Cell Division and Chromosomal Distribution

Differences Between Prokaryotic and Eukaryotic Cells

Understanding the structural and functional differences between prokaryotic and eukaryotic cells is fundamental to genetics and cell biology.

• Prokaryotic Cells: Lack a membrane-bound nucleus and organelles; genetic material is located in the nucleoid region.

• Eukaryotic Cells: Possess a membrane-bound nucleus and various organelles; genetic material is enclosed within the nucleus.

• Example: Escherichia coli (prokaryote) vs. human cells (eukaryote).

Cellular Organization of Prokaryotes

Prokaryotes are simpler in structure, with essential components for survival and reproduction.

• Nucleoid: Region containing circular DNA.

• Cell membrane: Controls transport and maintains cell integrity.

• Ribosomes: Sites of protein synthesis (smaller than eukaryotic ribosomes).

• Cell wall: Provides structural support.

Cellular Organization of Eukaryotes (Plant and Animal)

Eukaryotic cells are complex, with specialized organelles for various functions.

• Nucleus: Contains genetic material; site of transcription.

• Nucleolus: Produces ribosomal RNA.

• Centrosome: Organizes microtubules; important in cell division.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosome: Digests cellular waste (mainly in animal cells).

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplast: Site of photosynthesis (in plant cells).

• Peroxisome: Breaks down fatty acids and detoxifies.

• Vesicles: Transport materials within the cell.

• Vacuoles: Storage and structural support (large in plant cells).

• Ribosomes: Protein synthesis (larger than prokaryotic ribosomes).

Types of Cell Division

Cells divide to reproduce, grow, and repair tissues. The main types are binary fission, mitosis, and meiosis.

• Binary Fission: Occurs in prokaryotes.

• Mitosis: Occurs in somatic cells of eukaryotes.

• Meiosis: Occurs in germ cells of eukaryotes.

Binary Fission

Binary fission is the primary method of reproduction in prokaryotes.

• Organisms: Bacteria and archaea.

• Process: DNA replicates, cell elongates, and divides into two identical daughter cells.

• Example: E. coli dividing by binary fission.

Cell Cycle in Eukaryotes

The cell cycle is a series of phases that eukaryotic cells undergo for growth and division.

• Phases:

  • G1 (Gap 1): Cell grows and prepares for DNA replication.

  • S (Synthesis): DNA is replicated.

  • G2 (Gap 2): Cell prepares for mitosis.

  • M (Mitosis): Cell divides.

• Checkpoints:

  • G1/S Checkpoint: Ensures cell is ready for DNA synthesis.

  • G2/M Checkpoint: Ensures DNA is fully replicated and undamaged.

  • Spindle Checkpoint: Ensures chromosomes are properly attached to spindle fibers before anaphase.

DNA Molecules and Chromosomes After Replication

After DNA replication, each chromosome consists of two sister chromatids held together.

• Number of DNA molecules: Doubles after replication.

• Number of chromosomes: Remains the same until cell division.

• Example: A cell with 4 chromosomes will have 8 DNA molecules after replication.

Protein Keeping Sister Chromatids Together

The protein complex cohesin holds sister chromatids together after DNA replication.

• Cohesin: Ensures proper chromosome segregation during mitosis and meiosis.

Mitosis

Mitosis is the process by which eukaryotic cells divide to produce two genetically identical daughter cells.

• Stages of Mitosis:

  • Prophase: Chromosomes condense, spindle forms.

  • Metaphase: Chromosomes align at the metaphase plate.

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

  • Telophase: Nuclear envelope reforms, chromosomes decondense.

• Mitotic Spindle Binding: Spindle fibers attach to chromosomes at the kinetochore region.

• Correct Orientation: Chromosomes must be attached to spindle fibers from opposite poles.

• DNA Molecules and Chromosomes in Each Stage:

  • Before mitosis: Chromosomes are duplicated (each consists of two sister chromatids).

  • After anaphase: Chromatids separate, each considered a chromosome.

• Cytoskeleton Fibers in Mitosis:

  • Microfilaments: Involved in cytokinesis (cell division).

  • Microtubules: Form the mitotic spindle, move chromosomes.

Meiosis

Meiosis is a specialized cell division process that produces gametes (sperm and egg) with half the chromosome number of the parent cell.

• Key Terms:

  • Haploid (n): One set of chromosomes.

  • Diploid (2n): Two sets of chromosomes.

  • Gametes: Reproductive cells (sperm, egg).

  • Zygote: Fertilized egg (diploid).

  • Fertilization: Fusion of gametes to form a zygote.

• Homologous Chromosomes: Chromosomes with the same genes but possibly different alleles; one from each parent.

Meiosis I

Meiosis I reduces chromosome number by half and introduces genetic variation.

• Stages:

  • Prophase I: Chromosomes condense, homologous chromosomes pair (synapsis), crossing over occurs.

  • Metaphase I: Homologous pairs align at the metaphase plate.

  • Anaphase I: Homologous chromosomes separate.

  • Telophase I: Cells divide, each with half the chromosome number.

• Homologous DNA Recombination: Exchange of genetic material between homologous chromosomes.

• Two Ways of Recombination:

  • Holliday Model: Involves single-strand break, strand invasion, branch migration, and resolution of Holliday junction.

  • Double Strand Break Model: Involves double-strand break, end resection, strand invasion, formation of two Holliday junctions, and resolution.

• Complex of Proteins: Synaptonemal complex mediates recombination.

• Outcome: Two cells, each with half the chromosome number (haploid).

• DNA Molecules and Chromosomes in Each Stage:

  • After S phase: Chromosomes are duplicated (two sister chromatids per chromosome).

  • After Meiosis I: Each cell has half the chromosome number, but each chromosome still consists of two sister chromatids.

• Interkinesis: Period between Meiosis I and II; no DNA replication occurs.

Meiosis II

Meiosis II separates sister chromatids, similar to mitosis.

• Stages:

  • Prophase II: Chromosomes condense again.

  • Metaphase II: Chromosomes align at the metaphase plate.

  • Anaphase II: Sister chromatids separate.

  • Telophase II: Cells divide, resulting in four haploid cells.

• Outcome: Four genetically unique haploid cells.

• DNA Molecules and Chromosomes in Each Stage:

  • After Meiosis II: Each cell has one set of chromosomes (haploid), each chromosome is a single DNA molecule.

• Genetic Variation: Meiosis generates different combinations of chromosomes in daughter cells due to independent assortment and crossing over.

Comparison of Mitosis and Meiosis

Mitosis and meiosis are distinct processes with different outcomes and purposes.

Key Formulas and Concepts

• Chromosome Number After Replication:

  • Let n = number of chromosomes in a diploid cell.

  • After DNA replication: Number of DNA molecules =

  • Number of chromosomes remains until cell division.

• Genetic Variation in Meiosis:

  • Number of possible combinations due to independent assortment: (where n is the haploid number).

Summary Table: Stages of Mitosis and Meiosis

Additional info: Academic context was added to clarify the steps of recombination, the function of cytoskeletal fibers, and the comparison tables. These notes are suitable for exam preparation and provide a comprehensive overview of cell division relevant to genetics.