The Cell Cycle

The ability of organisms to reproduce distinguishes living organisms from nonliving matter.
Continuity of life is maintained through reproduction of cells, also known as cell division.

Definition: The cell cycle is a series of events leading to the division of a cell and the replication of its DNA to form two daughter cells.
It describes the events that occur in one round of cell division.

All life begins as a single cell (fertilized egg).
Critical for:
- Embryonic development.
- Growth and development of organisms.
- Generation of cells that are genetically identical to the parent cell.

Unicellular organisms: Division of a single cell results in reproduction of the organism.
Multicellular eukaryotes rely on cell division for:
- Development from a fertilized cell.
- Growth.
- Repair of tissues.

Most cell division results in daughter cells with identical genetic information (DNA).
Exception: Meiosis is a specialized type of division that generates sperm and egg cells with half the genetic content.

All DNA in a cell constitutes the cell’s genome.
A genome may consist of:
- A single DNA molecule (common in prokaryotic cells).
- Multiple DNA molecules (common in eukaryotic cells).
DNA in eukaryotic cells is organized into chromosomes.

Composed of chromatin, a complex of DNA and proteins.
Chromatin condenses during cell division.
Each eukaryotic species has a specific number of chromosomes in its nucleus.
Somatic cells contain two sets of chromosomes; gametes have half the number of chromosomes as somatic cells.

DNA Replication: In preparation for cell division, DNA is replicated and chromosomes condense.
Sister Chromatids: Each duplicated chromosome consists of two sister chromatids joined by cohesins.
Centromere: This is the region where two chromatids are most closely attached.
Separation of Sister Chromatids: During cell division, sister chromatids separate and move into two nuclei; once separate, they are termed chromosomes.

Eukaryotic cell division involves:
- Mitosis: Division of genetic material in the nucleus.
- Cytokinesis: Division of the cytoplasm.
Meiosis: Variability leads to nonidentical daughter cells that have half the chromosome number of the parent.

The cell cycle is composed of:
- Interphase: Accounts for approximately 90% of the cycle.
- Mitotic (M) Phase.
- Cytokinesis.
Interphase includes:
- G1 phase (First gap): Cell grows and performs normal functions; protein synthesis occurs.
- S phase: DNA is replicated; each chromosome consists of two sister chromatids.
- G2 phase (Second gap): Production of proteins necessary for mitosis; DNA begins to wind tightly around histones; errors in duplicated chromosomes are checked before division.

Definition: Division of the cytoplasm into two cells, each forming identical daughter cells with 46 chromosomes.
In animal cells, it occurs through a process called cleavage furrow formation.
In plant cells, a cell plate forms during this process.

Prokaryotic organisms (such as bacteria) reproduce through binary fission:
- Replication of the chromosome starts at the origin of replication.
- Plasma membrane pinches inward, dividing the cell into two.

Mitosis is thought to have evolved from binary fission in prokaryotes.
Certain protists display types of cell division intermediary between binary fission and mitosis.

The frequency of cell division varies among different cell types, reflecting molecular level regulation.
Cancer cells evade normal controls on the cell cycle.

The cell cycle operates via specific chemical signals in the cytoplasm, suggesting a control system analogous to a clock.
The system has specific checkpoints where the cycle can pause until a go-ahead signal is received.

Two regulatory proteins: Cyclins and Cyclin-dependent kinases (Cdks).
The activity of Cdks fluctuates with the levels of cyclins.
Maturation-promoting factor (MPF): A cyclin-Cdk complex that triggers advancement into the M phase.

Internally and externally derived signals regulate checkpoints.
The G1 checkpoint is deemed the most crucial; if a go-ahead signal is received, the cell usually completes S and M phases.
Without a go-ahead signal, a cell exits the cycle to enter a non-dividing state called G0 phase.

Internal signal: Cells must wait for all chromosomes to be properly attached before anaphase.
External signal: Growth factors, such as platelet-derived growth factor (PDGF), stimulate cell division; density-dependent inhibition encourages cells to stop dividing when crowded.
Anchorage dependence: Cells require attachment to a substrate to divide, checking cell growth at optimal density.

Cancer cells neglect normal growth controls; they may:
- Produce their own growth factors.
- Convey signals in the absence of growth factors.
- Display an abnormal cell cycle control system.
Cells capable of indefinite division undergo transformation.
Tumors can be benign (localized) or malignant (invading surrounding tissues). Malignant tumors can metastasize to form additional tumors in distant parts of the body.

Understanding the cell cycle has informed cancer treatment, facilitating personalized medicine approaches that consider specific DNA sequencing of tumors.