The Cell Cycle and Cell Division

The Cell Cycle and Cell Division

Introduction to the Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It is fundamental for growth, development, and tissue repair in multicellular organisms, and for reproduction in unicellular organisms.

• Cell division enables unicellular organisms to reproduce and allows multicellular organisms to grow, repair, and renew tissues.

• Most cell division results in the distribution of identical genetic material to two daughter cells.

Cellular Organization of DNA

Genetic information is stored in the DNA of cells, which is organized into structures called chromosomes.

• Genome: All the DNA in a cell.

• Chromosomes: DNA molecules packaged with proteins (chromatin).

• Each eukaryotic species has a characteristic number of chromosomes in each cell nucleus.

• Somatic cells: Nonreproductive cells with two sets of chromosomes.

• Gametes: Reproductive cells (sperm and eggs) with one set of chromosomes.

Chromosomes

Before a cell divides, it duplicates its DNA, resulting in duplicated chromosomes composed of two sister chromatids.

• Sister chromatids: Identical copies of a chromosome, joined at a region called the centromere.

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

• Once separated, chromatids are considered individual chromosomes.

The Cell Cycle

The cell cycle consists of interphase (cell growth and DNA replication) and the mitotic (M) phase (mitosis and cytokinesis).

• Interphase: About 90% of the cell cycle; divided into three subphases:

  • G1 phase: Cell growth

  • S phase: DNA synthesis (replication)

  • G2 phase: Preparation for mitosis

• Mitotic (M) phase: Includes mitosis (division of the nucleus) and cytokinesis (division of the cytoplasm).

Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical sets. It is divided into five phases:

• Prophase: Chromosomes condense, spindle apparatus begins to form.

• Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids separate and move toward opposite poles.

• Telophase: Nuclear envelopes reform around the two sets of chromosomes.

Cytokinesis overlaps with the latter stages of mitosis, dividing the cytoplasm and forming two daughter cells.

Mitotic Spindle

The mitotic spindle is a structure made of microtubules and proteins that segregates chromosomes during mitosis.

• Centrosome: Microtubule organizing center in animal cells.

• Aster: Radial array of short microtubules extending from each centrosome.

• Kinetochores: Protein complexes on chromosomes where spindle fibers attach.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells.

• In animal cells, cytokinesis occurs by cleavage, forming a cleavage furrow.

• In plant cells, a cell plate forms during cytokinesis.

Binary Fission in Bacteria

Bacteria reproduce by binary fission, a process in which the cell grows, replicates its DNA, and divides into two identical daughter cells.

1. Chromosome replication begins at the origin of replication.

2. Each copy of the origin moves to opposite ends of the cell.

3. Replication finishes and the cell divides.

Cell Cycle Control System

The cell cycle is regulated by internal and external controls, ensuring proper division and preventing errors.

• Checkpoints: Control points where the cell cycle can be stopped until conditions are favorable.

• Regulated by proteins such as cyclins and cyclin-dependent kinases (Cdks).

• Growth factors: Proteins that stimulate cell division.

Factors Influencing Cell Division

• Density-dependent inhibition: Crowded cells stop dividing.

• Anchorage dependence: Cells must be attached to a surface to divide.

• Cancer cells do not exhibit density-dependent inhibition or anchorage dependence.

Cancerous Cells

Cancer results from uncontrolled cell division due to the breakdown of regulatory mechanisms.

• Transformation: Conversion of a normal cell to a cancerous cell.

• Benign tumors: Abnormal cells remain at the original site.

• Malignant tumors: Invade surrounding tissues and can metastasize (spread to other parts of the body).

Summary Table: Key Differences in Cell Division

Process	Organisms	Purpose	Key Features
Mitosis	Eukaryotes	Growth, repair, asexual reproduction	One division, two identical daughter cells
Binary Fission	Prokaryotes (Bacteria)	Asexual reproduction	Simple division, no mitotic spindle