Cell Cycle - High School Biology

Cell Cycle

Background Information: Cell Division

  • Cell division is part of the cell cycle.
  • It's a process where one parent cell divides into two or more daughter cells.
  • There are two types of cell division:
    • Mitosis: Division of the nucleus to produce somatic cells.
    • Meiosis: Division of the nucleus to produce gametes.
  • Reasons cells divide:
    • To create cells needed for growth and development.
    • To create cells to repair damaged cells.
    • To create cells to replace dead or dying cells.
    • To create sex cells for sexual reproduction.
    • To create offspring during asexual reproduction.

Background Information: Important Terms

  • Chromatin: Thin strands of DNA in the nucleus before cell division, when the cell is growing, developing, and carrying out daily processes.
  • Histones: Proteins in the nucleus with chromatin; DNA wraps around them when condensing into chromosomes.
  • Chromosome: Formed when DNA wraps around histones just before cell division.
    • Can have 1 chromatid or 2 chromatids.
    • 2 chromatids are called sister chromatids.
  • Chromatid: Lengthwise side or half of a chromosome.
  • Sister Chromatids: Two identical copies of a chromosome, due to DNA replication, so each daughter cell gets a copy of DNA.
  • Centromere: A constricted region of a chromosome that holds sister chromatids together.
  • Centrosomes: Organelle that contains centrioles and produces spindle fibers.
    • Each centrosome contains 2 centrioles.
    • Doubles during Interphase of the cell cycle.
  • Centrioles: Organelles that form part of the centrosome.
    • Organize microtubules for spindle fibers.
    • Ensure accurate chromosome segregation and distribution to daughter cells.
  • Spindle Fibers: Pull sister chromatids and homologous chromosomes apart during cell division.
    • Protein structures made of microtubules.
    • Attach to the centromere of chromosomes.
  • Human Chromosomes consist of XX (female) or XY (male).

Background Information: Types of Reproduction

Asexual Reproduction
  • Reproduction by which offspring arise from a single parent.
  • Requires only 1 parent.
  • Offspring are genetically identical.
  • Happens through mitosis.
  • Types:
    • Binary Fission: When a parent cell elongates and then splits into 2 identical daughter cells.
    • Fragmentation: When a parent organism breaks into pieces and each piece becomes a new organism.
    • Budding: When a parent cell develops a bud that grows and then breaks away from the parent cell.
Sexual Reproduction
  • Reproduction by which offspring arise from two parents.
  • Requires 2 parents.
  • Offspring are NOT genetically identical.
  • Gametes are produced through meiosis.
  • Fetus develops through mitosis.
  • Fertilization: Process by which the nucleus of a male gamete (sperm cell) fuses with the nucleus of a female gamete (egg/ovum cell) to form a zygote.
  • After fertilization, the zygote continues to divide through mitosis until a fetus is fully developed.

Cell Cycle - Mitosis

Cell Cycle
  • Orderly sequence of events used by biological systems to coordinate cell division.
  • Cells grow, develop, and carry out daily processes.
  • Cells divide to make new cells.
  • 3 Main phases:
    • Interphase
    • M phase
    • Cytokinesis
Interphase
  • Cell may be growing.
  • Cell is carrying out its daily processes.
  • May not be dividing.
  • May be preparing for division.
M Phase
  • Division of the nucleus of a cell.
  • Can happen in Mitosis or Meiosis.
Cytokinesis
  • Division of the cytoplasm of the cell.
  • Completes cell division and results in daughter cells.

Cell Cycle - Mitosis: Subphases of Interphase

G1 Phase
  • G = gap
  • Cell growth occurs.
  • Accumulating building blocks of DNA for the S Phase.
  • Protein synthesis is occurring to prepare for cell division.
  • Cell Checkpoint:
    • The cell is checked to determine if it has accumulated enough materials and is ready to enter the S Phase.
S Phase
  • S = Synthesis
  • This is where DNA replication (synthesis) happens.
  • Each daughter cell needs a copy of DNA to make a total of 46 chromosomes.
  • The centrosome containing centrioles is also replicated.
G2 Phase
  • G = gap
  • More cell growth.
  • Cell replenishes its energy stores.
  • Organelles are duplicated.
  • Cell Checkpoint:
    • Ensures DNA replication is complete and undamaged.
    • Prevents errors from moving to daughter cells.
G0 Phase
  • Cells are arrested after exiting the cell cycle.
  • Some cells enter this phase temporarily.
  • Some cells remain in this phase permanently (e.g., nerve cells, cardiac muscle cells).

Cell Cycle - Mitosis: Mitosis Definition

  • Division of the nucleus in somatic cells.
Phases
  • Prophase, Metaphase, Anaphase, Telophase
Purpose
  • Asexual reproduction
  • Provide cells for growth and development
  • Provide cells to replace damaged cells
  • Provide cells to replace dead or dying cells

Cell Cycle - Mitosis: Mitosis Phases

Prophase
  • Chromatin condenses into chromosomes and becomes visible.
  • Nuclear envelope breaks down.
  • Centrosomes move towards opposite poles.
  • Spindle fibers emerge from centrosomes and attach to centromeres of chromosomes.
Metaphase
  • Chromosomes are lined up at the metaphase plate (cell equator).
  • Each sister chromatid is attached to a spindle fiber originating from opposite poles.
Anaphase
  • Centromeres split in two.
  • Sister chromatids are pulled towards opposite ends of the cell.
  • The cell elongates.
Telophase
  • Two new nuclear envelopes begin to form around both sets of chromosomes.
  • Chromosomes are at opposite poles and begin to decondense back into chromatin.

Cell Cycle - Mitosis: Cytokinesis

  • Occurs in conjunction with telophase in Mitosis.
  • Division of the cytoplasm to produce 2 daughter cells.
  • A contractile ring of actin filaments (protein strands) pinches the cytoplasm and forms a cleavage furrow.
  • The cleavage furrow deepens until the cell is cleaved in two.
Animal and Plant Cell Differences
  • Plant cells do not have centrosomes.
  • Microtubule organization centers produce spindle fibers.
  • Plant cells have a cell wall, so a cleavage furrow is not possible.
  • A new cell wall must be formed to separate the cells.
  • The Golgi apparatus releases vesicles containing molecules to build a new cell wall.
  • The vesicles fuse at the cell plate (center of the cell) to form the cell wall that divides the cell into 2 daughter cells.
At the End of Mitosis
  • Diploid: a cell containing 2 complete sets of chromosomes, one from each parent.
  • Mitosis produces diploid cells.
  • In human cells, each cell will have 46 chromosomes.
  • The cells are genetically identical.