Chapter 12: The Cell Cycle – Structure, Phases, and Regulation

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. All cells arise from preexisting cells through cell division. In multicellular organisms, development begins with a single-celled embryo that grows through repeated cell divisions.

• Mitosis produces all somatic (body) cells, which are genetically identical to the parent cell.

• Meiosis produces reproductive cells, called gametes, which have half the hereditary material and are genetically different from the parent cell.

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

Comparison of Mitosis and Meiosis

Process	Genetic Material	Daughter Cells
Mitosis	Copied and divided equally	Genetically identical to parent and each other
Meiosis	Half the amount of hereditary material	Genetically different from parent and each other

Phases of the Cell Cycle

Overview of Cell Cycle Phases

The cell cycle consists of an orderly sequence of events from the formation of a eukaryotic cell through the duplication of its chromosomes to cell division. It is divided into four main phases:

• G1 phase (First gap)

• S phase (DNA synthesis)

• G2 phase (Second gap)

• M phase (Mitosis and cytokinesis)

Interphase includes G1, S, and G2 phases, during which the cell grows, replicates organelles, and duplicates DNA.

Cell Cycle Diagram

Phase	Main Events
G1	Cell growth, preparation for DNA replication
S	DNA replication
G2	Preparation for mitosis, growth
M	Mitosis (nuclear division) and cytokinesis (cytoplasmic division)

Interphase and M Phase

• Interphase: Chromosomes are uncoiled; cell grows and fulfills specialized functions.

• M phase: Chromosomes condense and the cell divides.

• Cells spend most of their time in interphase.

Discovery of S Phase

Chromosome replication occurs during the S (Synthesis) phase of interphase. Although DNA is in the form of chromatin, this phase is conventionally referred to as chromosome replication.

Chromosomes and Chromatin

Structure of Chromosomes

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

• The DNA-protein complex 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 carry the same genes but may have different alleles.

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

Karyotypes Before and After S Phase

• Before S phase: Each chromosome is single (unreplicated).

• After S phase: Each chromosome consists of two sister chromatids.

Mitosis: The M Phase

Overview of M Phase

M phase consists of two distinct events:

• Mitosis: Division of the replicated chromosomes.

• Cytokinesis: Division of the cytoplasm.

Humans have 46 chromosomes. During mitosis, the chromosome number remains the same, but the chromatid number changes as sister chromatids separate.

Subphases of Mitosis

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

Prophase

• Chromosomes condense and become visible.

• The spindle apparatus forms to move chromosomes and pull chromatids apart.

Prometaphase

• Nuclear envelope breaks down.

• Microtubules attach to chromosomes at kinetochores (structures at the centromere).

• Kinetochore microtubules push and pull chromosomes to the spindle's center.

Metaphase

• Mitotic spindle is complete.

• Chromosomes align on the metaphase plate.

• Each chromosome is held by kinetochore microtubules from opposite poles.

• Astral microtubules hold spindle poles in place.

Anaphase

• Cohesins holding sister chromatids split.

• Sister chromatids are pulled to opposite poles, creating two identical sets of daughter chromosomes.

• Two forces: Kinetochore microtubules shrink; motor proteins of polar microtubules push poles apart.

Telophase

• A new nuclear envelope forms around each set of chromosomes.

• Chromosomes decondense.

• Mitosis is complete when two independent nuclei have formed.

Cytokinesis

• Occurs immediately after mitosis.

• Cytoplasm divides to form two daughter cells.

Cytokinesis in Plants vs. Animals

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

Bacterial Cell Division

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

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

Regulation of the Cell Cycle

Control of the Cell Cycle

Cell cycle length varies among cell types, mostly due to variation in the G1 phase. Rapidly dividing cells may eliminate G1, while nondividing cells remain in G0 (a resting state). Division rate can also vary in response to changing conditions, indicating regulation.

Key Regulators: Kinases and Phosphatases

• 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, turning the protein OFF.

M Phase Promoting Factor (MPF)

• MPF is a key regulator of the cell cycle, composed of cyclin-dependent kinase (Cdk) and cyclin.

• Cdk concentration remains constant; cyclin concentration cycles up and down during the cell cycle.

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

Turning Off MPF

• Negative feedback slows or shuts down processes by their products.

• Destruction of specific proteins is a common regulatory method.

• An enzyme complex (APC/C) tags cyclin with ubiquitin (Ub) for destruction, inactivating MPF.

Cell Cycle Checkpoints

Major Checkpoints

Three main checkpoints regulate progression through the cell cycle:

• G1 Checkpoint: Determines if the cell will continue or exit the cycle. Factors include cell size, nutrient availability, social signals, and DNA damage.

• G2 Checkpoint: Ensures proper chromosome replication and DNA integrity before mitosis.

• M-phase Checkpoint: Two points—between metaphase and anaphase (ensures all kinetochores are attached) and between anaphase and telophase (ensures chromosomes have fully separated).

Role of p53 and Tumor Suppressors

• p53 protein activates repair or initiates apoptosis if DNA is damaged.

• p53 is a tumor suppressor; mutations in p53 can lead to uncontrolled cell division and cancer.

Checkpoint Table

Checkpoint	Main Function	Key Factors
G1	Cell growth, DNA damage check	Size, nutrients, signals, p53
G2	DNA replication check	DNA integrity, MPF activation
M	Chromosome attachment and separation	Kinetochore attachment, chromosome separation

Summary

• The cell cycle is tightly regulated to ensure proper cell division and prevent the propagation of damaged cells.

• Key regulatory proteins and checkpoints maintain genomic integrity and prevent diseases such as cancer.

Additional info: The notes include expanded definitions, regulatory mechanisms, and checkpoint details for academic completeness.