Cell Cycle: G0 and G1 Phases

G0 Phase

• Cells in G0 are alive, metabolically active, but cannot divide.
• Some cells enter G0 temporarily; others are terminally differentiated and remain there.
• Examples of cells that typically remain in G0 include eye cells, mature nerve cells, mature blood cells, and mature cardiac cells. Damage to these cells is usually irreparable.
• Liver cells and stem cells can be recalled from G0.
• Recent research indicates that neurogenesis (the birth of new neurons) occurs in the hippocampus throughout life, contrary to previous beliefs.

G1 (First Gap) Phase

• Cell increases in size.
• Proteins and organelles are synthesized.
• DNA replication does not occur in this phase; DNA remains as chromatin.
• Some cells remain in G1 for extended periods (e.g., neurons, muscle cells).
• Cells in G1 possess totipotency (ability to differentiate into any cell type), a crucial concept in cloning.
• Cells may enter a quiescent phase called G0 due to lack of growth factors or nutrients.

S (Synthesis) Phase

• DNA replication occurs, creating two identical sister chromatids joined at the centromere.
• Centrosomes are duplicated.
• Entry into S phase usually commits the cell to completing the rest of the cell cycle.

G2 (Second Gap) Phase

• Further cell growth and protein synthesis occurs, particularly proteins needed for microtubule formation.
• Chromosomes are replicated but remain as loosely packed chromatin fibers.
• The cell prepares for mitosis – the demand on transport mechanisms increases.

Mitosis

• Mitosis is the division of the cell's nucleus (karyokinesis).
• It's divided into four (or five) distinct stages: prophase, prometaphase, metaphase, anaphase, and telophase. Prometaphase is sometimes considered part of prophase.
• Nearly 2 trillion human cells divide daily.

Prophase

• Longest phase of mitosis.
• Chromosomes condense and become visible.
• The nuclear envelope dissolves.
• The mitotic spindle begins to form, centrosomes separate and migrate to opposite ends of the cell. Each centrosome has two centrioles.

Prometaphase

• Kinetochores form at the centromeres, providing attachment sites for spindle fibers.
• The nuclear envelope fragments. This allows the spindle fibers to connect with the kinetochores and subsequently move the chromosomes.

Metaphase

• Chromosomes align at the cell's equator (metaphase plate).
• Centrosomes are positioned at opposite poles of the cell.

Anaphase

• Sister chromatids separate and move to opposite poles of the cell

Telophase

• Chromosomes arrive at the poles and begin to decondense.
• Nuclear envelopes reform around each chromosome set.
• The mitotic spindle disassembles.

Cytokinesis

• Cytoplasm divides, resulting in two genetically identical daughter cells.

Chromosome Structure

• A chromatid is a single strand of a replicated chromosome.
• Two sister chromatids are connected at the centromere.

Hippocampus

• Region of the brain involved in long-term memory and learning. Displays continual neurogenesis.

Hematopoietic Stem Cells

• Found in bone marrow and blood.
• Can differentiate into all types of blood cells (red blood cells, white blood cells, platelets).

Cell Differentiation & Examples

• During a cell's life cycle, cells can differentiate – that is, become specialized into cells that perform different jobs in the body.
• Examples of terminally differentiated cells that usually can’t divide include: eye cells, nerve cells, blood cells, and heart cells.
• Other cells are able to exit the cell cycle temporarily and re-enter (such as liver cells, stem cells, and some other adult cells).

Explore the cellular processes of the G0 and G1 phases in this quiz. Understand the characteristics of cells in G0, including their metabolic activity and the implications of being terminally differentiated. Delve into the significance of the G1 phase, where cells increase in size and prepare for division without DNA synthesis.