Cell Division and Cell Death

Introduction

Normal growth and development in multicellular organisms require a precise balance between cell division and cell death. These processes are essential for tissue formation, maintenance, and the removal of damaged or unnecessary cells.

• Mitosis: The process of cell division that produces two genetically identical somatic cells from one parent cell.

• Apoptosis: Programmed cell death, a genetically regulated sequence that eliminates cells in a controlled manner.

Mitosis: Cell Division

Definition and Purpose

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells. This process is fundamental for growth, tissue repair, and asexual reproduction in multicellular organisms.

• Produces somatic cells (body cells, not gametes).

• Ensures genetic consistency across cells.

• Occurs in all tissues requiring growth or repair.

Stages of Mitosis

Mitosis is divided into several distinct phases, each characterized by specific cellular events:

• Interphase: Cell grows, replicates DNA, and prepares for division.

• Prophase: Chromatin condenses into visible chromosomes; spindle fibers begin to form.

• Prometaphase: Nuclear envelope breaks down; spindle fibers attach to chromosomes.

• Metaphase: Chromosomes align at the cell's equatorial plate (metaphase plate).

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

• Telophase: Chromosomes arrive at poles; nuclear envelope reforms; chromatin relaxes.

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

Key Terms: Chromosome, chromatid, spindle fiber, centromere, diploid, cytokinesis, interphase, homologous chromosomes.

Example

• Skin cells dividing to replace those lost due to injury.

Apoptosis: Programmed Cell Death

Definition and Biological Role

Apoptosis is a tightly regulated process of programmed cell death that removes cells in a controlled manner. It is essential for proper development, tissue homeostasis, and defense against disease.

• Eliminates cells that are no longer needed or are potentially harmful.

• Prevents the accumulation of damaged or abnormal cells.

• Involves a genetically programmed sequence of events.

Examples and Applications

• Formation of fetal fingers and toes: Apoptosis removes tissue between developing digits.

• Menstruation: Shedding of the uterine lining involves apoptosis of endometrial cells.

• Neural development: Apoptosis eliminates excess neurons to refine neural connections.

Protective Functions of Apoptosis

Apoptosis helps maintain organismal integrity by removing cells that pose a threat:

• Cells infected with viruses: Apoptosis prevents the spread of infection.

• Cells with DNA damage: Prevents development of cancer or embryonic defects.

• Cancer cells: Cancer therapies often induce apoptosis in malignant cells.

• Immune system cells: Defects in apoptosis can lead to autoimmune diseases.

Mechanism of Apoptosis

Apoptosis is a choreographed process controlled by specific genes and signaling pathways:

• Activation of apoptotic enzymes (caspases).

• Destruction of DNA replication and repair enzymes.

• DNA fragmentation.

• Collapse of the cytoskeleton and nucleus.

• Destruction of mitochondria.

• Loss of cell adherence; cell fragments are encapsulated.

• Phagocytes engulf and remove cell remnants.

Comparison: Mitosis vs. Apoptosis

Cell Types: Prokaryotes vs. Eukaryotes

Overview

Cells are classified as either prokaryotic or eukaryotic based on their structure and organization.

• Prokaryotes: Lack a nucleus; have a single circular chromosome; no membrane-bound organelles.

• Eukaryotes: Have a nucleus; multiple linear chromosomes; contain membrane-bound organelles (e.g., mitochondria, chloroplasts).

Chromosome Structure

• Prokaryotic chromosomes: Circular DNA molecule.

• Eukaryotic chromosomes: Linear DNA molecules associated with histone proteins.

Human Genome

• Humans are diploid (2N): Two sets of chromosomes.

• 46 chromosomes: 22 pairs of autosomes, 1 pair of sex chromosomes.

• Homologous chromosomes: Chromosome pairs with the same genes but possibly different alleles.

Key Terms and Concepts

• Chromosome: DNA molecule with associated proteins, carrying genetic information.

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

• Spindle fibers: Microtubules that separate chromosomes during cell division.

• Centromere: Region where sister chromatids are joined.

• Diploid: Cell with two sets of chromosomes (2N).

• Interphase: Cell cycle phase of growth and DNA replication.

• Homologous chromosomes: Chromosome pairs with similar structure and gene content.

• Apoptosis: Programmed cell death.

• Nucleosome: DNA wrapped around histone proteins.

Cell Cycle Overview

Phases of the Cell Cycle

• G1 phase: Cell growth.

• S phase: DNA replication.

• G2 phase: Preparation for mitosis.

• M phase: Mitosis and cytokinesis.

• G0 phase: Non-dividing state.

Checkpoints ensure proper progression and fidelity of cell division.

Relevant Equations

• Number of chromosomes after DNA replication:

• Diploid number:

Additional info: Some terminology and context have been expanded for clarity and completeness.