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 is fundamental to the continuity of life, allowing organisms to grow, develop, and maintain their tissues.

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

• This process ensures that genetic information is accurately passed from one generation of cells to the next.

• Most cell division results in genetically identical daughter cells, except during the formation of gametes (meiosis).

Roles of Cell Division

Functions in Living Organisms

Cell division serves several essential roles in both unicellular and multicellular organisms.

• Reproduction: In unicellular organisms, cell division is a form of asexual reproduction, producing new individuals.

• Growth and Development: In multicellular organisms, cell division enables growth from a single cell (zygote) to a complex organism.

• Tissue Renewal and Repair: Cell division replaces damaged or dead cells, maintaining tissue health.

• Cell division is highly regulated and remarkably accurate, minimizing errors in genetic transmission.

Cellular Organization of Genetic Material

Genome and Chromosomes

The genome of a cell is its complete set of DNA, containing all genetic instructions.

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

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

• Each species has a characteristic number of chromosomes (e.g., humans have 46 in somatic cells).

• Somatic cells are non-reproductive cells; gametes (sperm and eggs) have half as many chromosomes as somatic cells.

Chromatin and Chromosome Structure

• Chromatin is the complex of DNA and proteins (mainly histones) that forms chromosomes.

• During most of the cell cycle, chromosomes exist as long, thin chromatin fibers.

• Before cell division, DNA is replicated and chromosomes condense, becoming visible under a microscope.

• Each duplicated chromosome consists of two sister chromatids joined at a region called the centromere.

Distribution of Chromosomes During Eukaryotic Cell Division

Overview of Mitosis and Cytokinesis

Cell division in eukaryotes involves two main processes:

• Mitosis: Division of the genetic material in the nucleus, resulting in two identical nuclei.

• Cytokinesis: Division of the cytoplasm, producing two separate daughter cells.

• Meiosis is a variation of cell division that produces gametes with half the number of chromosomes.

Process of Chromosome Duplication and Separation

• During interphase, DNA is replicated, and chromosomes are duplicated.

• Each duplicated chromosome has two sister chromatids.

• During mitosis, sister chromatids separate and are distributed to two daughter nuclei.

• Once separated, chromatids are considered individual chromosomes.

Phases of the Cell Cycle

Main Phases

The cell cycle consists of two major phases:

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

• Interphase: Period of cell growth and DNA replication, accounting for about 90% of the cell cycle.

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.

• Chromosome duplication occurs only during the S phase.

Phases of Mitosis

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

• Cytokinesis overlaps with the latter stages of mitosis.

The Mitotic Spindle

Structure and Function

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

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

• During prophase and prometaphase, centrosomes move to opposite poles of the cell.

• The spindle includes centrosomes, spindle microtubules, and asters (radial arrays of short microtubules).

Role of Kinetochores

• Kinetochores are protein complexes assembled at the centromeres of chromosomes.

• Spindle microtubules attach to kinetochores during prometaphase.

• Microtubules from each pole attach to the kinetochores of each duplicated chromosome, exerting tension and aligning chromosomes at the metaphase plate.

Chromosome Movement

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

• During anaphase, proteins holding sister chromatids together 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 that separates the daughter cells.

Binary Fission in Prokaryotes

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.

• After replication, the plasma membrane pinches inward, dividing the cell into two genetically identical cells.

Evolutionary Context

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

• Certain protists exhibit intermediate forms of cell division, with the nuclear envelope remaining intact.

Regulation of the Eukaryotic Cell Cycle

Molecular Control System

The frequency and timing of cell division are regulated by a molecular control system involving both internal and external signals.

• Key regulatory proteins include cyclins and cyclin-dependent kinases (Cdks).

• The cell cycle has checkpoints (G1, G2, and M) where the cycle can be halted until conditions are favorable.

• The G1 checkpoint (restriction point) is especially important; cells that do not pass this checkpoint enter a nondividing state called G0 phase.

Internal and External Signals

• Internal signals ensure that key processes (e.g., chromosome attachment to spindle) are complete before proceeding.

• External signals include growth factors (e.g., PDGF), density-dependent inhibition (cells stop dividing when crowded), and anchorage dependence (cells must be attached to a substrate to divide).

Loss of Cell Cycle Controls in Cancer

Cancer Cell Characteristics

• Cancer cells do not respond to normal regulatory signals.

• They may produce their own growth factors, have abnormal signaling pathways, or defective 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 (spread to other parts of the body).

Cancer Treatment and Research

• 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 based on tumor genetics.

Key Terms Table