Chapter 12: The Cell Cycle

Learning Objectives

• Explain how mitosis results in genetically identical cells, using the terms chromosome, chromatid, and chromatin.

• Describe the events of interphase and the M phase.

• Identify three checkpoints that control the cell cycle and explain how they operate.

Concept 12.1: Most Cell Division Results in Genetically Identical Daughter Cells

Overview of Cell Division

Cell division is a fundamental process that enables organisms to reproduce, grow, and repair tissues. It is a defining characteristic of living things and ensures the continuity of life by producing new cells from pre-existing ones.

• Reproduction: Single-celled organisms reproduce by dividing to form new organisms.

• Development: Multicellular eukaryotes undergo embryonic development through repeated cell divisions.

• Renewal and Repair: Cell division maintains tissue health in multicellular organisms by replacing old or damaged cells.

• Genetic Continuity: During mitosis, identical copies of genetic material (DNA) are distributed to each daughter cell, ensuring genetic consistency.

Examples of Cell Division

• Asexual reproduction: Formation of new organisms from a single parent cell.

• Growth and development: Increase in cell number during organismal growth.

• Tissue renewal: Replacement of cells in tissues such as skin or blood.

Cellular Organization of the Genetic Material

Definition of a Genome

The genome is the complete set of DNA in a cell, containing all the genetic information required for cellular structure, function, and regulation.

• Prokaryotic cells: Typically possess a single, circular DNA molecule as their genome.

• Eukaryotic cells: Contain multiple, linear DNA molecules organized into chromosomes.

DNA Packaging into Chromosomes

DNA molecules are packaged into chromosomes to efficiently organize, protect, and distribute genetic material during cell division.

• Chromosomes: Structures composed of DNA and proteins (mainly histones) that condense during cell division.

• Function: Chromosome packaging allows for accurate segregation of DNA to daughter cells.

Chromosomes and Genes

Each chromosome contains hundreds to thousands of genes, which are segments of DNA that code for proteins or functional RNA.

• Chromatin: The combination of DNA and protein that makes up eukaryotic chromosomes. Chromatin condenses to form visible chromosomes during cell division.

Chromosome Number in Eukaryotic Cells

Each species has a characteristic number of chromosomes in its cells. For example, humans have 46 chromosomes in most cells.

• Somatic cells: Non-reproductive body cells containing two sets of chromosomes (diploid, 2n), one set from each parent.

• Gametes: Reproductive cells (sperm and eggs) containing half the number of chromosomes (haploid, n).

Comparison of Somatic Cells and Gametes

Key Terms

• Chromosome: A threadlike structure of nucleic acids and protein found in the nucleus, carrying genetic information.

• Chromatin: The material of which chromosomes are composed; consists of protein, RNA, and DNA.

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

• Genome: The complete set of genes or genetic material present in a cell or organism.

Example: Human Chromosome Number

• Humans have 46 chromosomes in somatic cells (diploid) and 23 chromosomes in gametes (haploid).

Additional info: Later sections of the chapter would cover the cell cycle phases, checkpoints, and the molecular mechanisms of mitosis and cytokinesis.