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Honors Biology
Table of Contents
Unit 6 Overview: Cell Division
6.1 Cell Cycle and Mitosis
6.2 Meiosis and Sexual Reproduction
6.3 Comparison of Mitosis and Meiosis

Cell division is the heart of growth and reproduction. The cell cycle, featuring interphase and mitosis, ensures genetic material is copied and divided accurately. This process is crucial for organisms to develop, repair tissues, and create new life.

Mitosis, a key part of cell division, involves several phases: prophase, metaphase, anaphase, and telophase. Each step plays a vital role in separating chromosomes and creating two identical daughter cells. Understanding mitosis helps explain how organisms grow and maintain themselves.

Cell Cycle Phases

Interphase and Preparation for Cell Division

  • Cell cycle consists of interphase and mitotic phase (mitosis and cytokinesis) which a cell goes through as it grows and divides
  • Interphase is the longest part of the cell cycle when the cell grows and prepares for cell division
    • G1 phase is the first gap phase where the cell increases in size and synthesizes proteins and organelles
    • S phase is the synthesis phase where the cell replicates its DNA
    • G2 phase is the second gap phase where the cell continues to grow and prepares for mitosis

Mitosis and Cytokinesis Stages

  • Mitosis is the process of nuclear division which produces two genetically identical daughter nuclei
    • Involves the separation of duplicated chromosomes into two separate nuclei
  • Cytokinesis is the division of the cytoplasm to form two separate daughter cells
    • Occurs concurrently with mitosis and varies between plant and animal cells (cell plate forms in plant cells vs. cleavage furrow forms in animal cells)

Mitotic Phases

Early Mitotic Phases

  • Prophase is the first phase of mitosis where chromatin condenses into chromosomes, nuclear envelope breaks down, and spindle fibers begin to form
    • Chromosomes become visible and centrioles move to opposite poles of the cell
  • Metaphase is the second phase where chromosomes line up along the metaphase plate at the center of the cell
    • Spindle fibers attach to the centromeres of the chromosomes

Later Mitotic Phases

  • Anaphase is the third phase where sister chromatids separate and move to opposite ends of the cell
    • Centromeres divide and spindle fibers shorten, pulling chromatids to cell poles
  • Telophase is the final phase where chromosomes arrive at opposite poles and begin to decondense
    • Nuclear envelope re-forms around each set of chromosomes and nucleoli reappear
    • Spindle fibers disperse and cytokinesis begins

Chromosomal Structures

Chromatin and Chromosomes

  • Chromatin is the complex of DNA and proteins that makes up chromosomes
    • Exists in relaxed form during interphase and condenses during prophase of mitosis
  • Chromosome is a condensed structure containing a single DNA molecule and associated proteins
    • Duplicated chromosomes are held together at the centromere and have an X shape

Centromeres and Chromatids

  • Centromere is the constricted region of a duplicated chromosome where the two sister chromatids are held together
    • Point of attachment for spindle fibers during mitosis
  • Sister chromatids are the identical copies of a chromosome formed by DNA replication, held together by the centromere
    • Separate during anaphase to become individual chromosomes in the daughter cells

Mitotic Apparatus

Spindle Fibers

  • Spindle fibers are protein structures that form during mitosis
    • Attach to the centromeres of chromosomes and help to separate sister chromatids
    • Consist of microtubules that originate from the centrioles at the poles of the cell
  • Metaphase plate is the imaginary line at the center of the cell along which chromosomes line up during metaphase
    • Ensures equal distribution of chromosomes to each daughter cell

Centrioles

  • Centrioles are cylindrical cell structures that organize the development of spindle fibers during cell division
    • Composed of microtubules arranged in a 9+0 pattern
  • Centrosomes are organelles that serve as the main microtubule organizing center
    • Each centrosome contains two centrioles positioned at right angles to each other
    • Migrate to opposite poles during prophase and organize the spindle fibers

Key Terms to Review (22)

