Mitosis, Meiosis, and Cell Differentiation: Human Biology Study Notes

Cell Reproduction: Mitosis and Meiosis

Overview of Cell Division

Cell division is a fundamental process in human biology, essential for growth, tissue repair, and reproduction. Two main types of cell division are mitosis and meiosis, each serving distinct biological roles.

• Mitosis: Produces two genetically identical diploid daughter cells for growth and repair.

• Meiosis: Produces four genetically unique haploid gametes for sexual reproduction.

Mitosis

Phases of Mitosis

Mitosis is the process of nuclear division, followed by cytokinesis (cytoplasmic division), resulting in two identical daughter cells. It consists of four main phases:

1. Prophase:

   • Centrioles move to opposite poles of the cell.

   • Mitotic spindle forms from microtubules.

   • Nuclear membrane disappears.

   • Chromosomes condense and become visible; centromeres develop into two structures.

2. Metaphase:

   • Duplicated chromosomes align at the cell's equator (metaphase plate).

   • Mitotic spindle attaches to centromeres.

3. Anaphase:

   • Centromeres split, separating sister chromatids (now called daughter chromosomes).

   • Chromatids move to opposite poles, pulled by spindle fibers using ATP.

4. Telophase:

   • Chromosomes arrive at poles and decondense into chromatin.

   • Nuclear membranes reform around each set of chromosomes.

   • Mitotic spindle disassembles.

Cytokinesis

• Division of the cytoplasm occurs after mitosis.

• A contractile ring forms, creating a cleavage furrow that pinches the cell into two daughter cells.

• Daughter cells are genetically identical and diploid (46 chromosomes).

Significance of Mitosis

• Responsible for growth, tissue repair, and replacement of worn-out cells (e.g., epithelial cells, stomach lining, red blood cells).

• All somatic (body) cells except gametes undergo mitosis.

Abnormal Cell Division

Malignant Cancer Cells

• Result from uncontrolled cell division.

• Causes include mutations from carcinogens or viruses.

• Treatments: Surgical removal, radiation, chemotherapy (inhibits cell division).

Meiosis

Overview and Purpose

Meiosis is a two-stage division process that reduces the chromosome number by half, producing haploid gametes (sperm and ova). This ensures genetic diversity and maintains chromosome number across generations.

• Haploid: Cells with one set of chromosomes (23 in humans).

• Diploid: Cells with two sets of chromosomes (46 in humans).

Stages of Meiosis

Meiosis consists of two successive divisions: Meiosis I and Meiosis II, each with four phases.

Meiosis I

• Prophase I: Homologous chromosomes pair up to form tetrads; crossing over (exchange of genetic material) occurs, increasing genetic variation.

• Metaphase I: Tetrads align at the cell equator independently.

• Anaphase I: Homologous chromosomes separate and move to opposite poles (random assortment).

• Telophase I and Cytokinesis: Cell divides into two non-identical haploid cells, each with duplicated chromosomes.

Meiosis II

• Proceeds similarly to mitosis but without prior DNA replication.

• Sister chromatids separate during anaphase II.

• Results in four non-identical haploid cells (gametes).

Meiosis in Males vs. Females

Feature	Males (Spermatogenesis)	Females (Oogenesis)
Number of gametes produced	4 sperm per meiosis	1 ovum + 2-3 polar bodies per meiosis
Cytoplasm distribution	Equal among all sperm	Most cytoplasm to one ovum; polar bodies receive little
Timing of meiosis completion	Continuous after puberty	Meiosis II completed only after fertilization
Genetic diversity	High (due to crossing over and independent assortment)	High (due to crossing over and independent assortment)

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetic identity	Identical to parent cell	Genetically unique
Function	Growth, repair, asexual reproduction	Sexual reproduction (gamete formation)

Regulation of Cell Reproduction

Cell Cycle Control and Checkpoints

• Cell division is regulated by internal mechanisms (e.g., cyclins and regulatory proteins).

• Checkpoints ensure the cell is ready to proceed to the next phase:

  • Middle of G1 phase

  • End of G2 phase

  • Middle of M phase

• External factors such as nutrient and hormone availability also influence cell division.

Cell Differentiation

Mechanisms and Influences

• Cells become specialized by expressing specific genes, leading to differences from parent or sister cells.

• Differentiation can occur at any stage of development and is influenced by environmental factors.

Factors Affecting Fetal Development

• Exposure to harmful substances during development can cause abnormalities or fetal death.

• Examples of teratogens and risks:

  • Cigarettes: Increased cancer risk in offspring

  • Alcohol: Fetal alcohol syndrome

  • Legal drugs: Thalidomide, marijuana

  • Illegal drugs: Heroin, cocaine

  • Chemicals: DDT, formaldehyde, PCB’s, lead

  • Radiation: Ionizing and non-ionizing

  • Infections: HIV, syphilis, rubella

Key Terms and Definitions

• Diploid (2n): Cell with two sets of chromosomes (46 in humans).

• Haploid (n): Cell with one set of chromosomes (23 in humans).

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

• Centromere: Region where sister chromatids are joined.

• Homologous chromosomes: Chromosome pairs, one from each parent, similar in shape and gene content.

• Tetrad: Structure formed by pairing of homologous chromosomes during prophase I of meiosis.

• Crossing over: Exchange of genetic material between homologous chromosomes during meiosis I.

• Polar body: Small cell produced during oogenesis that usually degenerates.

• Zygote: Fertilized egg cell, diploid.

Key Equations and Concepts

• Chromosome Number in Humans:

  • Diploid: $46 = 2n$

  • Haploid: $23 = n$

• Genetic Variation in Meiosis:

  • Crossing over and independent assortment increase genetic diversity among gametes.

Additional info: The process of cell differentiation is crucial for the development of specialized tissues and organs. Environmental factors can influence gene expression, leading to variations in cell fate and function. Understanding the mechanisms of mitosis and meiosis is essential for comprehending human development, inheritance, and the basis of many diseases, including cancer.