Cell Division: Mitosis, Meiosis, and Regulation in Eukaryotes

Cell Division in Eukaryotes

Intended Learning Outcomes

This section outlines the core objectives for understanding cell division in eukaryotic organisms, focusing on the mechanisms, regulation, and consequences of mitosis and meiosis.

• Differentiate between asexual and sexual cell division, including biological uses.

• Describe key events in prokaryotic (binary fission) and eukaryotic cell division (mitosis and meiosis).

• Explain how cell division mechanisms contribute to genetic variation and discuss consequences of variation.

• Analyze errors in cell division (mutations, non-disjunction, checkpoint failures) and their link to disease.

• Apply knowledge to interpret diagrams and experimental data related to cell cycle phases.

Mitosis

Overview and Biological Significance

Mitosis is the process by which eukaryotic cells divide to produce two genetically identical daughter cells. It is essential for tissue growth, homeostasis, repair, and asexual reproduction in unicellular and some multicellular eukaryotes.

• Daughter cells are genetic clones of the parent cell.

• Applications: Yeast budding, growth of hyphae, and clonal expansion in aspen groves.

Meiosis

Overview and Biological Significance

Meiosis is a specialized form of cell division that produces gametes (sperm and eggs) with half the chromosome number of the parent cell, ensuring genetic diversity in sexually reproducing organisms.

• Daughter cells (gametes) are genetically unique.

• Occurs in: Most multicellular eukaryotes for sexual reproduction.

Mechanism of Meiosis

• Two rounds of nuclear and cell division without an intervening interphase.

• Meiosis I: Reduction division; homologous chromosomes separate, producing two haploid cells. $1 imes ext{Diploid} (2n) ightarrow 4 imes ext{Haploid} (n)$

• Meiosis II: Equational division; sister chromatids separate, similar to mitosis.

Stages of Meiosis

• Prophase I: Chromatin condenses, homologous chromosomes pair to form tetrads, crossing over occurs, increasing genetic diversity.

• Metaphase I: Tetrads align at the cell equator, synaptonemal complex breaks down, kinetochores assemble on one side of chromatids.

• Anaphase I: Homologous chromosomes separate to opposite poles; sister chromatids remain attached.

• Telophase I & Cytokinesis: Cell divides into two haploid daughter cells, each chromosome still has two sister chromatids.

• Meiosis II: Chromosomes line up (Metaphase II), sister chromatids separate (Anaphase II), and cells divide (Telophase II & Cytokinesis) to produce four genetically unique haploid gametes.

Genetic Variation in Meiosis

Genetic diversity is generated by:

• Crossing Over: Exchange of genetic material between non-sister chromatids during Prophase I.

• Independent Assortment: Random distribution of homologous chromosome pairs in Anaphase I and sister chromatids in Anaphase II.

Example Calculation

For humans (2n = 46, 23 pairs):

• Number of possible gametes by independent assortment: $2^{23}$

• Number of possible zygotes from random fertilization: $2^{23} imes 2^{23} = 2^{46}$

Errors in Chromosomal Segregation

Non-disjunction and Aneuploidy

Failure to correctly separate chromosome pairs or sister chromatids during mitosis or meiosis leads to non-disjunction, resulting in aneuploidy (abnormal chromosome number).

• Examples: Down syndrome (trisomy 21), cancer, and other genetic disorders.

Regulation of Cell Division

Cell Cycle Checkpoints

Cell division is tightly regulated to maintain tissue growth and homeostasis. Checkpoints ensure proper progression and prevent errors.

• G1 Checkpoint (Restriction Point): Assesses cell size, nutrients, DNA integrity, and mitogenic signals before entering S phase.

• G2/M Checkpoint: Ensures DNA replication is complete and error-free before mitosis.

• Spindle Assembly Checkpoint (SAC): Verifies all chromosomes are properly attached to the spindle before anaphase.

Regulatory Factors: Cyclins and CDKs

• Cyclin-Dependent Kinases (CDKs): Enzymes that phosphorylate target proteins to drive cell cycle progression.

• Cyclins: Regulatory proteins whose levels fluctuate throughout the cell cycle, activating CDKs.

• CDKs are only active when bound to cyclins.

• Expression of cyclins is regulated by transcription, translation, and degradation.

Equation

Phosphorylation reaction catalyzed by CDK:

$ext{Substrate} + ext{ATP} ightarrow ext{Phosphorylated Substrate} + ext{ADP}$

Experimental Evidence

Cell fusion experiments in the 1960s revealed the existence of regulatory factors controlling cell cycle progression, leading to the discovery of cyclins and CDKs.

Recommended Reading

• Scott et al. (2022) Biological Science: Chapter 10 (Cell division in prokaryotes and eukaryotes)

• Alberts et al. (2022) Molecular Biology of the Cell: Chapter 17 (The Cell Cycle)

Summary Table: Mitosis vs. Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	1	2
Daughter Cells	2 (identical)	4 (unique)
Chromosome Number	Diploid (2n)	Haploid (n)
Genetic Variation	None	High (crossing over, independent assortment)
Function	Growth, repair, asexual reproduction	Sexual reproduction

Additional info:

• Some forms of meiosis differ between males and females (e.g., timing and arrest in oogenesis).

• Checkpoint failures can result in uncontrolled cell division and tumor formation.