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.