Cell Cycle and Cellular Reproduction

Overview of Cell Cycle and Cellular Reproduction

• The cell cycle is a series of events that lead to cell division, creating two genetically identical daughter cells.
• Eukaryotic cell division involves mitosis (nuclear division) and cytokinesis (cytoplasm division).
• The cell cycle is roughly 20 hours in mammals.

Interphase

• The longest part of the cell cycle (about 90% of the cycle).
• Divided into three stages:
  • G₁ (Gap 1): Cell growth, organelle duplication (mitochondria and ribosomes). Nerve and muscle cells remain in G₀.
  • S (Synthesis): DNA replication, synthesizing proteins associated with DNA. Each chromosome duplicates into two identical chromatids.
  • G₂ (Gap 2): Cell synthesizes proteins necessary for cell division (e.g., microtubule proteins).

Sister Chromatids

• Chromosomes duplicated during the S phase consist of two identical chromatids.
• Attached at the centromere, forming an X shape.
• Essential for accurate chromosome segregation during mitosis.

Mitotic Stage

• Follows interphase.
• Mitosis: Division of the nucleus, creating two genetically identical nuclei.
• Cytokinesis: Division of the cytoplasm, forming two separate daughter cells.
• Results in two genetically identical daughter cells.

Control of the Cell Cycle

• Controlled by internal and external signals.
• External signals: Growth factors, signaling proteins received at the plasma membrane.
• Internal signals: Cyclins, proteins that increase and decrease throughout the cell cycle. Specific cyclins are needed for the cell to progress from one stage to the next. Without cyclins the cell cycle stops at checkpoints (G₁, M, or G₂).

Cell Cycle Checkpoints

• Critical control points to ensure accurate DNA replication and cell division.
  • G₁ checkpoint: p53 checks for DNA damage, initiating DNA repair or apoptosis if repair isn't possible. RB checks for nutrient availability.
  • G₂ checkpoint: Checks if DNA replication was completed correctly or if DNA is damaged.
  • M checkpoint: Monitors proper alignment and attachment of chromosomes to the mitotic spindle.

Apoptosis

• Programmed cell death caused by enzymes called caspases.
• A natural process to eliminate abnormal or damaged cells.
• Controlled by inhibitors, but can be unleashed by external or internal signals.
• Opposes cell division (which increases somatic cell number), keeping the cell count appropriate.

Eukaryotic Chromosomes

• Consist of a single DNA molecule condensed by histone proteins (DNA + histones = chromatin).
• Nucleosomes: DNA wrapped around histone proteins.
  • Nucleosomes are joined by "linker" DNA.
• Euchromatin: Active chromatin, able to be transcribed.
• Heterochromatin: More compact, inactive chromatin.

Mitosis and Cytokinesis in Eukaryotic Cells.

• Mitotic stages involve: prophase, prometaphase, metaphase, anaphase, telophase.
• Centrioles move to opposite poles, forming the mitotic spindle (microtubule fibers).
• Each fiber is a cylindrical bundle of microtubules.
• The spindle will allow chromosomes to move to opposite poles, so each new cell has a full set of chromosomes.

Mitosis in Animal Cells, Plant cells, and Prokaryotes

• Animal cells: Utilize cleavage furrow (contractile ring of actin filaments) during cytokinesis, a process that constricts and separates the daughter cells, while plant cells produce a cell plate that separates the daughter cells.
• Prokaryotes: Binary fission is the process where DNA replication results in two complete chromosomes that will be replicated and the cell divides into two daughter cells.
  • The chromosome is a ring of DNA.
  • Chromosome replicates into two rings.
  • The rings attach to the cell membrane.

The Cell Cycle and Cancer

• Cancer results from abnormal cell growth, developing uncontrolled cell division, caused by mutations in genes regulating the cell cycle.
• Abnormal cell growth is called a tumor.
  • Benign tumors are not cancerous and encapsulated.
  • Malignant tumors are cancerous and readily invade neighboring tissue and spread (metastasis).
• Carcinogenesis: Gradual cancer development, can take years.

Characteristics of Cancer Cells

• Lack differentiation (not specialized).
• Immortal (enter the cell cycle repeatedly).
• Abnormal nuclei and chromosomes with extra copies of genetic material.
• Do not undergo apoptosis.
• No contact inhibition (tumors form).
• Undergo metastasis (spread) and angiogenesis (formation of new blood vessels).

Cancer Cells vs. Normal Cells (table)

• Compare and contrast characteristics between cancer and normal cells.

Progression of Cancer

• Tumors grow rapidly, requiring more blood vessels for oxygen and nutrients.
• Cancer in situ (tumor remains at the origin).
• Metastasis: Cancer cells invade lymphatic and blood vessels to spread throughout the body.

The Origin of Cancer

• Normal cell growth and tissue maintenance balance between signals promoting and inhibiting cell division.
• Two mutated genes:
  • Proto-oncogenes (become oncogenes): Genes that promote the cell cycle. Mutations lead to increased cell division.
  • Tumor suppressor genes (inactivation): Genes that inhibit cell cycle and/or promote apoptosis. Mutations result in loss of inhibition or increased cell growth.

Proto-oncogenes Become Oncogenes

• Proto-oncogenes are stimulatory pathway components like receptors and signaling molecules.
• Mutations can produce an abnormal protein.
• Cancer arises from uncontrolled high levels of a normal product.

Tumor Suppressor Genes Become Inactive

• Mutations cause the cell cycle to accelerate.
• Examples are the RB and p53 genes.
• RB gene mutations associated with retinoblastoma (inherited condition).
• p53 gene activates other cell cycle proteins.

Causes of Cancer

• External and hereditary influences can also contribute to cancer, such as radiation, heredity, pesticides, herbicides, viruses, and various other influences.

The Cell Cycle and Telomeres

• Telomeres: Protective caps at the ends of chromosomes.
• Telomeres shorten after each cell division, causing a cell to no longer divide.
• Telomerase: Enzyme that maintains telomere length, and cancer cell-telomerase enables continuous cell division.
• Mutations in telomerase cause cancer cells maintain continuous cell division.

Prokaryotic Cell Division (Binary Fission)

• Prokaryotes reproduce asexually through binary fission.
• Chromosome is a ring of DNA.
• Chromosomes replicate into two separate rings.
• Rings attach to the membrane. The membrane grows inward, separating the two daughter cells.

Comparing Prokaryotes and Eukaryotes

• Prokaryotic division: Binary fission, with identical daughter cells; Eukaryotic division (mitosis), allows more complex cells growth and repair.

Table 9.3 Functions of Cell Division

• Describe division types for various organisms.