AP Bio Unit 5 Study Guide

AP Bio Unit 5 Study Guide Chapter 13 - Meiosis What is meiosis? Meiosis is the specialized process by which gametes (sex cells) are produced. It starts with a diploid (2n) cell, meaning the cell contains two sets of chromosomes (one from each parent), and results in haploid (n) cells, which contain only one set of chromosomes. These haploid cells become sperm or egg cells, and during fertilization, two haploid gametes fuse to restore the diploid chromosome number in the zygote. Stages of meiosis Meiosis consists of interphase, meiosis I, and meiosis II. ●​ During interphase, the cell grows (G1), replicates its DNA (S phase), and prepares for division (G2). This ensures each chromosome consists of two sister chromatids before division begins.​ chromatid per chromosome.​ Meiosis I (Reduction Division) - Prophase I: Homologous chromosomes pair up (synapsis), forming tetrads. This is where crossing over (recombination) occurs at chiasmata, exchanging genetic material between non-sister chromatids, creating recombinant chromosomes while others remain parental chromosomes. This is a major source of genetic variation.​ Meiosis produces genetically diverse haploid gametes, which is essential for sexual reproduction and evolution. Nondisjunction Nondisjunction occurs when chromosomes fail to separate properly during Anaphase I or II, leading to gametes with abnormal chromosome numbers (aneuploidy). Examples: -Down Syndrome (Trisomy 21): Extra chromosome 21 due to autosomal nondisjunction.​ -Metaphase I: Homologous chromosome pairs line up at the metaphase plate, and their orientation is random, contributing to independent assortment.​ -Anaphase I: Homologous chromosomes separate, but sister chromatids stay attached.​ -Telophase I + Cytokinesis: Two haploid cells form, each with duplicated chromosomes (still two chromatids).​ Meiosis II (Mitosis-like) -Similar to mitosis, but starting with haploid cells.​ Increasing genetic diversity in offspring Genetic variation in offspring comes from three main mechanisms: - Crossing over (Recombination): Exchange of DNA between homologous chromosomes during Prophase I, creating new allele combinations.​ -Independent assortment: Random orientation of homologous pairs during Metaphase I, meaning genes on different chromosomes are inherited independently.​ -Random fertilization: Any sperm can fuse with any egg, dramatically increasing possible genetic combinations.​ Together, these processes ensure that each offspring is genetically unique. Mitosis vs. Meiosis Mitosis produces genetically identical diploid cells used for growth and repair in somatic cells.​ -Klinefelter Syndrome (XXY): Extra sex chromosome, often affecting males.​ -Turner Syndrome (XO): Missing one X chromosome in females.​ These conditions highlight the importance of proper chromosome segregation. Oogenesis vs. Spermatogenesis Spermatogenesis Oogenesis Produces sperm Produces eggs Occurs in testes Occurs in ovary Lifelong process All eggs created before birth -Prophase II → Metaphase II → Anaphase Chapters 46 & 47 - Development II → Telophase II results in four genetically Embryonic Stages/Events unique haploid cells, each with one Fertilization is the fundamental process where haploid gametes (sperm and egg) unite to form a diploid zygote, restoring the full chromosome complement and initiating development. This occurs as a sperm cell penetrates the egg's extracellular matrices—first the corona radiata and then the zona pellucida—to fuse with the egg's plasma membrane, triggering cortical reactions that prevent polyspermy. The newly formed zygote then undergoes cleavage, a series of rapid mitotic divisions without intervening growth phases. This results in a multicellular structure still enclosed within the original zona pellucida. The first stages produce a solid ball of cells known as the morula. As cell division continues, the embryo compacts and differentiates into two distinct cell populations: the inner cell mass (embryoblasts), which will form the organism, and the outer layer (trophoblasts), which will contribute to placental structures. Fluid accumulates, creating a blastocoel cavity, and upon dissolution of the zona pellucida, the embryo is now termed a blastocyst. The blastocyst is characterized by the embryoblasts condensed at one pole, adjacent to the blastocoel and surrounded by trophoblasts. The next critical phase is gastrulation, a dramatic reorganization involving the inward migration of cells from the blastula surface, forming the three primary germ layers: ectoderm, mesoderm, and endoderm. Following gastrulation, the process of neurulation forms the neurula, marked by the thickening of the dorsal mesoderm and its induction of the overlying ectoderm to roll into the neural tube, the precursor to the central nervous system. ●​ Ectoderm (outer layer): Gives rise to the nervous system (including the neural tube and subsequent brain/spinal cord), the skin epidermis, and epithelial linings of the mouth and rectum. ●​ Mesoderm (middle layer): Gives rise to muscle cells, skeletal tissue (bone and cartilage), the circulatory system (heart and blood vessels), and the reproductive organs. Endoderm (inner layer): Gives rise to the epithelial linings of the digestive and respiratory tracts and associated organs like the liver and pancreas. AP Bio Unit 5 Study Guide Differentiation Differentiation occurs when cells become specialized due to cytoplasmic determinants and gene expression differences.