7.1 Cell Division and Growth notes
Cell Growth and Division
Protein Synthesis: Causes cells to grow.
Why Cells Must Divide
Strength of the Plasma Membrane: Cannot accommodate the volume of the cytoplasm (volume exceeds membrane capacity).
Surface Area to Volume Ratio: Insufficient diffusion rate through the plasma membrane to sustain life functions.
Nuclear Control of the Cell: Messages must travel quickly from the nucleus to all parts of the cell.
Regeneration: Replace injured cells in multicellular organisms.
Reproduction: Ensures continuity of species in unicellular organisms (e.g. binary fission).
Cell Division
Occurs in all somatic (body) cells.
Process increases the total number of cells; key to growth.
Triggers asexual reproduction, forming two identical daughter cells.
Characteristics passed via chromosomes - made of Deoxyribonucleic Acid (DNA) containing hereditary codes.
Chromosomes must be duplicated before division; one set goes to each daughter cell.
The Cell Cycle
Life cycle has three phases: Interphase, Mitosis, and Cytokinesis.
A) Interphase
Time for cell growth and development; longest phase.
Divided into 3 Stages:
G1 Phase (First Growth Phase): Follows mitosis, cell grows and develops.
S Phase (Synthesis Phase): DNA replicates (chromosomes make exact copies).
G2 Phase (Second Growth Phase): Cell completes maturation and prepares for division.
B) M Phase (Mitosis)
Division of nucleolus; divided into stages:
Prophase: Replicated chromosomes become visible, nuclear membrane breaks down, centrioles move to opposite poles.
Metaphase: Chromosomes line up at spindle's equator, attached at centromere, spindle fibers form.
Anaphase: Centromeres divide; sister chromatids pulled apart by spindle fibers.
Telophase: Nuclear membrane reforms, chromosomes de-condense, cytokinesis begins.
C) Cytokinesis
Division of cytoplasm.
Stages of Mitosis in Animal Cells
Prophase: Chromatids visible, nuclear membrane dissolves, centrioles move.
Metaphase: Chromosomes align at spindle equator.
Anaphase: Chromatids separated and pulled apart.
Telophase: Reformation of nuclear membranes, cytokinesis begins.
Mitosis in Plant Cells
Differences from Animal Cells:
No centrosomes or formation of asters.
Cell plate forms during cytokinesis, grows outward forming cell wall.
Rate of Cell Division
Varies with tissue type and organism complexity:
Simpler organisms divide faster (e.g., Amoeba every 20 minutes).
Skin cells divide every few days.
Factors Affecting Rate of Cell Division
Injury to Cells: E.g., cutting grass or skin.
Tissue Separation: E.g., during surgery.
Nutrient Absence: Lack of nutrients prevents growth.
Presence of Poisons: E.g., chemotherapy inhibits growth.
Growth Factors: Hormones leading to division.
Cell Density: Limited division capacity impacts aging and death.