The Cell Cycle

Introduction to the Cell Cycle

The cell cycle is a fundamental process by which cells grow, duplicate their genetic material, and divide to produce new cells. This process is essential for growth, development, maintenance, and reproduction in all living organisms.

• Cell division is the process by which a parent cell divides into two or more daughter cells.

• In multicellular organisms, the cell cycle enables growth from a single cell to a complex organism and allows for tissue renewal and repair.

• Most cell division results in genetically identical daughter cells, ensuring continuity of genetic information.

Key Terms

• Anaphase: A stage of mitosis where sister chromatids separate.

• Binary fission: A type of cell division in prokaryotes.

• Cell cycle: The ordered sequence of events in the life of a cell, from its origin to its own division.

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

• Chromosome: A DNA molecule with part or all of the genetic material of an organism.

• Cytokinesis: Division of the cytoplasm to form two separate daughter cells.

• Genome: The complete set of genes or genetic material present in a cell or organism.

• Interphase: The phase of the cell cycle when the cell is not dividing, consisting of G1, S, and G2 phases.

• Meiosis: A type of cell division that reduces the chromosome number by half, producing gametes.

• Mitosis: Division of the nucleus, resulting in two genetically identical daughter nuclei.

• Somatic cells: Non-reproductive cells in multicellular organisms.

• Gametes: Reproductive cells (sperm and eggs) with half the number of chromosomes of somatic cells.

• Centromere: The region of a chromosome where the two sister chromatids are most closely attached.

• Centrosome: The microtubule organizing center in animal cells.

• Spindle: The structure made of microtubules that separates chromosomes during cell division.

• Kinetochore: A protein complex at the centromere that attaches chromosomes to spindle microtubules.

• Cleavage furrow: The indentation that begins the process of cytokinesis in animal cells.

• Cell plate: The structure that forms during cytokinesis in plant cells.

• Checkpoints: Control points in the cell cycle where stop and go-ahead signals regulate the cycle.

• Benign: Describes a tumor that does not invade surrounding tissue.

• Malignant: Describes a tumor that invades surrounding tissue and can metastasize.

• Metastatic: Refers to cancer cells that have spread from the original site to other parts of the body.

• Tumor: A mass of abnormal cells within otherwise normal tissue.

Roles of Cell Division

Functions in Organisms

Cell division is crucial for various biological processes in both unicellular and multicellular organisms.

• Reproduction: In unicellular organisms, cell division produces new individuals.

• Growth and Development: In multicellular organisms, it enables growth from a single cell to a complex organism.

• Tissue Renewal and Repair: Replaces damaged or dead cells in tissues.

• Cell division is highly accurate, ensuring faithful transmission of genetic material.

Genetic Material and Chromosomes

Organization of Genetic Material

The genetic material of a cell, its genome, is organized into chromosomes.

• In prokaryotes, the genome is usually a single circular DNA molecule.

• In eukaryotes, the genome consists of multiple linear DNA molecules packaged into chromosomes.

• Chromatin is the complex of DNA and proteins that forms chromosomes in eukaryotic cells.

• Humans have 46 chromosomes in somatic cells (23 pairs) and 23 chromosomes in gametes.

Chromosome Duplication and Distribution

Before a cell divides, its chromosomes are duplicated, resulting in two identical sister chromatids joined at the centromere.

• During cell division, sister chromatids separate and are distributed to two daughter cells.

• After separation, each chromatid is considered an individual chromosome.

The Cell Cycle Phases

Overview of the Cell Cycle

The cell cycle consists of two main stages: Interphase and the Mitotic (M) phase.

• Interphase (about 90% of the cycle): Includes cell growth and DNA replication.

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

Subphases of Interphase

• G1 phase (First Gap): Cell grows and carries out normal functions.

• S phase (Synthesis): DNA is replicated; chromosomes are duplicated.

• G2 phase (Second Gap): Cell prepares for division; additional growth occurs.

• Chromosomes are duplicated only during the S phase.

