The Cell Cycle: Regulation and Mechanisms of Eukaryotic Cell Division

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 that genetic material is accurately replicated and distributed to daughter cells. Cell division is fundamental for growth, development, and tissue repair in multicellular organisms.

• Cell division arises from preexisting cells, as observed in developing embryos of plants and animals.

• Meiosis produces reproductive cells called gametes (sperm and eggs).

• Mitosis produces all other cell types, known as somatic cells.

• Both mitosis and meiosis are usually accompanied by cytokinesis, the division of the cytoplasm into daughter cells.

Comparison of Mitosis and Meiosis

	Mitosis	Meiosis
Genetic Material	Copied and divided equally between two cells	Produces cells with half the amount of hereditary material as the parent cell
Daughter Cells	Genetically identical to the parent cell and to each other	Genetically different from the parent cell and from each other

How Do Cells Replicate?

Basic Steps of Cellular Replication

1. Copying the DNA

2. Separating the copies

3. Dividing the cytoplasm to create two complete cells

These steps ensure that each daughter cell receives a complete set of genetic instructions.

The Four Phases of the Cell Cycle

Overview of the Cell Cycle

The cell cycle is an orderly sequence of events from the formation of a eukaryotic cell through the duplication of its chromosomes to the time it undergoes cell division.

• M phase (Mitotic phase): Cell divides (mitosis and cytokinesis)

• Interphase: Nondividing phase, includes:

  • G1 (First gap)

  • S (Synthesis)

  • G2 (Second gap)

Phases of the Cell Cycle

Phase	Description
G1 (First Gap)	Cell grows and performs normal functions
S (Synthesis)	DNA is replicated
G2 (Second Gap)	Cell prepares for division
M (Mitotic Phase)	Division of nucleus (mitosis) and cytoplasm (cytokinesis)

Interphase vs. M Phase

• Interphase (G1, S, G2): Chromosomes are uncoiled; cell grows, replicates organelles, and prepares for division.

• M Phase: Chromosomes condense and cell divides.

Discovery of S Phase

• Chromosome replication occurs during the S (Synthesis) phase of interphase.

• DNA is replicated as chromatin, but this is still referred to as chromosome replication.

Chromosomes and Their Structure

What Is a Chromosome?

• A chromosome is a single long double helix of DNA wrapped around proteins called histones.

• DNA-protein material is called chromatin.

• DNA encodes the cell's genetic information.

• A gene is a section of DNA that codes for a specific RNA and, therefore, a specific protein.

Homologous Chromosomes and Sister Chromatids

• Homologous chromosomes have the same genes but may have different alleles (versions of a gene).

• After replication, each chromosome consists of two sister chromatids joined at the centromere.

Karyotypes

• A karyotype is an organized profile of an individual's chromosomes.

• Before S-phase: chromosomes are unreplicated.

• After S-phase: chromosomes are replicated, each consisting of two sister chromatids.

M Phase: Mitosis and Cytokinesis

Overview of M Phase

• M phase consists of two events:

  1. Mitosis – division of the replicated chromosomes

  2. Cytokinesis – division of the cytoplasm

• Humans have 46 chromosomes; chromosome number remains constant through mitosis.

Phases of Mitosis

• Prophase: Chromosomes condense and become visible; spindle apparatus forms to move chromosomes.

• Prometaphase: Nuclear envelope breaks down; microtubules attach to chromosomes at kinetochores (structures at the centromere).

• Metaphase: Chromosomes align at the metaphase plate; spindle is complete; astral microtubules hold spindle poles in place.

• Anaphase: Cohesins holding sister chromatids split; chromatids are pulled to opposite poles by spindle fibers; two identical sets of chromosomes are created.

• Telophase: New nuclear envelopes form around each set of chromosomes; chromosomes decondense; mitosis is complete when two independent nuclei have formed.

Cytokinesis

• Typically occurs immediately after mitosis.

• Divides the cytoplasm to form two daughter cells.

Cytokinesis in Plants	Cytokinesis in Animals
Vesicles from the Golgi apparatus bring membrane and cell wall components to the middle of the cell, forming a cell plate.	A ring of actin and myosin filaments contracts inside the cell, forming a cleavage furrow that pinches the cell in two.

Bacterial Cell Replication

• Bacteria divide via binary fission, a process similar to eukaryotic M phase.

• Bacterial chromosomes are replicated, and proteins attach to chromosomes to separate them; other proteins divide the cytoplasm.

Control of the Cell Cycle

Regulation of the Cell Cycle

• Cycle length varies among cell types, mostly due to variation in G1 phase.

• Rapidly dividing cells may eliminate G1 phase; nondividing cells are permanently in G0 phase.

• Division rate can change in response to environmental conditions, suggesting the cycle is regulated.

Key Regulatory Molecules

• Kinase: An enzyme that attaches a phosphate group from ATP to a protein, turning the protein ON.

• Phosphatase: An enzyme that removes a phosphate group from a protein, turning the protein OFF.

• MPF (M Phase Promoting Factor): A key regulator composed of cyclin-dependent kinase (Cdk) and cyclin.

• Cyclin concentration cycles up and down during the cell cycle, while Cdk concentration remains constant.

Activation and Deactivation of MPF

• When cyclin concentrations are high, MPF is active, phosphorylating target proteins and initiating mitosis.

• MPF is turned off by negative feedback and destruction of cyclin via ubiquitin tagging and proteasome degradation.

Cell Cycle Checkpoints

Overview of Checkpoints

• Checkpoints are regulatory points where the cell decides whether to proceed with division.

• If checkpoints fail, cells may divide uncontrollably, leading to tumor formation.

Major Cell Cycle Checkpoints

Checkpoint	Location	Purpose
G1 Checkpoint	End of G1 phase	Determines if cell will divide or enter G0; checks size, nutrients, social signals, and DNA damage
G2 Checkpoint	End of G2 phase	Ensures DNA replication is complete and undamaged before mitosis
M Checkpoint	During M phase (metaphase/anaphase and anaphase/telophase)	Ensures chromosomes are properly attached to spindle and fully separated before division completes

Role of p53 and Tumor Suppressors

• p53 protein is a tumor suppressor that halts the cell cycle if DNA is damaged, allowing for repair or triggering apoptosis (programmed cell death).

• Mutations in p53 are found in 50-60% of human cancers, leading to loss of cell cycle control.

Summary Table: Cell Cycle Phases and Key Events

Phase	Main Event
G1	Cell growth, normal function
S	DNA replication
G2	Preparation for mitosis
Mitosis	Division of chromosomes (prophase, prometaphase, metaphase, anaphase, telophase)
Cytokinesis	Division of cytoplasm

Key Equations and Terms

• Phosphorylation: $\text{Protein} + \text{ATP} \rightarrow \text{Phosphorylated Protein} + \text{ADP}$

• Ubiquitination: $\text{Cyclin} + \text{Ubiquitin} \rightarrow \text{Cyclin-Ub} \rightarrow$ degradation

Example Application

• During tissue repair, cells at the wound site are stimulated to re-enter the cell cycle and divide, ensuring proper healing.

• Loss of checkpoint control can result in cancer, where cells divide uncontrollably.

Additional info: The notes above integrate and expand upon the provided slides and text, ensuring a comprehensive, self-contained summary suitable for college-level General Biology students.