Chapter 9: The Cell Cycle – Structure, Function, and Division of Cells

Chapter 9 | The Cell Cycle

Key Roles of Cell Division

Cell division is a fundamental biological process that ensures the continuity of life by enabling organisms to reproduce, grow, and repair tissues. All cells arise from pre-existing cells through division.

• Unicellular organisms: Cell division serves as a means of asexual reproduction, producing genetically identical offspring.

• Multicellular organisms: Cell division is essential for growth, development, and tissue removal/repair.

• Principle: Cells originate only from pre-existing cells, supporting the cell theory.

Example: The division of a fertilized egg leads to the development of a multicellular organism; tissue repair occurs via cell division in wound healing.

Cellular Organization of Genetic Material

Genetic material is organized within cells to ensure accurate transmission during cell division. The structure and packaging of DNA differ between prokaryotes and eukaryotes.

• DNA (deoxyribonucleic acid): The molecule that contains genetic instructions for growth, function, and response to stimuli.

• Genome: The complete set of DNA in a cell or organism.

• Gene: A segment of DNA that codes for a specific protein and determines cell function.

• Locus: The specific location of a gene on a chromosome.

Additional info: In eukaryotes, DNA is linear and organized into multiple chromosomes; in prokaryotes, DNA is typically circular.

DNA Packaging and Chromosome Structure

DNA is highly organized and compacted to fit within the cell nucleus and facilitate accurate division.

• Nucleosome: DNA wrapped around a core of eight histone proteins, forming the basic unit of chromatin structure.

• Chromatin: The loose complex of DNA and proteins found in the nucleus during interphase.

• Chromosome: A tightly wound structure of chromatin, visible during cell division.

• Homologous chromosomes: Pairs of chromosomes with the same genes but possibly different alleles.

Example: Human somatic cells have 46 chromosomes (23 pairs), while gametes have 23 chromosomes.

Prokaryotic Cell Division: Binary Fission

Prokaryotes, such as bacteria, divide by a process called binary fission, which is simpler than eukaryotic mitosis.

• Binary fission: The circular DNA molecule is replicated, and each copy attaches to different parts of the cell membrane.

• The cell elongates, and the DNA copies are separated to opposite ends.

• A cross wall forms, dividing the cell into two genetically identical daughter cells.

Additional info: Binary fission allows rapid population growth in prokaryotes.

Eukaryotic Cell Cycle

The eukaryotic cell cycle describes the life of a cell from its formation to its division into two daughter cells. It consists of interphase and the mitotic (M) phase.

• Interphase: Period of cell growth and DNA replication, subdivided into G1, S, and G2 phases.

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

Interphase

• G1 phase (First Gap): Cell grows and performs normal metabolic functions.

• S phase (Synthesis): DNA replication occurs; each chromosome is duplicated, forming two sister chromatids joined at a centromere.

• G2 phase (Second Gap): Final preparations for cell division; cell grows and synthesizes proteins.

Example: After S phase, a human cell has 46 chromosomes, each consisting of two sister chromatids.

Mitotic (M) Phase

• Mitosis: Division of the nucleus to produce two genetically identical daughter nuclei.

• Cytokinesis: Division of the cytoplasm to form two separate daughter cells.

Mitosis: Stages and Key Events

Mitosis is a multi-step process that ensures equal distribution of genetic material to daughter cells.

1. Prophase: Chromatin condenses into visible chromosomes, each composed of two sister chromatids. The nuclear envelope begins to disassemble, and the mitotic spindle forms as centrioles move to opposite poles.

2. Metaphase: Chromosomes align along the metaphase plate (equatorial plane) of the cell, guided by kinetochore microtubules. Chromosomes are at their most condensed state.

3. Anaphase: Sister chromatids separate at the centromere and are pulled to opposite poles by spindle fibers. Once separated, chromatids are considered individual chromosomes.

4. Telophase: Daughter chromosomes arrive at the poles and decondense back into chromatin. Nuclear envelopes reform, and the mitotic spindle disassembles.

5. Cytokinesis: Division of the cytoplasm. In animal cells, a cleavage furrow forms; in plant cells, a cell plate develops.

Additional info: The result of mitosis and cytokinesis is two genetically identical daughter cells.

Summary Table: Stages of Mitosis

Stage	Main Events
Prophase	Chromatin condenses, spindle forms, nuclear envelope breaks down
Metaphase	Chromosomes align at metaphase plate
Anaphase	Sister chromatids separate and move to poles
Telophase	Chromosomes decondense, nuclear envelope reforms
Cytokinesis	Cytoplasm divides, forming two daughter cells

Key Terms and Concepts

• Centromere: Region where sister chromatids are joined; site of kinetochore formation.

• Sister chromatids: Identical copies of a chromosome connected at the centromere.

• Kinetochore: Protein structure on chromatids where spindle fibers attach during cell division.

• Mitotic spindle: Structure made of microtubules that segregates chromosomes during mitosis.

Equations and Formulas

• Chromosome number in humans:

$\text{Somatic cells: } 2n = 46$ $\text{Gametes: } n = 23$

• General equation for DNA replication:

$\text{Original DNA} \xrightarrow{\text{Replication}} \text{Two identical DNA molecules}$

Recap of Eukaryotic Cell Division

The eukaryotic cell cycle ensures that each daughter cell receives a complete set of genetic material. After division, cells return to interphase and may repeat the cycle as needed for growth, development, or repair.

• G1: Growth and normal function

• S: DNA replication

• G2: Growth and preparation for mitosis

• Mitosis: Nuclear division

• Cytokinesis: Cytoplasmic division

Example: Skin cells divide regularly to replace lost or damaged cells.