Chapter 4 - The Eukaryotic Cell Cycle, Mitosis and Meiosis

Eukaryotic Cell Cycle, Mitosis, and Meiosis

• The human body contains between 10 to 50 trillion cells.
• DNA sequences in all chromosomes in all cells are the same, except for rare mutations.
• Cell reproduction ensures genetic material integrity through mitosis and meiosis.
• A cell cycle is a highly regulated series of events that lead to cell division.
• Chromosomes become compact enough to be viewed under a light microscope before cell division.
• Cytogenetics is the field of genetics that involves examining chromosomes microscopically.

Cell Division

• The cell cycle has distinct phases: G1 (first gap), S (synthesis of DNA), G2 (second gap), and M (mitosis & cytokinesis).
• G1: Cell growth and commitment to division, accumulating molecular changes for progression.
• S: Each chromosome replicates, forming sister chromatids.
• G2: Cell synthesizes proteins needed for chromosome sorting and division, and some growth might occur.
• M: Mitosis divides a cell's nucleus into two, distributing duplicated chromosomes, and cytokinesis divides the cytoplasm into two daughter cells.
• A cell may also exit the cell cycle and enter a nondividing phase G0.

Cell Cycle Length

• Cell cycle length varies among cell types. Embryos might have cell cycles measured in minutes, while adult cells can take several months to divide.
• Fast dividing mammalian cells (like skin cells) have cell cycles between 10 to 24 hours.
• Specific phases (G1, S, G2, and M) have specific durations within a 24 hour cell cycle.

Control of the Cell Cycle

• Cyclin/CDK protein complexes regulate cell cycle progression.
• Cyclins fluctuate in levels during the cell cycle.
• CDKs (cyclin-dependent kinases) are enzymes that activate other proteins—becoming active during specific phases.

Cell Cycle Checkpoints

• Checkpoint proteins control cell cycle progression to ensure proper conditions for division.
• G1 checkpoint (restriction point) checks for favorable conditions and DNA damage.
• G2 checkpoint checks for DNA damage and completeness of replication.
• Metaphase checkpoint ensures proper spindle apparatus connection to chromosomes.

Karyotype

• Karyotype: A photographic representation of chromosomes.
• Diagrams arranged according to chromosome size and banding patterns to enable identification of abnormalities or genetic defects.
• For a diploid human cell, two sets of chromosomes from one cell constitute a karyotype of that cell.
• Human cells are diploid, containing 46 chromosomes. 2 complete sets.
• Gametes (sperm and eggs) are haploid, with one set of chromosomes= 23

Homologs

• In diploid species, members of a chromosome pair are called homologs or homologous chromosomes.
• Homologous pairs are virtually identical in size and genetic composition, but have slight sequence differences responsible for genetic variation.
• Most homologs differ by less than 1%.
• Sex chromosomes (X & Y) differ significantly in size and genetic content.

Mitosis

• Mitosis produces two genetically identical daughter cells.
• Mitosis involves the division of one nucleus into two nuclei and is followed by cytokinesis (division of the cell into two).
• Asexual reproduction frequently uses mitosis in single-celled organisms. Mitosis is essential for development of multicelluar organisms.

Mitosis Stages

• Interphase—chromosome replication
• Prophase—chromatid condensation, nuclear envelope fragmentation.
• Prometaphase—spindle apparatus formation, attaching kinetochore microtubules to sister chromatids.
• Metaphase—chromosomes align at the metaphase plate.
• Anaphase—sister chromatids separate, with one going to each pole.
• Telophase—chromosomes decondense, nuclear envelopes reform.
• Cytokinesis—division of the cytoplasm.

Cytokinesis

• Cytokinesis is the division of the cytoplasm to form two daughter cells.
• Cytokinesis differs in animals and plants.
• Animal cells utilize a cleavage furrow.
• Plant cells use a cell plate, that develops from Golgi vesicles and leads to a new cell wall.

Meiosis

• Meiosis produces four genetically unique haploid (n) daughter cells from one diploid (2n) cell.
• Meiosis involves two rounds of cell division.
• Meiosis is the process by which gametes are produced.

Meiosis versus Mitosis

• Meiosis and mitosis involve similar stages, however meiosis' unique stages increase genetic variation.
• Mitosis involves one round of division, produces 2 diploid cells, and DNA replicates before the division.
• Meiosis involves two rounds of division, produces 4 haploid cells, and DNA replicates only before the first division.

Crossing Over

• Crossing over is the exchange of genetic material between homologous chromosomes.
• Crossing over occurs during the prophase I of Meiosis I.

Meiosis I Stages

• Prophase I: Homologous chromosomes pair up and exchange genetic material(crossing-over).
• Prometaphase I: The nuclear envelope breaks down, and spindle fibers attach to chromosomes.
• Metaphase I: Homologous pairs are aligned at the metaphase plate, in a double row.
• Anaphase I: Homologous chromosomes separate and move toward opposite poles. Sister chromatids remain attached.
• Telophase I: Chromosomes arrive in separate poles; nuclear membranes are reformed. Cytokinesis follows, resulting in 2 cells—each with one set of homologous chromosomes.

Meiosis II Stages

• Prophase II: Chromosomes condense. Nuclear envelope begins to dissociate.
• Prometaphase II: Spindle apparatus forms, and kinetochores attach to sister chromatids.
• Metaphase II: Sister chromatids are aligned at the metaphase plate.
• Anaphase II: Sister chromatids separate, and each separated chromatid can now be considered a chromosome.
• Telophase II/ Cytokinesis: Chromosomes arrive at opposite poles; nuclear membranes form. Cytokinesis separates the two cells into four new haploid cells.

Chromosome Theory of Inheritance

• Meiosis explains Mendel's laws of inheritance.
• The chromosome theory of inheritance states that genes are located on chromosomes which are passed down from parent to offspring.
• Homologous chromosomes segregation occurs during meiosis.
• The pairing and segregating of homologs explain Mendelian traits.

Sexual Reproduction

• Sexual reproduction combines genetic material from two parents to create a diploid zygote.
• After fertilization, mitosis produces numerous diploid cells, resulting in a multicelluar organism.
• Sexual reproduction offers more genetic diversity.
• Alternation in generations occurs in species where the life cycle alternates between a haploid cell or organism stage and a diploid cell or organism stage.