🐾General Biology II Unit 3 – Cell Division: Mitosis and Meiosis

Key Concepts and Terminology

• Cell cycle consists of interphase and mitotic phase (M phase) which includes mitosis and cytokinesis
• Interphase is the period between cell divisions when the cell grows, replicates its DNA, and prepares for division
  • Divided into G1, S, and G2 phases
• Mitosis is the process of nuclear division that produces two genetically identical daughter cells
  • Divided into prophase, metaphase, anaphase, and telophase
• Meiosis is a specialized cell division that produces four haploid gametes or spores
  • Consists of two divisions: meiosis I and meiosis II
• Cytokinesis is the division of the cytoplasm that follows mitosis or meiosis
• Chromosomes are structures that carry genetic information and are composed of DNA and proteins
• Centromere is the constricted region of a chromosome where sister chromatids are attached
• Spindle fibers are protein structures that attach to chromosomes and help separate them during cell division

The Cell Cycle Overview

• Cell cycle is an ordered series of events that leads to cell growth and division
• Interphase is the longest phase of the cell cycle and is divided into three stages: G1, S, and G2
  • G1 (Gap 1) phase is a period of cell growth and preparation for DNA replication
  • S (Synthesis) phase is when DNA replication occurs, doubling the cell's genetic material
  • G2 (Gap 2) phase is a period of further growth and preparation for mitosis
• Mitotic phase (M phase) is the shortest phase of the cell cycle and includes mitosis and cytokinesis
• Checkpoints throughout the cell cycle ensure proper progression and prevent errors
  • G1 checkpoint ensures the cell is ready for DNA replication
  • G2 checkpoint ensures the cell is ready for mitosis
• Cell cycle is regulated by cyclins and cyclin-dependent kinases (CDKs)
  • Cyclins are proteins that regulate the cell cycle by activating CDKs
  • CDKs are enzymes that phosphorylate other proteins to control cell cycle progression

Mitosis: Stages and Process

• Mitosis is divided into four stages: prophase, metaphase, anaphase, and telophase
• Prophase is the first stage of mitosis characterized by chromatin condensation, nuclear envelope breakdown, and spindle formation
  • Chromosomes become visible as two sister chromatids joined at the centromere
• Metaphase is the second stage of mitosis where chromosomes align at the cell's equatorial plane
  • Spindle fibers attach to the centromeres of each chromosome
• Anaphase is the third stage of mitosis where sister chromatids separate and move towards opposite poles of the cell
  • Spindle fibers shorten, pulling the chromatids apart
• Telophase is the final stage of mitosis where chromosomes decondense, nuclear envelopes reform, and cytokinesis begins
  • Cleavage furrow forms in animal cells, while a cell plate forms in plant cells
• Cytokinesis is the division of the cytoplasm that follows mitosis, resulting in two genetically identical daughter cells

Meiosis: Stages and Process

• Meiosis is a specialized cell division that produces four haploid gametes or spores
• Consists of two divisions: meiosis I and meiosis II, each with four stages (prophase, metaphase, anaphase, and telophase)
• Meiosis I is a reductional division that separates homologous chromosomes, reducing the chromosome number by half
  • Prophase I is the longest stage and includes synapsis (pairing of homologous chromosomes) and crossing over (exchange of genetic material)
  • Metaphase I has homologous pairs aligned at the equatorial plane
  • Anaphase I separates homologous chromosomes, which move to opposite poles
  • Telophase I results in two haploid daughter cells
• Meiosis II is an equational division that separates sister chromatids, similar to mitosis
  • Prophase II, metaphase II, anaphase II, and telophase II are similar to their mitotic counterparts
• Cytokinesis follows each division, resulting in four haploid daughter cells (gametes or spores)

Comparing Mitosis and Meiosis

• Mitosis produces two genetically identical daughter cells, while meiosis produces four genetically diverse haploid cells
• Mitosis maintains the chromosome number, while meiosis reduces it by half
• Mitosis occurs in somatic cells for growth and repair, while meiosis occurs in germ cells for sexual reproduction
• Crossing over and independent assortment during meiosis increase genetic variation, while mitosis preserves genetic uniformity
• Mitosis has one division, while meiosis has two divisions (meiosis I and meiosis II)
• Mitosis results in diploid daughter cells, while meiosis results in haploid gametes or spores
• Mitosis is shorter in duration compared to meiosis, which is a longer and more complex process

Regulation and Control of Cell Division

• Cell cycle is tightly regulated to ensure proper cell division and prevent uncontrolled growth
• Checkpoints monitor the cell cycle and halt progression if conditions are not met
  • G1 checkpoint ensures the cell is ready for DNA replication and checks for DNA damage
  • G2 checkpoint ensures the cell is ready for mitosis and checks for DNA replication completion
  • Spindle assembly checkpoint (SAC) ensures proper chromosome attachment to spindle fibers during metaphase
• Cyclins and cyclin-dependent kinases (CDKs) are key regulators of the cell cycle
  • Cyclins accumulate and degrade at specific points in the cell cycle, activating CDKs
  • CDKs phosphorylate target proteins to promote cell cycle progression
• Tumor suppressor genes (e.g., p53) and proto-oncogenes (e.g., Ras) play crucial roles in regulating cell division
  • Mutations in these genes can lead to uncontrolled cell growth and cancer
• External factors such as growth factors, cell density, and nutrient availability also influence cell division

Genetic Implications and Variations

• Mitosis maintains genetic stability by producing identical daughter cells, while meiosis generates genetic diversity through crossing over and independent assortment
• Crossing over during prophase I of meiosis allows for the exchange of genetic material between homologous chromosomes
  • Results in new combinations of alleles on the chromosomes
• Independent assortment during metaphase I and II of meiosis leads to random distribution of chromosomes to daughter cells
  • Contributes to the genetic diversity of gametes
• Nondisjunction is an error in chromosome segregation during meiosis or mitosis, leading to aneuploidy (abnormal chromosome number)
  • Down syndrome (trisomy 21) and Turner syndrome (monosomy X) are examples of aneuploidies
• Polyploidy is the presence of more than two sets of chromosomes in an organism
  • Can occur naturally (e.g., in some plants) or through errors in cell division
• Variations in chromosome structure, such as translocations, inversions, and deletions, can also arise during cell division and impact genetic inheritance

Real-World Applications and Disorders

• Understanding cell division is crucial for the development of cancer treatments, as cancer is characterized by uncontrolled cell growth and division
  • Chemotherapy drugs often target rapidly dividing cells to halt tumor growth
• Stem cell research relies on the principles of cell division to develop therapies for regenerative medicine
  • Stem cells can self-renew and differentiate into various cell types, making them valuable for tissue repair and regeneration
• Reproductive technologies, such as in vitro fertilization (IVF), depend on the understanding of meiosis and gamete formation
  • Manipulating the meiotic process can help overcome fertility issues
• Chromosomal disorders, such as Down syndrome and Turner syndrome, arise from errors in cell division (nondisjunction)
  • Prenatal screening and genetic counseling can help identify and manage these conditions
• Plant breeding and crop improvement utilize the principles of meiosis and genetic recombination to develop new varieties with desirable traits
  • Crossing over and independent assortment are exploited to create novel gene combinations in hybrid plants
• Cell cycle research has implications for understanding the aging process and age-related diseases
  • Cellular senescence, a state of permanent cell cycle arrest, is associated with aging and age-related pathologies