Cell Division and Differentiation Notes

Genetically Identical Cells

• Definition: Genetically identical cells are those that arise from a single zygote and share the same genetic material.

• Example: When two genetically identical cells are separated, they can develop into identical twins, as they are functionally equivalent.

Meiosis vs. Mitosis

• Meiosis:

  • Purpose: Reshuffling of chromosomes; produces gametes (egg and sperm) with half the genetic material.

  • Result: Cells undergo division and reduce chromosome number by half.

• Mitosis:

  • Purpose: Cell division for growth, repair, and replacement; maintains the same genetic material.

  • Result: Two genetically identical daughter cells.

Development of the Embryo

• After fertilization, the zygote divides repeatedly as it moves down the fallopian tube to the uterus. Before implantation, little is known about the embryo's development.

• If the embryo fails to implant, it may stop dividing and undergo apoptosis (programmed cell death).

Somatic Cells vs. Germ Cells

• Somatic Cells:

  • Definition: Cells that make up the body (from Greek 'soma').

  • Function: Primarily undergo mitosis for growth and replacing dead cells (e.g., skin cells, digestive tract cells).

• Germ Cells:

  • Definition: Cells that give rise to gametes (egg and sperm).

  • Properties: Can undergo both mitosis (to create more germ cells) and meiosis (to produce gametes).

Cell Division Types

• Symmetric Cell Division:

  • Definition: A type of mitosis that produces two identical daughter cells of the same cell type (e.g., skin cells).

  • Importance: Essential for growth and tissue replacement.

• Asymmetric Cell Division:

  • Definition: A form of cell division producing daughter cells with different fates; occurs in stem cell differentiation.

  • Example: In embryos, one daughter cell may become a neural cell while the other becomes a skin cell.

  • Cell Differentiation: Process by which cells become specialized in structure and function.

Stem Cells

• Stem cells are undifferentiated cells set aside in tissues capable of dividing and producing specialized cells.

• They can either remain as stem cells or differentiate into specific cell types necessary for tissue maintenance.

Ploidy and Genetics

• Ploidy: Refers to the number of copies of the entire genome in an organism.

  • Diploid (2n): Most multicellular organisms, including humans, with 23 pairs (46 total) of chromosomes.

  • Tetraploid: Rare cells with four copies of each chromosome (4n), observed in some organisms.

Cell Cycle Phases

• Interphase: Period of preparation before a cell division; consists of three stages:

  • G1 Phase (Gap 1):

  • Purpose: Cell growth and energy storage.

  • Cells prepare for DNA synthesis.

  • S Phase (Synthesis):

  • Purpose: DNA replication; chromosomes are duplicated.

  • Results in temporarily duplicated chromosomes.

  • G2 Phase (Gap 2):

  • Purpose: Final preparations for division; the cell synthesizes proteins and organelles needed for mitosis.

• M Phase: Includes both mitosis and cytokinesis, where the cell physically divides into two daughter cells.

  • Mitosis Stages:

  • Prophase: Chromosomes condense, spindle apparatus begins to form.

  • Metaphase: Chromosomes align at the metaphase plate.

  • Anaphase: Sister chromatids are pulled apart to opposite poles.

  • Telophase: Chromosomes de-condense, and the nuclear membrane re-forms.

Key Processes in Mitosis

• Karyokinesis: The division of the nucleus during mitosis, ensuring each daughter cell receives an identical set of chromosomes.

• Cytokinesis: The division of the cytoplasm that follows karyokinesis, resulting in two separate cells.

Importance of Cell Cycle Regulation

• Cells must receive growth signals to proceed from G1 to S phase.

• The restriction point in G1 is crucial; if a cell fails to meet the criteria, it may undergo apoptosis.

• Damaged cells within G2 cannot revert back to earlier phases and are usually targeted for cell death, maintaining integrity in cell division.

Conclusion on Cell Division

• Understanding the mechanisms of both dividing and non-dividing cells provides insight into growth, development, and potential regenerative medicine applications. The study of various cell types, including somatic and germ cells, helps elucidate how tissues maintain functionality and respond to damage.