Phases of Mitosis

Mitosis is conventionally divided into five phases:

1. Prophase

2. Prometaphase

3. Metaphase

4. Anaphase

5. Telophase

Cytokinesis overlaps with the latter stages of mitosis.

The Mitotic Spindle and Chromosome Movement

Structure and Function

The mitotic spindle is a structure made of microtubules and associated proteins that orchestrates the movement of chromosomes during mitosis.

• In animal cells, spindle assembly begins at the centrosome, which duplicates during interphase.

• Spindle microtubules grow out from centrosomes, which migrate to opposite poles of the cell.

• An aster is a radial array of short microtubules extending from each centrosome.

• The spindle includes centrosomes, spindle microtubules, and asters.

Kinetochore and Chromosome Movement

• Kinetochore: A protein complex at the centromere where spindle fibers attach.

• During prometaphase, some spindle microtubules attach to kinetochores (kinetochore microtubules).

• Microtubules from each pole attach to kinetochores of each duplicated chromosome, exerting tension.

• At metaphase, chromosomes align at the metaphase plate, an imaginary plane equidistant from the spindle poles.

• During anaphase, proteins holding sister chromatids are cleaved, and chromatids move toward opposite poles.

• Microtubules shorten by depolymerizing at their kinetochore ends, and chromosomes are "reeled in" by motor proteins.

• Nonkinetochore microtubules elongate the cell by pushing against each other.

Cytokinesis

Mechanisms in Animal and Plant Cells

• In animal cells, cytokinesis occurs by cleavage, forming a cleavage furrow that pinches the cell in two.

• In plant cells, a cell plate forms, eventually developing into a new cell wall between the daughter cells.

Cell Division in Prokaryotes: Binary Fission

Process of Binary Fission

Prokaryotes (such as bacteria) reproduce by binary fission, a simpler form of cell division.

• The single, circular chromosome replicates at the origin of replication.

• As the chromosome replicates, each origin moves to opposite ends of the cell.

• The plasma membrane pinches inward, dividing the cell into two genetically identical daughter cells.

Evolutionary Context

• Mitosis likely evolved from binary fission, as prokaryotes predate eukaryotes.

• Certain protists exhibit cell division processes intermediate between binary fission and mitosis.

Regulation of the Eukaryotic Cell Cycle

Molecular Control System

The frequency and timing of cell division are regulated by a molecular control system involving checkpoints and signaling molecules.

• Checkpoints are control points where the cell cycle can be halted until conditions are favorable.

• The G1 checkpoint (restriction point) is especially important; if a cell does not receive a go-ahead signal, it enters a nondividing state called G0 phase.

• Regulatory proteins, including cyclins and cyclin-dependent kinases (Cdks), control progression through the cell cycle.

• Internal signals (e.g., unattached kinetochores at the M checkpoint) and external signals (e.g., growth factors) regulate the cycle.

External Signals and Cancer

• Growth factors are proteins that stimulate cell division (e.g., platelet-derived growth factor, PDGF).

• Density-dependent inhibition: Crowded cells stop dividing.

• Anchorage dependence: Most animal cells must be attached to a surface to divide.

• Cancer cells do not exhibit density-dependent inhibition or anchorage dependence and can divide uncontrollably.

Loss of Cell Cycle Control and Cancer

Characteristics of Cancer Cells

• Cancer cells do not respond to normal regulatory signals.

• They may produce their own growth factors or have abnormal control systems.

• Cancer cells can divide indefinitely (immortalized) and evade apoptosis (programmed cell death).

• Uncontrolled division leads to tumors:

  • Benign tumors: Remain at the original site.

  • Malignant tumors: Invade surrounding tissues and can metastasize to other parts of the body.

Cancer Treatment

• Localized tumors may be treated with radiation, which damages DNA in cancer cells.

• Metastatic cancers are treated with chemotherapy, which targets rapidly dividing cells.

• Advances in understanding cell cycle regulation have led to more personalized cancer therapies.

Summary Table: Comparison of Mitosis and Binary Fission

Key Equations

• DNA content after S phase:

• Chromosome number after mitosis: