The Cell Cycle and Cell Division

Overview of the Cell Cycle

The cell cycle is the series of events that take place in a cell leading to its division and duplication. It ensures that genetic material is accurately replicated and distributed to daughter cells. The cell cycle consists of three main stages: Interphase, Mitosis, and Cytokinesis.

• Interphase: The cell grows, performs its normal functions, and duplicates its DNA.

• Mitosis (M phase): The nucleus divides, distributing duplicated chromosomes to two daughter nuclei.

• Cytokinesis: The cytoplasm divides, resulting in two genetically identical daughter cells.

Functions of Cell Division

• Reproduction: Single-celled organisms reproduce by dividing.

• Growth: Multicellular organisms grow by increasing their cell number.

• Repair: Damaged or dead cells are replaced through cell division.

• Healing: Wounds are healed by the production of new cells.

Organization of Genetic Material

Genome and Chromosomes

The genome is the complete set of DNA in a cell. In prokaryotes, the genome is typically a single circular DNA molecule, while in eukaryotes, it consists of multiple linear DNA molecules organized into chromosomes. Chromosomes are structures that package and organize DNA, making it manageable during cell division.

Chromatin and Chromosome Structure

In eukaryotic cells, DNA is wrapped around proteins called histones, forming a complex known as chromatin. Chromatin condenses to form visible chromosomes during cell division. Each chromosome consists of two identical sister chromatids joined at a region called the centromere.

Chromosome Number and Types

• Somatic cells: Non-reproductive cells with two sets of chromosomes (diploid).

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

• Humans have 46 chromosomes (23 pairs): 22 pairs of autosomes and 1 pair of sex chromosomes (XX or XY).

Homologous Chromosomes

Each pair of chromosomes in a diploid cell consists of homologous chromosomes, which have the same shape, size, centromere position, and gene loci, but may carry different alleles.

Key Vocabulary

• Chromosome: Condensed form of DNA visible during cell division.

• Chromatid: One of two identical halves of a duplicated chromosome.

• Centromere: The region where sister chromatids are most closely attached.

• Gene: A segment of DNA that codes for a protein.

Phases of the Cell Cycle

Interphase

Interphase is the longest phase of the cell cycle, during which the cell grows, performs its normal functions, and prepares for division. It is subdivided into three stages:

• G1 phase: Cell grows and synthesizes organelles.

• S phase: DNA is replicated.

• G2 phase: Cell prepares for mitosis by producing proteins and organelles needed for division.

Mitosis

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

1. Prophase: Chromosomes condense, nuclear envelope breaks down, spindle fibers form, and centrioles migrate.

2. Metaphase: Chromosomes align at the metaphase plate, spindle fibers attach to kinetochores.

3. Anaphase: Sister chromatids are pulled apart to opposite poles of the cell.

4. Telophase: Chromosomes decondense, nuclear envelopes reform, and the cell begins to divide.

Cytokinesis

Cytokinesis is the division of the cytoplasm, resulting in two separate daughter cells. In animal cells, this occurs via the formation of a cleavage furrow, while in plant cells, a cell plate forms to divide the cell.

Regulation of the Cell Cycle

The cell cycle is tightly regulated by checkpoints (G1, G2, and metaphase checkpoints) to ensure that each phase is completed accurately before the next begins. This regulation prevents errors such as incomplete DNA replication or chromosome missegregation.

Summary Table: Key Terms and Concepts

Review Questions

1. List the four phases of mitosis.

2. During which phase is DNA replicated?

3. During which phase do sister chromatids separate?

4. Name the final stage of cell division, which divides the cytoplasm in two, forming two new daughter cells.

Additional info: The cell cycle and mitosis are fundamental for understanding growth, development, and maintenance in multicellular organisms. Errors in these processes can lead to diseases such as cancer.