LECTURE 4 GENETICS

Definition and Context: Meiosis is a specific type of cell division occurrent only in sexually reproducing organisms.
Rounds of Division: Unlike mitosis, which involves a single round of division, meiosis consists of two successive rounds of cell division.
Genomic Outcome: Meiosis always results in the production of four daughter cells. These cells contain exactly half the number of genome copies as the original parent cell (haploid status).
Clinical Significance: Errors in the meiotic process can result in aneuploidy (an incorrect number of chromosomes). In humans, aneuploidy constitutes the most frequent cause of developmental disabilities.
Primary Academic Goals: * Comprehensive understanding of the key phases specific to meiosis. * Clear differentiation between the genetic and mechanical mechanisms of meiosis versus mitosis.

Initial Preparatory Stages: The steps taken to prepare a cell for meiosis are identical to those in mitosis, occurring during Interphase: * $G1$ Phase: The first gap phase involved in cell growth. * $S$ Phase: The synthesis phase where DNA replication occurs. * $G2$ Phase: The second gap phase for final preparations before division.
Sequence of Events: Interphase is followed immediately by Meiosis $I$ .
Interkinesis: * This is a transitional period that prepares the cell for Meiosis $II$ . * Mechanical Changes: The single spindle apparatus organized during Meiosis $I$ is disassembled. Subsequently, new microtubules are assembled to facilitate Meiosis $II$ . * Lack of DNA Synthesis: Interkinesis notably lacks an $S$ -phase; DNA is not replicated a second time between the two divisions to ensure the resulting gametes are haploid.

Bivalent: * Definition: A pair of two homologous chromosomes. * Timing: Found specifically during Prophase $I$ of Meiosis $I$ . * Composition: Consists of the two homologous chromosomes.
Tetrad: * Definition: A group consisting of four sister chromatids found within the homologous pair. * Origin: Formed via DNA replication during the $S$ -phase of Interphase. * Composition: Consists of four components, specifically the four sister chromatids that belong to a homologous chromosome pair.
Comparison Summary: * Bivalent: Refers to the pairing of the two homologs; occurs during Prophase $I$ . * Tetrad: Refers to the four constituent chromatids resulting from replication; originates in Interphase $S$ -phase.

Prophase $I$ is categorized as the longest phase of meiosis and is the primary stage for homologous recombination.

1. Leptotene: * Duplicated chromosomes begin to condense from diffuse chromatin into long, thin threads. * Chromosomes attach to the inner membrane of the nuclear envelope via their telomeres.
2. Zygotene: * Chromosomal Pairing: Known as the chromosomal pairing stage where chromosomal synapsis occurs. * Synaptonemal Complex: This proteinaceous structure forms to allow for synapsis between homologs. * Zygotene Bouquet: Telomeres gather at the nuclear periphery, pulling the chromosome loops outward into a bouquet shape. * Pairing Promotion: This arrangement reduces the "search space" for homologous chromosomes, facilitating alignment. * Homology Search: Dynamic movement and tethering on the nuclear envelope drive the search for matching partners.
3. Pachytene: * Crossing Over: The actual exchange of genetic material between non-sister chromatids occurs. * Chiasmata Formation: Physical points of contact called chiasmata form where the crossover events have occurred.
4. Diplotene: * Degradation: The synaptonemal complex begins to degrade. * Separation: Homologous chromosomes separate slightly but remain physically bound at the chiasmata.
5. Diakinesis: * Maximum Condensation: Chromosomes reach their peak state of shortness and thickness. * Structural Breakdown: The nucleolus and nuclear envelope disappear. * Centriole Migration: Centrioles move toward the equator of the cell to prepare for spindle attachment.

Metaphase I: * Homologous pairs (centromeres) line up at the equatorial plate. * Terminal Chiasmata: These structures serve to hold non-sister chromatids together as the cell prepares for separation.
Anaphase I: * Homologous Separation: The goal is the separation of homologous chromosomes. * Mechanical Dynamics: Separation involves a balance between "pushing" and "pulling" forces exerted on the microtubules. * Cohesin Preservation: Cohesin molecules remain intact around the centromeres of sister chromatids to keep them together. This is achieved by the protein Shugoshin, which protects cohesin from degradation. * Disjunction: Half of each tetrad (now called a dyad) is randomly pulled to opposite poles. * Nondisjunction: An error state where proper separation is not achieved.
Telophase I: * A nuclear membrane reappears around the dyads at each pole. * The nucleus enters a short interphase period. * Replication Note: Chromosomes do not replicate during this period because they already consist of two sister chromatids.

Meiosis $II$ is the process whereby sister chromatids are separated.

Structural State: Each dyad is composed of one pair of sister chromatids attached by a common centromere.
Prophase II: Preparation for the second alignment.
Metaphase II: Centromeres are positioned at the metaphase plate.
Anaphase II: * Centromeres finally divide. * Sister chromatids are pulled to opposite poles. * Monads: Once separated, the individual members of the homologous chromosomes are referred to as monads.
Telophase II and Cytokinesis: * Results in the formation of four haploid gametes. * These four cells are the final product of a single initial meiotic event.