Cell Cycle and Mitosis: Key Concepts and Mechanisms

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The genetic material in eukaryotic cells exists in different forms depending on the stage of the cell cycle and the level of organization.

Chromatin: The complex of DNA and proteins (mainly histones) that makes up chromosomes. Chromatin is loosely packed during interphase, allowing for gene expression and DNA replication.
Chromosomes: Highly condensed structures of chromatin visible during cell division. Each chromosome contains a single, long DNA molecule.
Sister Chromatids: Two identical copies of a chromosome, joined together at the centromere, produced during DNA replication in the S phase.
Example: Before cell division, each chromosome consists of two sister chromatids.

Cells in multicellular organisms are classified based on their role in reproduction.

Somatic Cells: All body cells except reproductive cells; they are diploid (contain two sets of chromosomes).
Gametes: Reproductive cells (sperm and egg); they are haploid (contain one set of chromosomes).
Example: Human skin cells are somatic, while sperm and eggs are gametes.

These structures play distinct roles in chromosome organization and cell division.

Centromere: The region of a chromosome where sister chromatids are joined and where spindle fibers attach during mitosis.
Centrosome: The microtubule-organizing center of the cell, important for forming the mitotic spindle.
Example: Each chromosome has one centromere; animal cells typically have two centrosomes during mitosis.

The cell cycle is a series of events that cells go through as they grow and divide.

G1 Phase (First Gap): Cell grows and carries out normal functions.
S Phase (Synthesis): DNA is replicated, resulting in sister chromatids.
G2 Phase (Second Gap): Cell prepares for division; checks for DNA damage and repairs.
Mitosis: Division of the nucleus into two genetically identical nuclei.
Cytokinesis: Division of the cytoplasm, resulting in two daughter cells.
Example: A skin cell undergoes the cell cycle to produce two identical daughter cells.

An aster is a star-shaped structure formed around each centrosome during mitosis.

Aster: Microtubules radiating from the centrosome, helping to position the spindle apparatus and anchor it to the cell membrane.
Importance: Ensures proper orientation and separation of chromosomes.

The kinetochore is a protein complex assembled on the centromere of each chromatid.

Kinetochore: Site of attachment for spindle microtubules, facilitating chromosome movement.
Importance: Ensures accurate segregation of chromosomes during cell division.

Mitosis is divided into distinct phases, each with specific events.

Prophase: Chromatin condenses into chromosomes; spindle begins to form.
Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move toward opposite poles.
Telophase: Nuclear envelopes reform; chromosomes decondense.
Example: During metaphase, all chromosomes are lined up in the center of the cell.

Cytokinesis differs between animal and plant cells due to structural differences.

Animal Cells: Cytokinesis occurs via cleavage furrow, where the cell membrane pinches in.
Plant Cells: Cytokinesis occurs via cell plate formation, where vesicles fuse to form a new cell wall.
Example: Animal cells "pinch" apart, while plant cells build a new wall between daughter cells.

Binary fission is a method of asexual reproduction used by prokaryotes and some single-celled eukaryotes.

Process: The cell grows, replicates its DNA, and divides into two identical cells.
Organisms: Bacteria and archaea primarily use binary fission.
Example: Escherichia coli divides by binary fission.

Checkpoints are control mechanisms that ensure the cell cycle progresses correctly.

Checkpoint	What is Checked
G1 Checkpoint	Cell size, nutrients, DNA damage
G2 Checkpoint	DNA replication completeness, DNA damage
M Checkpoint (Spindle Checkpoint)	Chromosome attachment to spindle fibers

The G0 phase is a resting state where cells exit the cell cycle and stop dividing.

Significance: Many differentiated cells (e.g., neurons) remain in G0 permanently.
Example: Mature nerve cells are typically in G0.

Cell cycle progression is regulated by proteins called cyclins and cyclin-dependent kinases (CDKs).

Cyclins: Proteins whose levels fluctuate during the cell cycle.
CDKs: Enzymes that, when bound to cyclins, phosphorylate target proteins to advance the cell cycle.
Example: The cyclin-CDK complex triggers the transition from G2 to M phase.
Equation: