The Cell Cycle: Mechanisms and Regulation

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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, enabling organisms to reproduce, grow, and repair tissues. The cell cycle ensures that genetic material is accurately duplicated and distributed to daughter cells.

Cell division distinguishes living organisms from nonliving matter.
It is essential for reproduction, growth, development, and tissue renewal.

Cell division in a sea urchin embryo, showing two and four cell stages

Key Roles of Cell Division

Single-celled organisms reproduce by cell division.
Multicellular eukaryotes use cell division for embryonic development, growth, and tissue repair.
Cell division is highly accurate in passing DNA from one generation to the next.

Examples of cell division: asexual reproduction, growth and development, tissue renewal

Cellular Organization of Genetic Material

Chromosomes and Chromatin

All the DNA in a cell constitutes its genome. In eukaryotes, DNA is organized into multiple linear chromosomes, each consisting of chromatin—a complex of DNA and protein.

Somatic cells have two sets of chromosomes.
Gametes (sperm and eggs) have one set of chromosomes.

Eukaryotic chromosome structure: chromatin in the nucleus

Chromosome Duplication and Distribution

Before cell division, chromosomes are duplicated, resulting in two sister chromatids joined at a region called the centromere. During division, sister chromatids separate, ensuring each daughter cell receives an identical set of chromosomes.

Electron micrograph of a duplicated chromosome with sister chromatids and centromeres Diagram of chromosome duplication and separation of sister chromatids

Phases of the Cell Cycle

Overview of the Cell Cycle

The cell cycle consists of two main phases: Mitotic (M) phase and Interphase. Interphase is subdivided into G1, S, and G2 phases.

G1 phase: Cell growth and normal metabolic roles.
S phase: DNA replication (chromosome duplication).
G2 phase: Preparation for mitosis.
M phase: Mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Diagram of the cell cycle with interphase and mitotic phase

Mitosis: The Division of the Nucleus

Stages of Mitosis

Mitosis is conventionally divided into five stages:

Prophase
Prometaphase
Metaphase
Anaphase
Telophase

Stages of mitosis: G2 of interphase, prophase, prometaphase Stages of mitosis: metaphase, anaphase, telophase and cytokinesis

The Mitotic Spindle

The mitotic spindle is a structure made of microtubules that orchestrates the movement of chromosomes during mitosis. It forms from centrosomes, which replicate and move to opposite poles of the cell.

Each sister chromatid has a kinetochore, a protein complex at the centromere.
Spindle microtubules attach to kinetochores and move chromosomes.
At metaphase, chromosomes align at the metaphase plate.

Diagram of the mitotic spindle with centrosomes, kinetochores, and microtubules

Mechanisms of Chromosome Movement

During anaphase, separase cleaves cohesins, allowing sister chromatids to separate. Chromosomes move toward spindle poles by motor proteins and microtubule depolymerization (the "Pac-man" mechanism).

Experiment showing chromosome movement along microtubules during anaphase

Cytokinesis

Cytokinesis is the division of the cytoplasm, which follows mitosis. In animal cells, it occurs by cleavage, forming a cleavage furrow. In plant cells, a cell plate forms, eventually becoming the new cell wall.

Cleavage in animal cells and cell plate formation in plant cells during cytokinesis Stages of mitosis in plant cells, including cell plate formation

Cell Division in Prokaryotes: Binary Fission

Binary Fission

Prokaryotes (bacteria and archaea) divide by binary fission. The chromosome replicates, and the cell splits into two genetically identical daughter cells.

Diagram of binary fission in bacteria

Evolution of Mitosis

Mitosis likely evolved from binary fission. Some unicellular eukaryotes show intermediate forms of cell division.

Comparison of cell division in bacteria, dinoflagellates, diatoms, and most eukaryotes

Regulation of the Cell Cycle

Cell Cycle Control System

The cell cycle is regulated by a molecular control system with checkpoints at G1, G2, and M phases. Progression through the cycle depends on internal and external signals.

Cyclins and cyclin-dependent kinases (Cdks) are key regulatory proteins.
MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers passage into M phase.

Checkpoint	Main Function
G1	Checks for cell size, nutrients, growth factors, DNA damage
G2	Checks for DNA replication completion, DNA damage
M	Checks for chromosome attachment to spindle

Additional info: The G1 checkpoint is often called the "restriction point" in animal cells.

Internal and External Signals

Cells will not begin anaphase until all chromosomes are properly attached to the spindle.
Growth factors stimulate cell division; e.g., PDGF (platelet-derived growth factor).
Density-dependent inhibition and anchorage dependence regulate normal cell growth.

Loss of Cell Cycle Control in Cancer

Cancer cells do not respond to normal cell cycle controls. They may divide uncontrollably, form tumors, and invade other tissues (metastasis). Treatments include radiation and chemotherapy, which target dividing cells.

Benign tumors remain at the original site.
Malignant tumors invade tissues and can spread (metastasize).

Additional info: Cancer research is increasingly focused on personalized medicine, targeting specific cell-signaling pathways in tumors.