Cyclins: Cyclins are a group of proteins that regulate the cell cycle by activating cyclin-dependent kinases (CDKs), which are essential for the progression through different phases of the cell cycle. These proteins ensure that the cell only moves on to the next stage when it is appropriate, coordinating cell growth, DNA replication, and division. Cyclins are produced and degraded in a cyclical manner, reflecting their crucial role in the timing of cellular events.
Cdks: Cyclin-dependent kinases (cdks) are a family of protein kinases that play a crucial role in regulating the cell cycle by controlling the progression through different phases. These enzymes need to bind to a specific regulatory protein, known as cyclin, to become active and trigger various processes essential for cell division and growth. By phosphorylating target proteins, cdks help ensure that the cell cycle proceeds in a coordinated manner, preventing errors that could lead to uncontrolled cell proliferation.
Checkpoints: Checkpoints are regulatory mechanisms in the cell cycle that ensure proper progression through the various phases of cell division. They act as surveillance systems to monitor the integrity of DNA, the completion of DNA replication, and proper spindle assembly before allowing the cell to proceed to the next stage. By controlling the flow of the cell cycle, checkpoints help prevent errors that could lead to issues such as uncontrolled cell growth or tumor formation.
Somatic cells: Somatic cells are any cells in the body that are not germ cells, meaning they are not involved in reproduction. These cells make up most of the body's tissues and organs and contain a full set of chromosomes, which carry the genetic information necessary for the growth and function of an organism. Understanding somatic cells is crucial as they play a key role in processes like growth, repair, and maintenance within the body, which is essential for overall health.
Sister chromatids: Sister chromatids are identical copies of a single chromosome, connected by a region called the centromere. They form during the S phase of the cell cycle, when DNA is replicated, ensuring that each new cell will receive an exact copy of the genetic material during cell division. Their existence is crucial for the accurate segregation of chromosomes during mitosis and meiosis.
Spindle fibers: Spindle fibers are specialized protein structures that form during cell division, specifically in mitosis and meiosis, to help segregate chromosomes into daughter cells. These fibers are essential for ensuring that each new cell receives the correct number of chromosomes, thus maintaining genetic stability. They are part of the spindle apparatus, which also includes the centrosomes and the metaphase plate, and work by attaching to the centromeres of chromosomes to facilitate their movement.
Germ cells: Germ cells are specialized reproductive cells that give rise to gametes, which are sperm and eggs in animals. They are crucial for sexual reproduction, as they undergo meiosis to produce haploid gametes that combine during fertilization to form a diploid zygote, thus ensuring genetic diversity.
Apoptosis: Apoptosis is a programmed cell death process that enables cells to self-destruct in a controlled manner, which is crucial for maintaining healthy tissue homeostasis and development. This process is essential for the removal of unnecessary, damaged, or potentially harmful cells, thereby preventing the development of diseases like cancer. By orchestrating cell death, apoptosis contributes to various biological processes, including embryonic development, immune system regulation, and tissue remodeling.
Centromere: A centromere is the region of a chromosome that joins two sister chromatids and plays a crucial role during cell division by ensuring proper segregation of chromosomes. It serves as the attachment point for spindle fibers during mitosis and meiosis, facilitating the movement of chromatids to opposite poles of the cell. This structure is vital for maintaining genetic stability across generations of cells.
Helicase: Helicase is an enzyme that plays a critical role in DNA replication by unwinding the double-stranded DNA helix into two single strands. This process is essential for allowing the replication machinery to access the genetic information contained in the DNA. Helicase initiates the separation of the strands by breaking the hydrogen bonds between base pairs, enabling other enzymes to synthesize new complementary strands during cell division.
Chromatid: A chromatid is one half of a duplicated chromosome, formed during the cell division process. When a cell prepares to divide, its chromosomes replicate and each chromosome consists of two sister chromatids held together at a region called the centromere. These sister chromatids separate during mitosis, ensuring that each new daughter cell receives an identical set of chromosomes, which is crucial for maintaining genetic stability.
