CH 12: Cell Cycle and Cell Division

Cell division produces two genetically identical daughter cells from one parent cell, ensuring that each new cell receives a complete set of DNA.
- Interphase: The longest phase, involving significant cell growth and the precise replication of DNA in preparation for division.
- Mitosis: The process of nuclear division where replicated chromosomes are separated into two new nuclei.
- Cytokinesis: The division of the cytoplasm, which results in the formation of two distinct daughter cells.

Essential for various biological processes in organisms:
- Reproduction: Asexual reproduction in single-celled organisms and sexual reproduction by forming gametes.
- Development: Growth from a fertilized egg (zygote) into a multicellular organism.
- Tissue Repair and Renewal: Replacing damaged or dead cells, and for overall growth and maintenance of tissues.
Ensures accurate and equal distribution of genetic material (chromosomes) to daughter cells, maintaining genetic continuity.

A cell's genome consists of all the genetic information (DNA) packaged into chromosomes.
In eukaryotes, DNA is highly organized: it is a double helix wrapped around histone proteins, forming chromatin. This chromatin condenses tightly into visible chromosomes during cell division.
The number of chromosome sets varies:
- Somatic cells (non-reproductive cells) are diploid ( $2n$ ) and have two sets of chromosomes (e.g., human somatic cells have $2n=46$ chromosomes).
- Gametes (reproductive cells like sperm and eggs) are haploid ( $n$ ) and have half the number of chromosomes (e.g., human gametes have $n=23$ chromosomes).

The cell cycle is divided into two major phases:
- M Phase: This includes both Mitosis (nuclear division) and Cytokinesis (cytoplasmic division).
- Interphase: The preparatory phase, further subdivided into:
  - G1 (first gap/growth): Cell grows and carries out normal metabolic functions; organelles replicate.
  - S (synthesis): DNA replication occurs, resulting in two sister chromatids for each chromosome.
  - G2 (second gap/growth): Cell continues to grow and synthesizes proteins and organelles in preparation for mitosis.
Interphase typically constitutes about $90\%$ of the entire cell cycle, with the M phase being much shorter.

Prophase: Chromatin condenses into visible sister chromatids (each composed of two identical DNA molecules). The mitotic spindle begins to form from centrosomes that start moving to opposite poles of the cell. Nucleoli disappear.
Prometaphase: The nuclear envelope completely breaks down. Microtubules from the spindle apparatus invade the nuclear area. Some microtubules (kinetochore microtubules) attach to kinetochores, protein structures at the centromere of each sister chromatid. Non-kinetochore microtubules overlap at the metaphase plate.
Metaphase: Chromosomes are precisely aligned along the metaphase plate, an imaginary plane equidistant from the two poles of the mitotic spindle. This alignment ensures equal distribution of genetic material.
Anaphase: Sister chromatids suddenly separate as the cohesin proteins holding them together are cleaved. Each chromatid is now considered an individual chromosome. These newly separated chromosomes are pulled by motor proteins along the kinetochore microtubules towards opposite poles of the cell. The cell elongates as non-kinetochore microtubules lengthen.
Telophase: Chromosomes arrive at the poles and begin to decondense. New nuclear envelopes form around the two sets of chromosomes. The mitotic spindle disassembles, and nucleoli reappear within the new nuclei. Mitosis, the division of one nucleus into two genetically identical nuclei, is now complete.

The physical division of the cytoplasm, which usually overlaps with the latter stages of mitosis.
In animal cells: A cleavage furrow forms, pinching the cell in two. This results from a contractile ring of actin microfilaments and myosin proteins.
In plant cells: A cell plate forms down the middle of the cell. Vesicles containing cell wall material fuse to form a new cell wall between the two daughter cells, separating them.

The cell cycle is meticulously controlled by specific signaling molecules in the cytoplasm, employing internal and external cues. Critical checkpoints (e.g., G1, G2, and M phases) ensure that the cell is ready to proceed.
Key regulatory proteins include:
- Cyclins: Proteins whose concentrations fluctuate throughout the cell cycle.
- Cyclin-dependent kinases (Cdks): Enzymes that are active only when bound to a specific cyclin. The cyclin-Cdk complex phosphorylates target proteins, triggering progression through the cell cycle stages.

Cancer is characterized by uncontrolled cell division and growth, resulting from a breakdown in cell cycle regulation.
Characteristics of cancer cells:
- Loss of cell cycle regulation: They ignore normal checkpoints and growth signals.
- Ability to divide indefinitely: They may continue to divide without limits (immortality).
- Formation of tumors: Abnormal masses of cells. These can be benign (non-spreading) or malignant (invasive and capable of spreading).
- Metastasis: Malignant tumor cells can spread to other parts of the body through the bloodstream or lymphatic system.
Treatments: Often involve targeting rapidly dividing cells. Chemotherapy drugs, for example, interfere with DNA replication or spindle formation, thereby inhibiting cell division in cancer cells, although they can also affect healthy rapidly dividing cells.