Chapter 9: The Cell Cycle

Key Roles of Cellular Division

• Cellular division allows organisms to produce more of their own kind, distinguishing living from non-living things.
• It is a characteristic of life to replicate and reproduce.
• In multicellular organisms (e.g., humans), it facilitates growth and repair.
• In unicellular organisms, it enables reproduction.
• The continuity of life is based on the reproduction of cells through cellular division.

Cell Cycle Overview

• Cellular division is part of the cell cycle, consisting of cell growth and cell division.
• Image description: one cell dividing into two genetically identical cells through stages like nuclear envelope dissolution and chromosome condensation.

Cellular Division in Organisms

• Unicellular: e.g., amoeba uses cellular division for reproduction (asexual reproduction).
• Multicellular: Humans develop from a single cell (zygote) and use cellular division for tissue renewal and repair.
• Cell division results in genetically identical daughter cells.

Genetic Material Organization

• DNA in a cell constitutes the genome.
• Human genome: 46 chromosomes (23 from each parent).
• Chromosomes are packaged DNA, made of chromatin (DNA + protein).
• Somatic cells: diploid number (46 chromosomes), Body cells.
• Gametes: haploid number (23 chromosomes), Sex cells.

Chromosome Distribution in Eukaryotic Division

• DNA replication and condensation occur in preparation for division.
• Sister chromatids (joined identical copies) are held together by centromeres.
• During division, chromatids separate, forming new chromosomes.

Eukaryotic Cell Division

• Consists of mitosis (nuclear division) and cytokinesis (cytoplasm division).
• Mitosis produces genetically identical daughter cells.
• Meiosis produces non-identical daughter cells (gametes).

Cell Cycle Phases

• Alternates between mitotic phase (mitosis and cytokinesis) and interphase.
• Interphase: G1 (growth), S (DNA synthesis), and G2 (preparation for mitosis).
• M phase: mitosis (division) and cytokinesis.

Mitosis Steps

• Prophase: Chromosomes are packed.
• Metaphase: Chromosomes align in the middle.
• Anaphase: Chromatids pulled apart.
• Telophase: Two nuclei form in one cell.
• Cytokinesis: Cytoplasm divides, forming two cells.

Mitotic Spindle

• Made of microtubules, controls chromosome movement.
• Centrosome: microtubule organizing center replicates during interphase.
• Kinetochores: proteins on centromeres where spindle fibers attach.

Cytokinesis

• Animal cells: Cleavage furrow forms, pinching cytoplasm.
• Plant cells: Cell plate forms, new cell wall divides the cell.

Binary Fission in Prokaryotes

• Bacterial reproduction through binary fission (asexual reproduction).
• Chromosome replicates, cell elongates, splits into two cells.

Evolution of Mitosis

• Mitosis evolved from binary fission.
• Protists display intermediary steps between binary fission and mitosis.

Cell Cycle Regulation

• Driven by signaling molecules in cytoplasm.
• Cancer cells evade normal controls, leading to uncontrolled growth.

Checkpoints in Cell Cycle

• G1 checkpoint: critical for continuing the cycle.
• G0 phase: non-dividing state if checkpoint is not passed.
• M checkpoint: ensures proper spindle attachment before anaphase.

Internal and External Controls

• Internal: Cyclin-dependent kinases (CDKs) regulate cell cycle progression.
• External: Growth factors like PDGF stimulate cell division.

Cancer Cells

• Exhibit uncontrolled growth, lack density-dependent inhibition and anchorage dependence.
• Benign tumors: remain localized.
• Malignant tumors: invade tissues, metastasize.

Cancer Treatment

• Advances in understanding cell cycle signaling aid cancer treatment.
• Treatments becoming more personalized due to cancer complexity.