Cancer, DNA Synthesis, Mitosis, and Meiosis

Overview

This chapter explores the cellular mechanisms underlying cancer, the processes of cell division (mitosis and meiosis), and the molecular basis of genetic inheritance. Understanding these topics is essential for grasping how cells grow, reproduce, and how errors in these processes can lead to diseases such as cancer.

Cancer: Cellular Basis and Risk Factors

Cellular Basis of Cancer

Cancer is a disease characterized by uncontrolled cell division. It arises when normal regulatory mechanisms of the cell cycle are disrupted, leading to the formation of abnormal masses of cells called tumors.

• Benign Tumors: Non-cancerous growths that do not invade surrounding tissues.

• Malignant Tumors: Cancerous growths that invade nearby tissues and can spread (metastasize) to other parts of the body.

• Metastasis: The process by which cancer cells break away from the original (primary) tumor and form new tumors in other locations.

Risk Factors for Cancer

Cancer risk is influenced by genetic and environmental factors, some of which are under individual control.

• Inherited Risk: Genetic mutations (e.g., BRCA1 gene) can increase susceptibility to certain cancers.

• Environmental Exposures: Contact with carcinogens (cancer-causing agents) such as tobacco smoke, UV radiation, and certain viruses (e.g., human papilloma virus).

• Synergistic Risk Factors: Combined behaviors (e.g., smoking and alcohol consumption) can amplify cancer risk beyond individual effects.

Reducing Cancer Risk

• Avoiding tobacco and excessive alcohol use

• Maintaining a healthy diet and weight

• Protecting skin from excessive sun exposure

• Receiving recommended vaccinations (e.g., HPV vaccine)

Cell Division: Mitosis and Meiosis

Normal Functions of Cell Division

Cell division is essential for growth, repair, and reproduction. There are two main types:

• Asexual Reproduction: Involves one parent; offspring are genetically identical to the parent (e.g., mitosis in somatic cells).

• Sexual Reproduction: Involves two parents; gametes combine to produce genetically unique offspring (e.g., meiosis in germ cells).

Chromosome Structure and Function

Chromosomes are structures made of DNA and proteins that carry genetic information.

• Genes: Segments of DNA that encode instructions for building proteins.

• Chromatin: Uncondensed, string-like form of DNA present before cell division.

• Chromatids: Duplicated chromosomes consist of two sister chromatids joined at the centromere.

DNA Replication

DNA replication is the process by which a cell copies its DNA before division.

• Structure: Double-stranded "twisted ladder" (double helix) with a sugar-phosphate backbone and nitrogenous base pairs (A-T, C-G).

• Process: The DNA helix splits in half; each strand serves as a template for a new complementary strand.

Equation:

Additional info: DNA polymerase is the enzyme responsible for synthesizing new DNA strands.

The Cell Cycle and Mitosis

Phases of the Cell Cycle

The cell cycle consists of three main steps:

• Interphase: Cell grows, duplicates organelles, and replicates DNA.

• Mitosis: Copied chromosomes are separated into two daughter nuclei.

• Cytokinesis: The parent cell splits into two daughter cells.

Events During Interphase

• Cell growth and organelle duplication

• DNA replication

• Protein synthesis required for cell division

Mitosis: Stages and Chromosome Movement

Mitosis ensures that each daughter cell receives an identical set of chromosomes.

• Prophase: Chromosomes condense; spindle forms; nuclear membrane breaks down.

• Metaphase: Chromosomes align at the cell's center.

• Anaphase: Sister chromatids are pulled apart to opposite sides.

• Telophase: Chromosomes decondense; nuclear membrane reforms.

Cytokinesis in Animal and Plant Cells

• Animal Cells: Cleavage furrow forms; contractile ring pinches the cell in two.

• Plant Cells: Cell plate forms between daughter cells; new cell wall is constructed.

Regulation of the Cell Cycle and Cancer Formation

Cell Cycle Regulation

Proteins called tumor suppressors and proto-oncogenes regulate the cell cycle.

• Tumor Suppressors: Proteins that inspect newly replicated DNA and prevent damaged cells from dividing (e.g., BRCA1).

• Proto-oncogenes: Genes that code for cell cycle control proteins; mutations can convert them to oncogenes, which overstimulate cell division.

Disregulation and Tumor Formation

• Mutations: Changes in DNA sequence can disrupt normal protein function, leading to uncontrolled cell division.

• Inherited or Environmental: Mutations may be inherited or caused by carcinogens.

Cancer Detection and Treatment

Detection Methods

• Early detection increases survival rates.

• Methods include surgical removal and analysis of cells or fluids.

• Benign tumors resemble normal tissue; malignant tumors do not.

Treatment Methods

• Chemotherapy: Uses chemicals to selectively kill dividing cells; combination therapy helps prevent resistance.

• Radiation Therapy: Uses high-energy particles to destroy cancer cells, especially after surgical removal.

Meiosis and Genetic Diversity

Cells Undergoing Meiosis

Meiosis is a specialized form of cell division that occurs in the gonads to produce gametes (egg and sperm).

• Reduces chromosome number by half (from diploid to haploid)

• Ensures each gamete receives one chromosome from each homologous pair

Major Events in Meiosis

• Meiosis I: Homologous chromosomes separate into two cells.

• Meiosis II: Sister chromatids separate, resulting in four haploid cells.

Genetic Diversity: Crossing Over and Random Alignment

• Crossing Over: Exchange of genetic material between homologous chromosomes during prophase I; creates new combinations of genes.

• Random Alignment: Homologous pairs align independently during metaphase I, resulting in genetically distinct gametes.

• Random Fertilization: The combination of gametes from two parents further increases genetic diversity.

Equation:

where is the number of chromosome pairs.

Errors in Meiosis: Nondisjunction

• Nondisjunction: Failure of chromosome pairs to separate properly, leading to gametes with abnormal chromosome numbers.

• Trisomy 21 (Down Syndrome): Most common human chromosome-number abnormality; three copies of chromosome 21.

Summary Table: Comparison of Mitosis and Meiosis

Additional info: The probability of any one sperm fertilizing any one egg is extremely high, contributing to genetic variation in offspring.