Cell Cycle and Genetics PDF

Cell Cycle Phases: ​ G1 (First Gap Phase): The cell grows, synthesizes proteins, and produces organelles necessary for division. This is the longest phase of the cell cycle. Cells can enter G0 if they do not need to divide. ​ S (Synthesis Phase): DNA replication occurs, doublin...

Cell Cycle Phases: ​ G1 (First Gap Phase): The cell grows, synthesizes proteins, and produces organelles necessary for division. This is the longest phase of the cell cycle. Cells can enter G0 if they do not need to divide. ​ S (Synthesis Phase): DNA replication occurs, doubling the genetic material. Each chromosome consists of two sister chromatids. ​ G2 (Second Gap Phase): The cell continues to grow, synthesizing proteins and checking for DNA errors before entering mitosis. Centrosomes duplicate. ​ M (Mitosis): The cell undergoes division, separating sister chromatids into two identical daughter cells. ​ G0 Phase: A non-dividing, resting state where cells perform normal functions but do not proceed through the cycle (e.g., neurons, muscle cells). Microtubules & Their Role in Mitosis ​ Spindle fibers, composed of microtubules, attach to kinetochores on chromosomes during mitosis and meiosis. ​ Microtubules shorten (depolymerize) to pull chromatids apart in anaphase. ​ They also elongate (polymerize) to push the poles of the cell apart, aiding cytokinesis. Time Spent in Cell Cycle Phases & G0  ​ Most cells spend the longest time in G1. ​ If a cell never enters S, G2, or M, it is in G0, a state of dormancy. ​ Cancer cells often bypass G0 and cycle continuously. Cyclin-CDK Complex & Cell Cycle Checkpoints ​ Cyclins regulate the cell cycle by activating Cyclin-Dependent Kinases (CDKs). ​ G1 Checkpoint (Restriction Point): Checks for DNA damage and cell size. ​ G2 Checkpoint: Ensures DNA replication is error-free before mitosis. ​ M (Spindle) Checkpoint: Ensures all kinetochores are attached to spindle fibers before anaphase begins. ​ If cyclin-CDK is inactive, MPF (Maturation Promoting Factor) is absent, halting mitosis. Cancer & Tumor Formation ​ Oncogenes (e.g., Ras, Myc): Mutated proto-oncogenes increase cell division, leading to uncontrolled growth. ​ Tumor Suppressor Genes (e.g., p53, RB1): Normally inhibit cell division; mutations cause unchecked growth. ​ Mutations in p53: Lead to cells bypassing apoptosis, allowing damaged cells to divide uncontrollably. Homologous Chromosomes & Crossing Over (Chiasmata Formation) ​ Homologous chromosomes pair during Prophase I of Meiosis I and exchange genetic material at chiasmata. ​ Crossing over increases genetic variation by shuffling alleles. Law of Independent Assortment & Meiosis ​ Occurs in Metaphase I of Meiosis I when homologous pairs align randomly. ​ Ensures random inheritance of maternal and paternal chromosomes. ​ Does not apply to linked genes that are close together on the same chromosome. DNA Quantification in Cell Cycle ​ Zygote (2N): One chromosome set from each parent. ​ S Phase (4N): DNA replication doubles chromosome content. ​ Mitosis (2N each): Two identical daughter cells retain full diploid content. ​ Gametes (N): Formed via meiosis; haploid (half the original DNA). Mendelian Genetics & Crosses  ​ Dominant vs. Recessive Alleles: Identified via Punnett squares and phenotypic ratios. ​ Test Cross: Crossing a homozygous recessive with an unknown genotype (e.g., Aa × aa). ​ Monohybrid Cross: One trait, 3:1 ratio in F2 (Aa × Aa → AA, Aa, Aa, aa). ​ Dihybrid Cross: Two traits, 9:3:3:1 ratio (AaBb × AaBb). ​ Epistasis: One gene modifies another (e.g., coat color in labs). Sex-Linked vs. Autosomal Inheritance ​ Autosomal Dominant: Trait appears in every generation. ​ Autosomal Recessive: Skips generations; carriers exist. ​ X-Linked Recessive: Males more affected, inherited from carrier mothers (e.g., hemophilia). ​ Mitochondrial Disorders: Passed only from the mother to all offspring. Chi-Square Test for Genetic Ratios ​ Compares observed vs. expected results in experiments. ​ Formula: Χ² = Σ [(O - E)² / E] ​ Degrees of Freedom (df): Number of categories minus one. ​ If p < 0.05, reject null hypothesis (differences are significant). Meiosis & Gamete Formation in Dihybrid Crosses ​ FOIL Method for AaBb × AaBb: Gametes = AB, Ab, aB, ab. ​ Creates a 16-box Punnett square. ​ Linked genes violate independent assortment (appear together more often). Pedigree Analysis & Inheritance Patterns ​ Autosomal Dominant: Appears in every generation. ​ Autosomal Recessive: Can skip generations. ​ Sex-Linked Traits: More common in males (X-linked recessive). ​ Mitochondrial Inheritance: Passed only from the mother. ​ Homozygous dominant cannot always be confirmed, use A? notation. FRQ Topics & Practice ​ Long FRQ: Explain stages of the cell cycle, checkpoints, and their roles. ​ Short FRQ: Given a dihybrid cross, determine gametes and phenotype ratios. ​ Pedigree Analysis: Identify inheritance type (autosomal, X-linked, mitochondrial). ​ Chi-Square Interpretation: Justify whether results align with Mendelian expectations. Key Terms to Know  ​ Cyclins/CDKs – regulate cell cycle checkpoints. ​ MPF (Maturation Promoting Factor) – allows mitosis progression. ​ p53 Gene – tumor suppressor; mutations lead to cancer. ​ Synapsis – pairing of homologous chromosomes in meiosis I. ​ Nondisjunction – failure of chromosomes to separate (causes Down syndrome). ​ Barr Body – inactivated X chromosome in female cells. ​ Linked Genes – genes located close together, inherited together. Final Tips for AP Exam: ​ Practice FRQs: Justify answers with biological reasoning. ​ Understand Diagrams: Cell cycle phases, meiosis, mitosis, pedigrees. ​ Review Experiments: Fruit fly genetics, Mendelian crosses. ​ Memorize Key Ratios: 3:1 (monohybrid), 9:3:3:1 (dihybrid), and chi-square interpretation.