Cell Cycle and Mitosis

Cell Cycle

• The cell cycle consists of three phases: Mitotic (M) phase, Interphase, and Cytokinesis
• Interphase can be divided into three subphases: G1 (first gap), S (synthesis), and G2 (second gap)
• The cell grows during all three phases, but chromosomes are duplicated only during the S phase

Mitosis

• Mitosis is divided into five phases: Prophase, Prometaphase, Metaphase, Anaphase, and Telophase
• Cytokinesis is well underway by late Telophase

Cytokinesis

• In animal cells, a ring of actin filaments forms at the metaphase plate, which contracts and divides the cell in two
• In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast, which grows into a cell plate that divides the cell in two

Chemosynthesis

• Chromosomes are duplicated during the S phase of the cell cycle

Cell Cycle Control System

• The cell cycle control system is regulated by both internal and external controls
• The cell cycle stops at specific checkpoints (G1, G2, and M) until a go-ahead signal is received
• If a cell receives a go-ahead signal at the G1 checkpoint, it will usually complete the S, G2, and M phases and divide

Chromosomes

• A chromosome consists of a DNA molecule packed together with proteins
• Bacterial chromosomes are double-stranded, circular DNA molecules associated with a small amount of protein
• Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein
• In eukaryotic cells, DNA is precisely combined with proteins in a complex called chromatin
• Chromatin undergoes changes in packing during the cell cycle
• Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis

Chromatin Structure

• Nucleosomes are the basic units of DNA packaging
• Each nucleosome is composed of two each of the four basic histone types, with DNA wrapped twice around the core of the eight histones
• Histone tails protrude from the nucleosome and are involved in the regulation of gene expression
• Chromatin is organized into a 10-nm fiber, which is further compacted into a 30-nm fiber through folding and looping
• During interphase, chromosomes occupy specific restricted regions in the nucleus, and the fibers of different chromosomes do not become entangled

Mitosis and Cytokinesis

• During mitosis, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
• In anaphase, the microtubules shorten by depolymerizing at their kinetochore ends

Kinetochore

• The kinetochore is a structure composed of several proteins that associate with the centromere region of a chromosome and can bind to spindle microtubules
• The kinetochore is responsible for chromosome movement during mitosis### Cell Division and the Cell Cycle
• The centromere region of a metaphase chromosome is where DNA binds with spindle proteins.

Cell Cycle Stages

• The cell cycle consists of multiple stages: G1, S, G2, and Mitotic (M) phase.
• Mitotic phase is further divided into:
  • Prophase
  • Prometaphase
  • Metaphase
  • Anaphase
  • Telophase and Cytokinesis

Cell Cycle Regulation and Cancer

• Loss of cell cycle control can lead to unregulated cell growth and cancer.
• Important terms related to loss of cell cycle control:
  • Transformation: a process by which a normal cell becomes cancerous.
  • Benign tumor: a non-cancerous growth that does not invade nearby tissues.
  • Malignant tumor: a cancerous growth that invades nearby tissues.
  • Metastasis: the spread of cancer cells from one part of the body to another.