The Cell Cycle: Study Guide and Key Concepts

The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It ensures the accurate distribution of genetic material to daughter cells and is fundamental to growth, development, and tissue repair in multicellular organisms.

• Genome: The complete set of an organism’s DNA, including all of its genes.

• Chromosomes: Structures within cells that contain DNA and protein; they carry genetic information.

• Chromatin: The complex of DNA and proteins that makes up chromosomes.

• Somatic cells: Any cells in the body other than gametes (sperm and egg cells); they are diploid.

• Gametic cells: Reproductive cells (sperm and eggs); they are haploid.

Sister Chromatids and Centromeres

• Sister chromatids: Identical copies of a chromosome connected by a centromere, formed during DNA replication.

• Centromere: The region of a chromosome where the two sister chromatids are joined and where spindle fibers attach during cell division.

Mitosis and Cytokinesis

• Mitosis: The process of nuclear division in eukaryotic cells that results in two genetically identical daughter nuclei.

• Cytokinesis: The division of the cytoplasm, resulting in two separate daughter cells.

Interphase: G1, S, and G2 Stages

Interphase is the period of the cell cycle between cell divisions, consisting of three stages:

• G1 phase: Cell grows and carries out normal functions.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for mitosis by producing proteins and organelles.

Phases of Mitosis

• Prophase: Chromatin condenses into visible chromosomes; spindle fibers begin to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

• Telophase: Nuclear envelopes reform around the two sets of chromosomes; chromosomes decondense.

Mitotic Spindle, Centrosome, Kinetochore, and Metaphase Plate

• Mitotic spindle: Structure made of microtubules that separates chromosomes during mitosis.

• Centrosome: Organelle that serves as the main microtubule organizing center.

• Kinetochore: Protein structure on chromatids where spindle fibers attach.

• Metaphase plate: Imaginary plane where chromosomes line up during metaphase.

Cytokinesis: Cleavage in Animal and Plant Cells

• Animal cells: Cleavage furrow forms, pinching the cell into two.

• Plant cells: Cell plate forms, leading to the separation of the two daughter cells.

Binary Fission in Prokaryotes

• Prokaryotic cells divide by binary fission, a process where the cell grows, replicates its DNA, and divides into two identical cells.

• Origin of replication: Specific sequence where DNA replication begins.

Cell Cycle Control and Checkpoints

• Not all cells divide continuously; cell division is regulated by the cell cycle control system.

• Checkpoints: Control points where stop and go-ahead signals regulate the cycle (G1, G2, and M checkpoints).

• Function: Ensure that critical processes are completed before the cell proceeds to the next stage.

Regulation of the Cell Cycle

• Regulatory molecules: Cyclins and cyclin-dependent kinases (Cdks) regulate the cell cycle.

• Growth factors: External signals that stimulate cell division.

• Density-dependent inhibition: Cells stop dividing when they come into contact with each other.

• Anchorage dependence: Cells must be attached to a substrate to divide.

Cancer and Apoptosis

• Cancer: Caused by uncontrolled cell division due to mutations in genes that regulate the cell cycle.

• Apoptosis: Programmed cell death; prevents damaged cells from dividing uncontrollably.

Benign vs. Malignant Tumors

Benign Tumor	Malignant Tumor
Non-cancerous, does not spread	Cancerous, can invade other tissues (metastasize)
Usually encapsulated	Not encapsulated
Rarely life-threatening	Often life-threatening