Cell Cycle

Cell & Molecular Biology: Cell Cycle

Overview

  • Cell division is a critical process for all living organisms, distinguishing them from nonliving matter.

  • It plays key roles in growth, repair, and reproduction.

Key Stages of the Cell Cycle

Interphase

  • Function: The cell prepares for division.

  • Chromosome Duplication: DNA is copied accurately during this phase.

Mitosis

  • Process: Chromosomal copies are separated and moved to opposing cell ends.

Cytokinesis

  • Following mitosis, the cell divides into two daughter cells, which are genetically identical.

Cell Division Significance

  • Essential for:

    • Single-celled organisms to reproduce.

    • Multicellular organisms for growth and development.

    • Renewal and repair in mature organisms.

Genetic Material in Cells

The Genome

  • Definition: All the DNA in a cell forms the genome.

  • Variations:

    • Prokaryotes have single DNA molecules.

    • Eukaryotes have multiple DNA molecules organized into chromosomes.

Chromosomes and Genes

  • Each chromosome contains hundreds to thousands of genes.

  • Eukaryotic chromosomes are composed of chromatin, condensing during cell division.

  • Somatic cells have two sets of chromosomes, while gametes have one set (half).

Chromosome Duplication and Structure

Preparation for Division

  • DNA is replicated before division.

  • Each chromosome has two sister chromatids joined at the centromere.

Chromatid Characteristics

  • Sister chromatids are identical copies of the original chromosome, linked by cohesins.

  • Centromere is the narrow section where chromatids are tightly attached.

Process of Cell Division

Stages of Mitosis

  • Mitosis encompasses five stages:

    1. Prophase

    2. Prometaphase

    3. Metaphase

    4. Anaphase

    5. Telophase

Eukaryotic Cell Division

  • Involves mitosis and cytoplasm division (cytokinesis).

  • Meiosis: A special type of cell division for gamete formation, producing nonidentical daughter cells.

The Cell Cycle

Phases of the Cell Cycle

  • Divided into:

    • Mitotic (M) phase: Includes mitosis and cytokinesis.

    • Interphase: Occupies about 90% of the cycle, further divided into:

      • G1 phase: Initial growth.

      • S phase: DNA synthesis and chromosome duplication.

      • G2 phase: Further growth and preparation for mitosis.

Mitotic Spindle

Structure and Function

  • The mitotic spindle is crucial for chromosome movement during mitosis.

  • Centrosome: Organizes microtubules; duplicates during interphase and migrates during mitosis.

Spindle Components

  • Aster: Radial array of microtubules from each centrosome.

  • Kinetochore: Protein complex at the centromere; spindle microtubules attach to kinetochores during prometaphase.

Anaphase and Cytokinesis

Chromatid Separation

  • The enzyme separase cleaves cohesins during anaphase, allowing sister chromatids to separate and move to opposite poles.

Cleavage in Animal Cells

  • Cytokinesis in animals results from cleavage furrow formation.

    • In plants, cytokinesis involves the formation of a cell plate.

Binary Fission in Prokaryotes

  • Prokaryotic reproduction occurs through binary fission, where the chromosome replicates, and the cell divides via inward plasma membrane pinch.

Cell Cycle Regulation

Control Mechanisms

  • Cell division frequency varies by cell type and is tightly regulated at the molecular level.

Cancer and Cell Cycle Control

  • Cancer cells bypass normal regulations, leading to uncontrolled division.

  • The cell cycle control system includes checkpoints that ensure orderly division.

Checkpoints and Signals

  • Three crucial checkpoints are located at the G1, G2, and M phases.

  • A go-ahead signal at the G1 checkpoint allows the cell to proceed through the cycle, while a lack of signal pushes the cell into G0 state.

External Factors in Cell Division

Influential Factors

  • Chemical and physical factors affect cell division, including growth factors and density-dependent inhibition.

  • Anchorage dependence is required for most animal cells to divide.

Cancer Cell Behavior

Transformation of Cells

  • Cancer cells can divide indefinitely and may form benign or malignant tumors.

  • Benign tumors remain localized, while malignant tumors invade tissues and can metastasize.