6.1 Cell Cycle and Mitosis

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

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• 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