Why cells undergo cell replication:

• Growth: Needs more cells, cells cannot become larger because size is limited

• Repair: The replacement of old and damaged cells is important to prevent diseases

• Procreation: Some species rely on cell replication to survive

Binary fission: Cell multiplication for prokaryotes

• Step 1: The circular DNA chromosome replicates and the cell elongates

• Step 2: The two circular chromosomes move to each side of the cell

• Step 3: The cell membrane splits into two, a new cell wall (called a septum) forms and breaks in the middle to form two daughter cells

Eukaryotic Cell cycle: The sequence of events of cell growth and reproduction for two daughter cells to form

• Interphase: period of DNA replication

  1. Gap 1 (G1) stage: Growth occurs which leads to an increased amount cytosol. Proteins are created. The mitochondria divides. If the cell does not go through this stage, it will be in the G0 or resting phase.

  2. Synthesis (S) Stage: The parent cell’s DNA is replicated and it will have two identical DNA copies.

  3. Gap 2 (G2) Stage: Further cell growth to prepare for cell division. More proteins are synthesised and the cell doubles in size.

• Mitosis: The organisation and separation of chromosomes

  1. Prophase: Chromosomes condense → visible as double stranded structures. The spindle forms + nuclear membrane breaks down

  2. Metaphase: Double stranded chromosomes line up at the cell’s equator

  3. Anaphase: Sister chromatids separate and are pulled to opposite sides of the spindle

  4. Telophase: Nuclear membrane forms around each group of single-stranded chromosomes → chromosomes decondense

• Cytokinesis: The cytoplasm divides, forming new membranes and enclosing the daughter cells

  • Animal cells during cytokinesis: Cytoplasm between two nuclei becomes narrow → cell membrane pinches to separate nuclei + cytoplasm into two new daughter cells

  • Plant cells during cytokinesis: Cell plate forms between chromosome groups → develops into one cell wall for each new cell

Regulation of Cell Cycle:

• G1 checkpoint: DNA is checked to be damaged or incomplete → if so, enters G0 stage to be destroyed

  • If it passes G1 it proceeds to the S stage and the DNA replicates

• G2 checkpoint: Replicated DNA is examined for completeness and lack of damage

  • Advances to mitosis stage afterwards

• M checkpoint: Ensures sister chromatids are attached to the correct microtubules of the spindle

Programmed Cell Death

• Apoptosis: Programmed cell death as a regular part of an organism’s development

  • Cells at the end of their natural life

  • Damaged or diseased cells

  • Excessive cells

Cell Differentiation

• Cell differentiation: cells become specialised for a specific function

• Stem cells: Undifferentiated cells that can differentiate into organ or tissue specific cells

  • Self renewable: Maintains stem cell pull

  • Differentiation: Replacement of dead + damaged cells

• Embryonic stem cells: Can differentiate into many different cell types

  • Found in the cells of embryos from zygote to blastocyst stage

• Adult tissue stem cells: Undifferentiated cells that can differentiate into related cell types

  • Found in the eye’s surface, brain, skin, breast, intestine, testicles, muscles, bone marrow

  • AKA somatic stem cells

  • eg. Blood stem cell found in bone marrow can differentiate into only specialised blood cell types - red blood cells, white blood cells, platelets

• Potency: Cell’s ability to differentiate into various cell types

• Totipotent stem cells: Can become any cell type or another embryo

• Pluripotent stem cells: Differentiate into any of the three germ layers - endoderm, mesoderm, ectoderm (blastocyst cells)

• Multipotent stem cells: Give rise to multiple cell types that are related to the family of cells. Ex. Blood stem cells becoming platelets or white blood cells

• Oligopotent stem cells: Differentiate into a few cell types. Ex. Adult lymphoid cells

• Unipotent: Differentiate into one cell type in a certain tissue repeatedly. Ex. skin epidermal stem cells