplicating and distributing genetic material
• Meiosis produces four genetically diverse haploid cells (gametes) with half the number of chromosomes as the parent cell
  • Occurs in reproductive cells (ovaries and testes) to produce eggs and sperm
  • Generates genetic variation through independent assortment and crossing over during prophase I
• Meiosis involves two rounds of cell division (meiosis I and II) with distinct phases
  • Prophase I: homologous chromosomes pair up, exchange genetic material (crossing over), and form tetrads
  • Metaphase I: tetrads align at the cell's equator
  • Anaphase I: homologous chromosomes separate and move towards opposite poles
  • Telophase I and cytokinesis: two haploid daughter cells form
  • Meiosis II is similar to mitosis, separating sister chromatids and resulting in four haploid gametes

Cellular Division Mechanisms

• Centrosomes, composed of centrioles, organize the mitotic spindle and ensure equal distribution of chromosomes
• Spindle fibers, made of microtubules, attach to chromosomes at the centromere and pull them apart during anaphase
• Kinetochores, protein structures on centromeres, serve as attachment points for spindle fibers
• Motor proteins (kinesins and dyneins) generate force to move chromosomes along spindle fibers
• Contractile ring, composed of actin and myosin filaments, forms during cytokinesis to divide the cytoplasm
  • In animal cells, the contractile ring forms a cleavage furrow that pinches the cell in two
  • In plant cells, vesicles containing cell wall material align at the cell's equator to form a cell plate

Regulation and Control

• Cell cycle checkpoints ensure proper progression and prevent errors in cell division
  • G1 checkpoint: assesses cell size, nutrient availability, and DNA integrity before entering S phase
  • G2 checkpoint: verifies DNA replication completion and cell readiness for mitosis
  • Spindle assembly checkpoint (metaphase checkpoint): ensures proper chromosome attachment to spindle fibers
• Cyclin-dependent kinases (CDKs) and cyclins regulate cell cycle progression
  • CDKs are enzymes that phosphorylate target proteins to initiate specific cell cycle events
  • Cyclins are regulatory proteins that bind to and activate CDKs at specific points in the cell cycle
• p53, a tumor suppressor protein, halts the cell cycle or triggers apoptosis in response to DNA damage
• Disruption of cell cycle regulation can lead to uncontrolled cell division and cancer development

Genetic Implications

• Mitosis maintains genetic stability by producing daughter cells with identical genetic material
• Meiosis generates genetic diversity through independent assortment and crossing over
  • Independent assortment: random distribution of homologous chromosomes during meiosis I
  • Crossing over: exchange of genetic material between homologous chromosomes during prophase I
• Errors in cell division can result in chromosomal abnormalities
  • Nondisjunction: failure of chromosomes to separate properly during anaphase, leading to aneuploid cells
  • Aneuploidy: cells with an abnormal number of chromosomes (e.g., trisomy 21 in Down syndrome)
• Mutations in genes involved in cell cycle regulation (proto-oncogenes and tumor suppressor genes) can contribute to cancer development

Real-World Applications

• Understanding cell reproduction is crucial for developing cancer treatments targeting uncontrolled cell division
• Stem cell research relies on knowledge of cell division to harness the potential of pluripotent cells for regenerative medicine
• Agricultural biotechnology utilizes cell reproduction principles to develop genetically modified crops with desirable traits
• Assisted reproductive technologies (in vitro fertilization) manipulate meiosis to help individuals with fertility issues
• Cell cycle research has led to the development of cell synchronization techniques used in various biological studies

Common Misconceptions

• Mitosis and meiosis are not the same processes; mitosis produces genetically identical cells, while meiosis generates genetic diversity
• Chromosomes do not replicate during mitosis; DNA replication occurs during the S phase of interphase
• Sister chromatids are not the same as homologous chromosomes; sister chromatids are identical copies of a single chromosome, while homologous chromosomes are similar but not identical
• Crossing over does not occur during mitosis; it is a unique feature of meiosis that contributes to genetic recombination
• Cell division is not always beneficial; uncontrolled cell division can lead to tumor formation and cancer
• Meiosis does not occur in all cells; it is restricted to reproductive cells (gametes) in sexually reproducing organisms