Biology 2020: Chapter 10 Cell Reproduction Flashcards

Fundamentals of Cell Division and Reproduction

• Definition of Cell Division: The foundational biological process by which a single parent cell divides to form new cells.

• Primary Types of Cell Division:     * Asexual Reproduction:         * Results in the creation of $2$ genetically identical cells.         * The offspring cells are clones of the original parent cell.     * Sexual Reproduction:         * Results in the creation of $4$ cells.         * These cells contain $\frac{1}{2}$ of the genetic material required to form a complete organism.         * This process leads to genetically unique offspring.

The Eukaryotic Cell Cycle and Interphase

• The Cell Cycle: Defined as an ordered series of events that occur in the life of a cell, ultimately leading to cell division.

• Interphase: A period of preparation and growth that occurs in three distinct stages where the cell is biochemically active even if external changes appear minimal.     * $G_1$ Phase (First Gap):         * Primary focus is cell growth.         * While physical change is not immediately evident (hence the label "gap"), the cell is highly active at the biochemical level.     * S Phase (DNA Synthesis):         * The period during which DNA replication occurs.         * Identical copies of DNA molecules, known as sister chromatids, are produced and joined together at the centromere.         * Centrosomes produce the mitotic spindles necessary for moving chromosomes.         * In animal cells, centrosomes are associated with centrioles, which assist in organizing the division process.     * $G_2$ Phase (Second Gap):         * A secondary period of growth.         * Energy reserves are replenished.         * Organelles reproduce or replicate.         * The cytoskeleton is broken down to provide components for the mitotic phase.

Structural Organization of Eukaryotic Chromosomes

• Chromosomes: Structures composed of a single DNA molecule paired with associated proteins.

• Chromatin: The condensed form of a chromosome found specifically in eukaryotic organisms. It is composed of DNA and proteins.

• Mechanisms of DNA Compaction:     * Histones: Specific proteins that group together to act as a scaffold or spool to help compact DNA.     * Nucleosome: The structure formed when DNA is wrapped around a group of histone proteins.     * Higher-Level Compaction: Loops of nucleosomes compact further to form chromatin.

• Functional States of Chromatin:     * Euchromatin:         * Characterized as more loosely compacted chromatin.         * Represents the "active" portion of the genome that is more likely to be transcribed into RNA.     * Heterochromatin:         * Characterized as highly compacted chromatin.         * Represents "inactive" regions of the genome that are typically not transcribed.

• Inheritance of Chromosomes:     * Chromosomes are inherited in sets.     * Diploid ($2n$): Cells that contain two complete copies of the entire genome.     * Homologues: The name given to two different versions of the same chromosome within a diploid cell.     * Homologous Chromosomes:         * These are very similar to one another and carry the same types of genetic information.         * In animals, one homologue is inherited from the biological mother and one from the biological father.

• Preparation for Division:     * Each chromosome is replicated and compacted prior to the start of division.     * Sister Chromatids: Refers to the two identical copies of a single homologue within a cell after replication but prior to division.

The Mitotic (M) Phase: Karyokinesis and Cytokinesis

• M Phase Overview: The phase of the cell cycle where the nucleus divides and the cytoplasm splits.

• Karyokinesis (Mitosis / Nuclear Division): The process of producing two genetically identical nuclei. It involves five defined stages:     1. Prophase:         * The nuclear envelope begins to break down.         * Organelles migrate toward the edges of the cell.         * The microtubule spindles and centrosomes form.         * Sister chromatids coil more tightly (DNA compaction).     2. Prometaphase:         * Sister chromatids develop specialized kinetochore proteins in their centromere regions.         * Kinetochores serve as the attachment point between chromatids and the spindle fibers.         * Centrosomes migrate to opposite poles of the cell.     3. Metaphase:         * Chromosomes align along the "metaphase plate," an imaginary plane in the center of the cell.     4. Anaphase:         * Sister chromatids separate from each other.         * The individual chromatids move in opposite directions toward the poles.         * The cell begins to elongate.     5. Telophase:         * Chromosomes reach the opposite poles and begin to decondense.         * The nuclear envelope reforms around each set of chromosomes.         * The mitotic spindles and centrosomes disappear.

• Cytokinesis: The second portion of the Mitotic phase involving physical separation.     * The cytoplasmic components physically separate into $2$ daughter cells.     * This differs between plant and animal cells (e.g., animal cells utilize a cleavage furrow).     * Results in two individual cells, each containing one nucleus.     * Each new daughter cell is genetically identical to the parent cell.

Cell Cycle Checkpoints and Control

• Purpose: To ensure the cell is prepared for division and to prevent the propagation of errors.

• $G_1$ Checkpoint:     * Checks for genomic DNA damage.     * Assesses if there are sufficient energy reserves and if the cell size is adequate to proceed to the S phase.

• $G_2$ Checkpoint:     * Blocks entry into the M phase if conditions are not met.     * Checks that all chromosomes have been fully replicated.     * Screens for DNA damage after the S phase.     * Ensures protein and energy reserves are sufficient.

• M Checkpoint (Spindle Checkpoint):     * Occurs near the end of metaphase.     * Determines if all sister chromatids are correctly attached to the spindle microtubules.

• Failure of Checkpoints: If a cell fails these checks, it will either attempt to fix the detected problems or halt the process entirely, leading to stalled division.

Sexual Reproduction and Meiosis

• Overview of Sexual Reproduction: Occurs when two cells (gametes) come together to create a genetically unique organism.

• Ploidy Levels:     * Diploid ($2n$): A cell containing two versions of the entire genome (e.g., somatic cells).     * Haploid ($1n$): A cell containing only one version of its genome.

• Gametes: Specialized haploid ($1n$) cells required for sexual reproduction (e.g., Egg and Sperm).

• Fertilization: The process of two haploid gametes ($1n$ + $1n$) joining together.     * This union results in a diploid ($2n$) product known as a Zygote.

• Meiosis (Gamete Production):     * The process of producing haploid cells from a diploid parent cell.     * Consists of two phases of the meiotic cycle.     * Functions of Meiosis:         1. Produce haploid cells ($1n$).         2. Separate homologous chromosomes and sister chromatids.         3. Increase genetic diversity through specific events such as crossover and independent assortment.