​ few related cell types within a specific tissue or lineage. Sources and Applications: Embryonic stem cells are derived from the inner cell mass of a blastocyst. While they hold immense therapeutic potential due to Induction is when one group of cells their pluripotency, their use raises influences another’s development through significant ethical considerations. signaling, guiding proper tissue formation. Adult stem cells are found in various Hox Genes: Master Regulators of tissues (bone marrow, fat, skin). They are Development less potent but can be harvested with fewer Hox genes are a subset of homeotic genes ethical concerns. They are already used in that act as master transcription factors, therapies, such as bone marrow transplants controlling the anterior-posterior (head-tail) to replenish blood cells after chemotherapy body axis and segment identity. They are or disease. characterized by a conserved Induced pluripotent stem cells (iPSCs) are 180-nucleotide DNA sequence called the adult somatic cells that have been homeobox, which encodes a protein genetically reprogrammed back into a pluripotent state, offering a promising avenue for regenerative medicine that bypasses some ethical issues of embryonic stem cells. Challenges: Despite their promise, stem cell therapies face hurdles including the risk of tumor formation (teratomas from undifferentiated pluripotent cells), the domain (the homeodomain) that binds to potential for inappropriate differentiation, DNA. and high costs that limit accessibility. Collinearity: A striking feature of Hox genes is the principle of colinearity: the Chapters 14 & 15 - Heredity order of genes on the chromosome is Vocabulary spatially and temporally correlated with Dominant Allele: An allele that produces their expression pattern along the body its phenotypic effect even when only one axis. Genes at one end of the cluster are expressed in the head region, while those at copy is present in a heterozygous genotype. Recessive Allele: An allele whose the other end are expressed in the phenotypic effect is masked in the presence posterior/tail region. of a dominant allele; its trait is only Conservation & Common Ancestry: Hox expressed in a homozygous genotype. gene clusters are found in a vast array of Genotype: The genetic makeup of an animal species, from flies to humans, and organism, referring to the specific alleles their organization and function are highly present at a given locus. conserved. This molecular similarity Phenotype: The observable physical or provides powerful evidence for common biochemical characteristics of an organism, ancestry, demonstrating that all animals resulting from the interaction of its share a fundamental genetic toolkit for genotype with the environment. building a body plan, even if the final Homozygous: Possessing two identical products are vastly different. alleles for a particular gene (e.g., AA or Stem Cells: Potency and Potential aa). Stem cells are defined by two key Heterozygous: Possessing two different properties: self-renewal (ability to divide alleles for a particular gene (e.g., Aa). and make more stem cells) and potency Patterns of Inheritance (ability to Complete Dominance: A pattern in which differentiate the heterozygote displays the phenotype of into the dominant allele exclusively (e.g., specialized dominant purple flowers). cell types). Incomplete Dominance: A pattern in which Potency exists the heterozygote displays a phenotype that on a spectrum: is an intermediate blend of the two Pluripotent homozygous phenotypes (e.g., red + white cells (e.g., = pink flowers). embryonic stem cells) can give rise to any cell type in Codominance: A pattern in which both alleles in a heterozygote are fully the body (all tissues from ectoderm, expressed, resulting in offspring with a mesoderm, and endoderm). Multipotent cells (e.g., adult stem cells like phenotype that shows both traits hematopoietic stem cells in bone marrow) simultaneously (e.g., red and white striped are more limited, typically giving rise to a flowers, or blood type AB). Human Blood Types Sex-Linked Gene: A gene located on a sex chromosome (usually the X chromosome). Inheritance patterns for X-linked genes differ between males and females, as males (XY) are hemizygous and express whatever allele is on their single X chromosome. Pedigree: A family tree diagram that uses standardized symbols to track the inheritance pattern of a specific trait or phenotype over multiple generations. Carrier: An individual who is heterozygous for a recessive allele and does not show the associated phenotype, but can pass the recessive allele to their offspring. Test Cross: A genetic cross used to determine the unknown genotype of an individual showing a dominant phenotype. The individual is crossed with a homozygous recessive individual, and the phenotypes of the offspring reveal the unknown genotype. Linked Genes: Genes located close together on the same chromosome that tend to be inherited together as a unit because they are less likely to be separated by crossing over during meiosis. Linkage Map: A genetic map that shows the relative order and linear distances along a chromosome. The distances are based on recombination frequencies—the higher the frequency of recombination between two genes, the farther apart they are assumed to be. Chi-Square (χ²) Analysis: A statistical test used in genetics to determine if the observed results of a cross differ significantly from the expected results based on a genetic hypothesis (e.g., a 9:3:3:1 ratio). The formula is χ² = Σ (observed – expected)² / expected. Null Hypothesis: The hypothesis that there is no real difference between the observed and expected data, and that any apparent variation is due purely to chance. A high chi-square value leading to a low p-value (typically <0.05) allows researchers to reject the null hypothesis, suggesting a factor other than chance is at play.