The Cell Cycle: Structure, Function, and Regulation

Ch. 12 – The Cell Cycle

Introduction to Cell Division

Cell division is the process by which a single parent cell divides to produce two or more daughter cells. This process is fundamental for growth, development, and maintenance in all living organisms.

• Fission: Division in prokaryotes (binary fission) and some single-celled eukaryotes.

• Mitosis: Division of somatic (body) cells, producing genetically identical daughter cells.

• Meiosis: Division in germ cells, producing gametes (sperm and egg) with half the number of chromosomes.

Example: In humans, somatic cells divide by mitosis, while gametes are produced by meiosis.

Asexual vs. Sexual Reproduction

Organisms reproduce either asexually (offspring from a single parent) or sexually (offspring from two parents).

• Asexual Reproduction: Offspring are genetically identical to the parent (clones). Example: binary fission in bacteria.

• Sexual Reproduction: Offspring inherit a combination of genes from both parents, resulting in genetic diversity.

Importance: Cell division is essential for reproduction, growth, tissue repair, and development.

Organization of DNA in the Cell

Genetic material is organized as DNA, which is packaged into chromosomes.

• Genome: The complete set of an organism’s DNA.

• Chromatin: DNA associated with proteins (histones) in a loose form in non-dividing cells.

• Chromosomes: Highly condensed chromatin, visible during cell division.

• Chromatid: Each of the two identical halves of a replicated chromosome.

Example: During cell division, chromatin condenses into chromosomes, each consisting of two sister chromatids joined at a centromere.

DNA Replication and Chromosome Structure

Before a cell divides, its DNA is replicated so that each daughter cell receives an identical set of chromosomes.

• DNA Replication: The process of copying DNA to produce two identical DNA molecules.

• Sister Chromatids: The two identical copies of a chromosome, attached at the centromere.

Equation:

$\text{Chromosome (before replication)} \xrightarrow{\text{DNA Replication}} \text{Chromosome with 2 sister chromatids}$

Introduction to the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase (cell growth and DNA replication) and the mitotic phase (mitosis and cytokinesis).

• Interphase: Includes G1 (cell growth), S (DNA synthesis), and G2 (preparation for mitosis).

• Mitotic Phase (M phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Example: Most cells spend the majority of their time in interphase.

Interphase

Interphase is the non-dividing stage where the cell grows, replicates its DNA, and prepares for division.

• G1 Phase: Cell growth and normal functions.

• S Phase: DNA replication, resulting in duplicated chromosomes (sister chromatids).

• G2 Phase: Further growth and preparation for mitosis.

Sub-phase: G0 is a resting phase where cells exit the cycle and do not divide.

Centrosomes and Mitotic Spindle

The centrosome is the microtubule-organizing center in animal cells, which duplicates during S phase and forms the mitotic spindle during mitosis.

• Mitotic Spindle: Structure made of microtubules that separates chromosomes during mitosis.

Phases of Mitosis

Mitosis is the process of dividing the nucleus and genetic material of a somatic cell. It consists of several phases:

• Prophase: Chromosomes condense, spindle apparatus forms, nuclear envelope begins to break down.

• Prometaphase: Nuclear envelope fragments, spindle fibers attach to chromosomes at kinetochores.

• Metaphase: Chromosomes align at the metaphase plate (center of the cell).

• Anaphase: Sister chromatids are pulled apart toward opposite poles of the cell.

• Telophase: Nuclear envelopes reform around separated chromosomes, which decondense.

Mnemonic: Prophase, Prometaphase, Metaphase, Anaphase, Telophase ("Please Pass Me Another Taco").

Cytokinesis

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

• Animal Cells: Cytokinesis occurs by cleavage, forming a cleavage furrow.

• Plant Cells: Cytokinesis occurs by formation of a cell plate, which develops into a new cell wall.

Cell Cycle Regulation

The cell cycle is regulated by checkpoints and signaling molecules to ensure proper division and prevent errors.

• Checkpoints: G1, G2, and M checkpoints monitor cell size, DNA integrity, and chromosome attachment to the spindle.

• Growth Factors: Proteins that stimulate cell division.

• Example: The G1 checkpoint is crucial for determining whether a cell will divide.

Cancer and Cell Cycle Control

Cancer results from uncontrolled cell division due to mutations in genes that regulate the cell cycle.

• Malignant Tumors: Invade surrounding tissues and can metastasize (spread).

• Benign Tumors: Remain localized and do not invade other tissues.

• Proto-oncogenes: Genes that promote cell division; mutations can convert them to oncogenes, leading to cancer.

• Tumor Suppressor Genes: Genes that inhibit cell division; loss of function can lead to uncontrolled growth.

Gene Type	Normal Function	Effect of Mutation
Proto-oncogene	Promotes cell division	Uncontrolled division (oncogene)
Tumor Suppressor	Inhibits cell division	Loss of inhibition, uncontrolled growth

Summary Table: Phases of the Cell Cycle

Phase	Main Events
G1	Cell growth, normal functions
S	DNA replication
G2	Preparation for mitosis
Mitosis	Nuclear division (prophase, prometaphase, metaphase, anaphase, telophase)
Cytokinesis	Cytoplasmic division

Additional info: The cell cycle is tightly regulated to prevent errors in DNA replication and chromosome segregation, which can lead to diseases such as cancer. Understanding the cell cycle is fundamental for fields such as genetics, developmental biology, and medicine.