ch 7Cell Division and the Cell Cycle: Structure, Function, and Regulation

Cell Division and Cell Cycle

Introduction

Cell division is a fundamental process in all living organisms, enabling growth, development, and tissue repair. The cell cycle is the ordered sequence of events that leads to cell division and the production of two genetically identical daughter cells. This guide covers the structure of genetic material, the phases of the cell cycle, mitosis, cytokinesis, binary fission, and the regulation of cell division.

DNA Structure and Organization

Genome, Chromatin, and Chromosomes

• Genome: The complete set of a cell's DNA, including all its genes.

• Chromatin: Long, thin strands of unwound DNA packaged with proteins (histones); not visible under a microscope in this state.

• Chromosomes: Condensed, visible strands of chromatin, each consisting of a single DNA molecule. Chromosomes are only visible during cell division.

Example: Human somatic cells contain 46 chromosomes, while gametes contain 23.

Types of Cells and Chromosome Number

• Prokaryotes: Usually have one circular chromosome.

• Eukaryotes: Possess multiple linear chromosomes located in the nucleus.

• Somatic Cells: All body cells except gametes; diploid (2n), containing two sets of chromosomes.

• Gametes: Sperm and egg cells; haploid (n), containing one set of chromosomes.

The Cell Cycle

Phases of the Cell Cycle

• M phase (Mitosis): Division of the nucleus; shortest phase (~1 hour).

• G1 phase: First "gap" phase; cell grows and carries out normal functions; longest phase.

• S phase: Synthesis phase; DNA is replicated.

• G2 phase: Second "gap" phase; cell prepares for mitosis.

• G0 phase: Non-dividing state; cells may enter this phase from G1 and remain quiescent.

• Interphase: Includes G1, S, and G2 phases; the cell is not undergoing mitosis.

Mitosis (M Phase)

Overview

• Mitosis is the process by which somatic cells divide to produce two genetically identical daughter cells.

• Gametes are produced by meiosis, not mitosis.

• Before mitosis, DNA is duplicated, and centrosomes are also duplicated.

Phases of Mitosis

• Prophase:

  • Chromosomes condense and become visible.

  • Sister chromatids are apparent.

  • Nucleoli disappear.

  • Mitotic spindle begins to form from centrosomes; asters appear.

  • Centrosome pairs separate.

• Prometaphase:

  • Nuclear envelope dissolves.

  • Chromosomes are fully condensed and form kinetochores at the centromere.

  • Spindle microtubules attach to kinetochores and begin to move chromosomes.

• Metaphase:

  • Longest phase of mitosis (~20 minutes).

  • Centrosomes are at opposite poles.

  • Chromosomes align at the metaphase plate (imaginary plane).

  • Kinetochores of sister chromatids attach to opposite spindle poles.

• Anaphase:

  • Shortest phase of mitosis.

  • Sister chromatids separate and move toward opposite poles.

  • Each chromatid is now considered an independent chromosome.

  • The cell elongates.

• Telophase:

  • Two daughter nuclei form with nuclear envelopes.

  • Chromosomes begin to de-condense.

• Cytokinesis:

  • Division of the cytoplasm, usually begins during telophase.

  • Cleavage furrow (in animals) or cell plate (in plants) forms to separate the two cells.

The Mitotic Spindle

Structure and Function

• Composed of microtubules (MTs) anchored to centrosomes (microtubule organizing centers, MTOCs).

• Centrosomes duplicate during S phase in preparation for mitosis.

• Spindle fibers attach to chromosomes at kinetochores (located at the centromere).

Cytokinesis: Animal vs. Plant Cells

• Animal Cells: Cytokinesis occurs via a cleavage furrow that pinches the cell into two.

• Plant Cells: Cytokinesis occurs via the formation of a cell plate, which develops into a new cell wall between daughter cells. Vesicles from the Golgi apparatus contribute to the cell plate formation.

Binary Fission in Prokaryotes

Process

• Bacteria divide by binary fission, producing two genetically identical offspring.

• After DNA replication, the two copies of the circular chromosome attach to opposite sides of the plasma membrane.

• The cell elongates, and the plasma membrane and cell wall pinch inward, dividing the cell into two.

Regulation of the Cell Cycle

Importance of Regulation

• Cell division must be tightly regulated to ensure proper growth, development, and tissue maintenance.

• Uncontrolled cell division can lead to cancer.

• Some cells rarely divide (e.g., nerve and muscle cells), some always divide (e.g., skin, gut, blood cells), and some divide only when needed (e.g., during wound repair).

Key Regulatory Mechanisms

• Oncogenes: Genes that promote cell division; mutations can lead to cancer.

• Tumor Suppressors: Genes that inhibit cell division; loss of function can result in uncontrolled growth.

Protein Families Involved in Cell Cycle Control

• Cyclins: Proteins whose levels fluctuate during the cell cycle; they activate CDKs.

• Cyclin-dependent kinases (CDKs): Enzymes that are always present but require cyclins to be active. The cyclin-CDK complex drives the cell through different phases of the cell cycle.

Cell Cycle Checkpoints

• Checkpoints are control mechanisms that ensure the cell is ready to proceed to the next phase.

• Major checkpoints: G1, G2, and M phase checkpoints.

• External signals (growth factors) can promote or inhibit cell cycle progression.

• If DNA is damaged, checkpoints can halt the cycle to allow for repair or trigger cell death if damage is irreparable.

Checkpoint	Main Function	Key Regulatory Proteins
G1 Checkpoint	Checks for cell size, nutrients, growth factors, and DNA damage	Cyclin D/CDK4, tumor suppressors (e.g., p53)
G2 Checkpoint	Checks for DNA replication completion and DNA damage	Cyclin B/CDK1 (MPF)
M Checkpoint	Ensures all chromosomes are attached to spindle before anaphase	Anaphase-promoting complex (APC/C)

Key Equations and Concepts

• Diploid Number (2n): Total number of chromosomes in somatic cells.

• Haploid Number (n): Number of chromosomes in gametes.

• Cell Cycle Progression: Driven by the accumulation and degradation of cyclins, which activate CDKs at specific checkpoints.

Example: The MPF (Maturation Promoting Factor) is a cyclin-CDK complex that triggers the cell's entry into mitosis.

Additional info: The cell cycle is a highly conserved process across eukaryotes, and its dysregulation is a hallmark of cancer. Understanding the molecular mechanisms of cell cycle control is crucial for developing targeted therapies in oncology.