Section 7: The Cell Cycle, Mitosis, and Meiosis

Key Roles of Cell Division

Cell division is a fundamental process in all living organisms, essential for reproduction, growth, development, and tissue repair. The mechanisms of cell division ensure the accurate transmission of genetic material from one generation of cells to the next.

• Reproduction: In single-celled organisms, cell division serves as a means of reproduction, producing two genetically identical offspring.

• Growth and Development: In multicellular organisms, cell division enables the development of an adult organism from a fertilized egg.

• Tissue Repair and Renewal: Cell division replaces damaged or dead cells in adult multicellular organisms, maintaining tissue health.

• Cell Cycle: The cell cycle is an ordered sequence of events in the life of a cell, from its origin to its own division into two daughter cells.

• Transmission of Genetic Material: Accurate distribution of genetic material (DNA) to daughter cells is a critical function of cell division.

Genetic Organization and the Genetic Material

The genetic material of a cell is organized into structures called chromosomes, which are composed of DNA and associated proteins. The organization and number of chromosomes differ between prokaryotes and eukaryotes.

• Genome: The complete set of genetic information in a cell. In prokaryotes, the genome is typically a single circular DNA molecule; in eukaryotes, it consists of multiple linear DNA molecules.

• Chromosomes: Structures that package DNA. Each eukaryotic chromosome is a single, linear DNA molecule associated with proteins.

• Chromatin: The complex of DNA and proteins that makes up chromosomes.

• Somatic Cells: All body cells except reproductive cells; contain two sets of chromosomes (diploid).

• Gametes: Reproductive cells (sperm and egg); contain one set of chromosomes (haploid).

Distribution of Chromosomes During Eukaryotic Cell Division

During cell division, chromosomes undergo a series of changes to ensure accurate segregation to daughter cells.

• When not dividing, chromosomes exist as loosely packed chromatin.

• After DNA replication, each chromosome consists of two sister chromatids, joined at a region called the centromere.

• During mitosis, sister chromatids separate and are distributed to two new nuclei, resulting in genetically identical daughter cells.

• Mitosis: Division of the nucleus and genetic material.

• Cytokinesis: Division of the cytoplasm, producing two separate cells.

The Cell Cycle

The cell cycle consists of a series of phases that prepare the cell for division and ensure the proper duplication and segregation of genetic material.

• Interphase: The period of cell growth and DNA replication, divided into three subphases:

  • G1 phase (First Gap): Cell growth and normal metabolic roles.

  • S phase (Synthesis): DNA replication; chromosomes duplicate.

  • G2 phase (Second Gap): Further growth and preparation for mitosis.

• Mitotic (M) Phase: Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division). This is the shortest part of the cell cycle.

Phases of Mitosis

Mitosis is a continuous process divided into five stages, ensuring the equal distribution of chromosomes to daughter cells.

1. Prophase: Chromatin condenses into visible chromosomes; mitotic spindle begins to form.

2. Prometaphase: Nuclear envelope breaks down; spindle fibers attach to kinetochores on chromosomes.

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

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

5. Telophase: Nuclear envelopes reform around the two sets of chromosomes; chromosomes begin to decondense.

Cytokinesis overlaps with the later stages of mitosis, dividing the cytoplasm and completing cell division.

The Mitotic Spindle: Structure and Function

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

• Centrosomes: Organize spindle formation and move to opposite poles of the cell.

• Microtubules: Three types are involved:

  • Kinetochore microtubules: Attach to kinetochores on chromosomes and move them.

  • Nonkinetochore microtubules: Interact with those from the opposite pole to elongate the cell.

  • Aster microtubules: Radiate from centrosomes and help anchor the spindle.

• Kinetochore: A protein structure at the centromere where spindle fibers attach.

• During anaphase, motor proteins and microtubule depolymerization move chromosomes toward the poles.

Cytokinesis: Animal vs. Plant Cells

Cytokinesis differs between animal and plant cells due to structural differences.

• Animal Cells: Cytokinesis occurs by cleavage. A contractile ring of actin filaments forms a cleavage furrow, pinching the cell in two.

• Plant Cells: Cytokinesis involves the formation of a cell plate. Vesicles from the Golgi apparatus coalesce at the center of the cell, forming a new cell wall that separates the daughter cells.

Binary Fission in Bacteria

Prokaryotes (bacteria and archaea) divide by a process called binary fission, which is simpler than mitosis.

• DNA replication begins at the origin of replication and proceeds bidirectionally.

• The cell elongates, and the plasma membrane pinches inward, dividing the cell into two genetically identical daughter cells.

• Binary fission does not involve mitosis.

Concepts of Heredity and Genetic Variation

While offspring inherit genes from their parents, genetic variation ensures that they are not identical to either parent. This variation is crucial for evolution and adaptation.

• Heredity: The transmission of traits from one generation to the next.

• Genetics: The scientific study of heredity and variation.

• Meiosis: A specialized form of cell division that produces haploid gametes in sexually reproducing organisms.

• Fertilization: The fusion of gametes restores the diploid chromosome number.

• Genetic Variation: Produced by meiosis and fertilization, contributing to evolution.

Summary Table: Comparison of Mitosis and Meiosis

Key Equations

• Chromosome Number in Gametes:

  • If a species has a diploid number of , then the haploid number in gametes is .

• DNA Replication:

  • During S phase:

Example

• If a chicken has 78 chromosomes in its somatic cells (2n = 78):

  • Each parent contributes 39 chromosomes (n = 39).

  • Each gamete contains 39 chromosomes.

  • Each somatic cell in the offspring will have 78 chromosomes.

Additional info: These notes expand on the provided slides and images, filling in standard academic context for General Biology students.