Homologous chromosomes: Homologous chromosomes are pairs of chromosomes in a diploid organism that contain the same genes, but may have different alleles, or variations of those genes. These chromosomes are crucial during processes such as cell division, where they ensure genetic diversity and proper segregation during meiosis and mitosis. Their alignment and exchange of genetic material is essential for the accurate distribution of genetic information to daughter cells.
Cytokinesis: Cytokinesis is the final stage of cell division, where the cytoplasm of a parent cell is divided into two daughter cells. This process occurs after mitosis or meiosis, ensuring that each new cell contains the necessary organelles and cytoplasmic components for survival and function. Cytokinesis is essential for maintaining cellular organization and contributing to tissue growth and repair.
Anaphase: Anaphase is a crucial stage in the cell division process where the sister chromatids of each chromosome are pulled apart towards opposite poles of the cell. This separation ensures that each new daughter cell receives an identical set of chromosomes. Anaphase is part of mitosis and meiosis, playing a critical role in maintaining genetic stability and diversity in cellular reproduction.
Prophase: Prophase is the first stage of mitosis and meiosis, where chromatin condenses into visible chromosomes and the mitotic spindle begins to form. During this phase, the nuclear envelope starts to break down, allowing the spindle fibers to interact with the chromosomes. Prophase sets the stage for the orderly separation of sister chromatids or homologous chromosomes, essential for accurate cell division.
DNA Replication: DNA replication is the biological process by which a cell makes an identical copy of its DNA before cell division. This process is crucial for ensuring that each daughter cell receives an exact copy of the genetic material, enabling proper functioning and growth. DNA replication occurs during the S phase of the cell cycle and involves several key enzymes and proteins that work together to unwind the double helix and synthesize new strands of DNA.
Interphase: Interphase is the longest phase of the cell cycle, during which a cell prepares for division by growing, replicating its DNA, and performing its normal functions. This phase is crucial as it encompasses three distinct subphases—G1, S, and G2—that allow the cell to grow, synthesize DNA, and ensure that all cellular components are ready for mitosis. Interphase sets the stage for successful cell division by ensuring the cell has the necessary resources and genetic material.
Mitotic phase: The mitotic phase is the stage of the cell cycle where a cell divides its copied DNA and cytoplasm to form two new daughter cells. This phase consists of several key processes, including prophase, metaphase, anaphase, and telophase, each playing a crucial role in ensuring that genetic material is accurately distributed to the daughter cells.
Metaphase: Metaphase is a critical stage in both mitosis and meiosis where chromosomes align at the cell's equatorial plane, known as the metaphase plate. This alignment ensures that each daughter cell will receive an identical set of chromosomes during cell division. Metaphase plays a vital role in the overall accuracy of chromosome segregation, minimizing the risk of genetic abnormalities.
Telophase: Telophase is the final stage of mitosis, where the chromosomes reach the opposite poles of the cell and begin to de-condense back into chromatin. During this phase, nuclear envelopes reform around each set of chromosomes, resulting in two distinct nuclei within the cell. Telophase plays a crucial role in ensuring that each daughter cell receives an identical set of genetic material as the cell prepares for cytokinesis.
Tumorigenesis: Tumorigenesis is the process by which normal cells transform into cancerous cells, leading to the formation of tumors. This transformation can occur due to a combination of genetic mutations, environmental factors, and disruptions in cellular regulation, which collectively drive uncontrolled cell division and growth. Understanding tumorigenesis is crucial because it links directly to the mechanisms of the cell cycle and mitosis, highlighting how errors in these processes can result in malignancy.
Cancer: Cancer is a group of diseases characterized by uncontrolled cell growth and division, leading to the formation of tumors that can invade surrounding tissues and spread to other parts of the body. This uncontrolled proliferation occurs when the regulatory mechanisms of the cell cycle, particularly during mitosis, fail due to genetic mutations or external factors. Cancer can disrupt normal cellular functions and has the potential to affect overall health severely.