The Cell Cycle Overview

The Cell Cycle

• Cell cycle: a sequence of events in a eukaryotic cell's life, from origin to division into two daughter cells.
• Cells divide for two reasons: reproduction of the organism and growth/repair of cells in a multicellular organism.

Stages of the Cell Cycle

• Interphase: The longest stage of a cell’s life (90%), where the cell performs its normal functions and prepares for division.
• Cell Division (Mitosis): The final stage of a cell's life cycle, where DNA is duplicated and divided into daughter cells.

Interphase

• G1 phase (gap 1): The first phase after a cell divides, where it takes in nutrients, produces proteins, and grows.
• S phase (synthesis): DNA is replicated during this phase, resulting in twice the amount of DNA in the cell.
• G2 phase (gap 2): Occurs after the S phase and before cell division. The cell prepares for division by ensuring raw materials are available.

Cell Cycle Checkpoints

• G1 Checkpoint: Stops the cell cycle if the cell is too small, lacks sufficient nutrients, or if DNA is damaged.
• G2 Checkpoint: Stops the cell cycle if the cell is too small, lacks sufficient nutrients, DNA is damaged, or chromosome duplication in the S phase is incomplete.

Tumors

• A mass of cells created by rapid, uncontrolled cell division.
• Often occur when cells ignore cell cycle checkpoints.
• Benign tumors: Cells remain confined to one site.
• Malignant tumors (cancerous): Cells become anchorage-independent, invading other tissues to form metastases.
• Metastases: Spread of cancer from one organ to another.
• Angiogenesis: Formation of new blood vessels, promoted by cancerous cells.

DNA

• The genetic blueprint containing the information for cells to function properly.
• Located within the nucleus of eukaryotic cells.
• Must be duplicated before a cell can divide.

Cell Division

• The final stage of a cell's life cycle, where DNA is duplicated and divided into daughter cells.

Cell Division in Prokaryotes

• Binary fission: Asexual reproduction process in prokaryotes and some single-celled eukaryotes.
  • The cell copies its chromosome.
  • Chromosomes move to separate ends of the cell.
  • The cell splits into two daughter cells, each receiving one chromosome.
  • Daughter cells are genetically identical clones.

Cell Division in Eukaryotes

• Mitosis: Asexual reproduction in somatic (body) cells that produces two identical daughter cells.

Mitosis: Stages

• Prophase: Chromosomes condense, the nuclear membrane dissolves, and the spindle forms.
• Metaphase: Chromosomes line up in the center of the cell.
• Anaphase: Spindle fibers pull chromatids apart, and chromatids migrate to opposite poles of the cell.
• Telophase/Cytokinesis: The nuclear membrane reforms, and the cell pinches in two, forming two daughter cells.

DNA Synthesis

• During Interphase's 'S' phase, DNA replicates.
• The cell has two identical copies of each chromosome, called sister chromatids.
• Sister chromatids are held together by a centromere.

Meiosis: Overview

• Cell division used in sexual reproduction to produce sperm and egg cells (gametes).
• Gametes are haploid, containing only one copy of each chromosome.
• Somatic cells are diploid, possessing two copies of each chromosome.

Gamete Formation

• Sperm are formed in the testes.
• Eggs are formed in the ovaries.
• Gametes are produced by meiosis.

Human Chromosomes

• Humans have 46 chromosomes organized into 23 homologous pairs (one from each parent).
• Meiosis halves the number of chromosomes, starting with a diploid cell and ending with four haploid cells.

Meiosis I: The First Round of Cell Division

• Interphase: Growth and chromosome replication.
• Prophase I: Chromosomes condense, the nuclear membrane dissolves, and the spindle forms.
• Metaphase I: Homologous chromosomes line up in the center of the cell.
• Anaphase I: Homologous chromosomes separate to opposite poles.
• Telophase I/Cytokinesis: Nuclear membranes form, and the cell pinches in two, resulting in two haploid daughter cells.

Meiosis II: The Second Round of Cell Division

• Prophase II: Chromosomes attach to the spindle.
• Metaphase II: Chromosomes line up in the center of the cell.
• Anaphase II: Sister chromatids of each chromosome separate.
• Telophase II/Cytokinesis: Nuclear membrane reforms, and the cell pinches in two, resulting in four haploid daughter cells.

Key Differences Between Mitosis and Meiosis

• Mitosis: Produces two identical diploid daughter cells.
• Meiosis: Produces four unique haploid daughter cells.

Importance of Haploid Gametes

• They provide the correct number of chromosomes for a new organism when a sperm cell fertilizes an egg cell, ensuring a diploid organism.