Chapter 9 - The Cell Cycle and Cellular Reproduction

Stages of Interphase:
- G1 Phase (Gap 1):
- Cells grow and increase in size.
- Longest phase of interphase, organelles double.
- Most affected by external factors (nutrients and growth factors).
- Prior to moving to S phase, must meet specific criteria at G1 checkpoint.
- G0 Phase:
  - Cells may exit the cell cycle temporarily or permanently.
  - Example cells that may enter G0: nerve and muscle cells, old cells, or cells not large enough to divide.
- S Phase (Synthesis):
- DNA replication occurs.
- Starts with 1 chromatid; ends with 2 sister chromatids (attached at the centromere).
- Sister chromatids remain connected at the centromere by cohesin proteins.
- Kinetochore: attachment site for microtubules.
- G2 Phase (Gap 2):
- Synthesis of microtubules and proteins that assist in division.
- Centrioles replicate (in animal cells only).
- Nuclear envelope and nucleolus remain intact before proceeding to the M phase.
- Cell size nearly doubles, and DNA is replicated and exists as chromatin.
Checkpoints Regulating the Cell Cycle:
- G1 Checkpoint: Evaluates cell size, nutrient availability, growth signals, and DNA integrity.
- Key Protein: p53, activates DNA repair or triggers apoptosis.
- G2 Checkpoint: Examines DNA for damage and completeness of replication before entering M phase.
- M Checkpoint: Ensures all sister chromatids are attached to the spindle before proceeding to anaphase.

Genome: All DNA present within a cell.
Chromosomes: Characterized as tightly coiled DNA wrapped around proteins.
Chromatin: The material making up chromosomes, consists of DNA, RNA, and proteins.
- Two types of chromatin:
- Euchromatin: Loose coiling; active form capable of transcription.
- Heterochromatin: Tight compaction; generally inactive form.
Gene: Basic unit of genetic information determining specific traits (e.g., hair color, eye color).
Levels of DNA Compaction:
- Nucleosomes: DNA wrapped around histone proteins.
- Chromosomes: Most tightly wound form during metaphase, assisted by condensin proteins.
Chromosome Numbers of Eukaryotes:
- Example organisms and their chromosome numbers:
- Yeast (Fungi): 32
- Garden Pea (Plants): 14
- Potato (Plants): 48
- Fruit Fly (Animal): 8
- Human (Animal): 46
- Goldfish (Animal): 94
- Homologous Chromosomes: Chromosomes of the same length, centromere position, and gene position; one inherited from each parent.

Mitosis and Cytokinesis Overview:
- Mitosis: The division of a cell into two genetically identical daughter cells.
- Cytokinesis: The process of dividing the cytoplasm.
- Karyokinesis: The division of the nucleus.
Phases of Mitosis:
- Prophase: Duplicated chromosomes become visible, chromatin condenses, nuclear envelope fragments, and spindle apparatus assembles.
- Prometaphase: Microtubules attach to chromosomes at the kinetochore; chromosomes move toward the cell center.
- Metaphase: Chromosomes align at the metaphase plate, and kinetochores attach to spindle fibers from opposite poles.
- Anaphase: Cohesin proteins dissolve, sister chromatids separate, becoming daughter chromosomes moving to opposite poles.
- Telophase: Nuclear envelope reforms, daughter cells forming, chromosomes uncoil into chromatin, spindle breaks down.
Cytokinesis:
- The division of the cytoplasm occurs alongside telophase.
- Differences between animal and plant cell cytokinesis:
- Animal Cells: Involves a cleavage furrow, constricted by actin filaments, forming a new cell membrane.
- Plant Cells: A cell plate forms between the nuclei, with vesicles from the Golgi apparatus merging to form a new cell wall.

Characteristics of Cancer Cells:
- Non-differentiated, immortal cells with abnormal nuclei.
- Lack of apoptosis; do not respond to contact inhibition; can metastasize.
Tumor Types:
- Benign Tumors: Non-cancerous; do not invade neighboring tissues or spread.
- Malignant Tumors: Cancerous; invade tissues and spread throughout the body.
- Metastasis: Tumors detach and settle in distant parts of the body.
- Angiogenesis: Formation of new blood vessels to supply tumors.
Mutations Leading to Cancer:
- Mutations in proto-oncogenes lead to uncontrolled growth.
- Tumor suppressor mutations prevent apoptosis and allow division of damaged cells.
Cell Cycling Regulation:
- Positive Regulators (e.g., cyclins and proto-oncogenes) activate progression through mitosis.
- Negative Regulators (e.g., tumor suppressors like p53, p21, Rb) inhibit cell division and promote apoptosis if necessary.
Mitotic Index:
- Ratio of actively dividing cells to total cells, expressed as a percentage.
- Higher mitotic indices in cancer correlate with aggressive types and faster division.
- Calculation:
  ext{Mitotic Index} = rac{ ext{Number of cells in mitosis}}{ ext{Total number of cells}} imes 100

Binary Fission:
- Method of asexual reproduction in prokaryotes.
- Rapid, requiring less energy, results in genetically identical daughter cells (clones).
- Examples: Escherichia coli has a generation time of about 20 minutes.
Steps of Binary Fission:
1. DNA in the nucleoid region attaches to a membrane site.
2. Cell wall and membrane enlarge; DNA replicates to create two identical chromosomes.
3. Inward growth of the cell wall and membrane forms a septum.
4. Elongation occurs, and chromosomes are pulled apart evenly.
5. Completion of new cell wall and membrane separates the daughter cells.
Comparison of Eukaryotic vs. Prokaryotic Division:
- Eukaryotes (including protists and some fungi) use mitosis for growth and development, while prokaryotes (bacteria and archaea) employ binary fission for asexual reproduction.