Mitosis and Cell Division

Mitosis

• Mitosis is a form of cell division that replaces dying cells and contributes to growth and renewal of cells.
• It occurs everywhere in the body except for gametes (reproductive cells: egg and sperm).
• Gamete production happens through meiosis (covered in Bio 169).
• Mitosis results in two daughter cells that are identical copies of the mother cell.
• If the daughter cells are not identical, it indicates a mutation.
• Cancer is essentially unregulated cell division where mitosis fails, leading to foreign growth within a tissue.

Cell Life Cycle

The life cycle of a cell consists of:

• G1 (Growth 1)

• S (Synthesis: DNA replication)

• G2 (Growth 2)

• M (Mitosis: division of the nucleus)

• C (Cytokinesis: division of the cytoplasm)

• Checkpoints exist to ensure everything is working correctly during the cell cycle.

• If a mistake occurs during DNA replication (S phase), the cell undergoes apoptosis (programmed cell death).

• Apoptosis is crucial for our survival, as it eliminates cells with errors.

• Red blood cells, for example, have a lifespan of 120 days and then undergo apoptosis.

• Uncontrolled cell growth without apoptosis leads to cancer.

G1 Phase

• The cell grows and carries out its normal functions (endocytosis, exocytosis).
• The cell receives a signal to undergo mitosis and divide, but the exact nature and regulation of this message are not fully understood.
• Research is focused on understanding these chemical signals to potentially control cell growth and induce apoptosis in cancer cells.
• North Carolina is a significant hub in the pharmaceutical and biotechnology industry (third in the world).
• Discovering and developing drugs is expensive due to extensive research, scientist salaries, and facilities.

S Phase

• DNA replicates, creating an exact copy.
• Human DNA contains approximately three billion nucleotides in one strand.
• The replicated DNA (six billion nucleotides total) is stored in chromosomes within the nucleus.
• The analogy of fitting three billion bricks into a small bedroom illustrates the complexity.

Organelle Doubling

• Organelles like the endoplasmic reticulum, Golgi apparatus, and mitochondria double during the G1 phase.
• For example, ten mitochondria become 20.

G2 Phase

• G2 is a resting stage where the cell replenishes energy after doubling its organelles and DNA.
• The process of doubling the organelles and DNA takes approximately 3 hours.

Mitosis Stages

Mitosis consists of four stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

• Following telophase, the cell undergoes cytokinesis.

• Interphase refers to the G1, S, and G2 stages before mitosis.

Prophase

1. Chromosomes condense and become visible.
2. Centrioles separate and move to opposite ends of the cell.
3. Spindle fiber microtubules form.
4. The nuclear envelope and nucleolus disappear.

• Centrioles act like cowboys, and spindle fibers are like their lassos.
• Microtubules connect to the centromere of sister chromatids.
• Sister chromatids are the identical copies of a chromosome formed during DNA replication, attached at the kinetochore.
• The centromere is the specific point where sister chromatids are most closely attached.
• Each centriole connects to one copy of each chromatid.

Metaphase

• Spindle fibers align the chromosomes at the metaphase plate (the center of the cell).
• Sister chromatids are lined up in the center, with one copy facing each centriole.

Anaphase

• Microtubules pull the sister chromatids away from each other towards opposite ends (poles) of the cell.
• The cowboys (centrioles) pull the lassos (microtubules) connected to the bulls (centromeres) to bring them closer.

Telophase

• Telophase is essentially prophase in reverse.
• A nucleus forms around each set of chromosomes.
• The nucleolus reappears.
• Spindle fibers disappear.
• The centrioles move back to their original location
• The result is that each new nucleus will include the newly replicated chromosome, which can be mixed with the original/older chromosome.

Cytokinesis

• Cytokinesis is the division of the cytoplasm.
• The cell splits into two daughter cells that are copies of the mother cell.
• This process is called cleavage furrow, where the cell membrane constricts in the middle to separate the two new cells.
• Furrow formation occurs due to signals and processes within the cell, causing constriction of the cell membrane.