Overview
This lecture covers the structure and function of chromosomes and the processes of cellular reproduction, focusing on differences between prokaryotic and eukaryotic cells, mitosis, and meiosis, and their genetic consequences.
Prokaryotic vs. Eukaryotic Cells
- Prokaryotic cells are small, lack a nucleus, and have one circular chromosome.
- Domains Bacteria and Archaea are prokaryotes; all are single-celled organisms.
- Prokaryotes have no membrane-bound organelles but do have ribosomes.
- Eukaryotic cells have a nucleus, multiple linear chromosomes, and many membrane-bound organelles (e.g., mitochondria, ER).
- Eukaryotes include both unicellular (protists) and multicellular organisms (plants, animals, fungi).
- Eukaryotic DNA is associated with histone proteins, forming chromatin; prokaryotes lack true histones.
Chromosomal Structure
- Eukaryotic chromosomes have a centromere (the "waist" where spindle fibers attach during division).
- Chromosome ends are called telomeres.
- Chromatin = DNA wrapped around histone octamers (nucleosomes).
- Number of chromosomes: humans have 46 (23 from each parent).
- Homologous chromosomes are similar but not identical, each inherited from one parent.
Cell Cycle and Mitosis
- Cell cycle: Interphase (G1: growth, S: DNA synthesis, G2: preparation) and M phase (mitosis + cytokinesis).
- Major checkpoints: G1/S (main checkpoint), G2/M, and spindle assembly.
- Mitosis stages: Prophase (chromosomes condense), Prometaphase (nuclear envelope breaks), Metaphase (chromosomes align), Anaphase (sister chromatids separate), Telophase (nuclei reform).
- Cytokinesis splits cytoplasm, resulting in two genetically identical diploid cells.
- Mitosis maintains chromosome number and produces identical daughter cells.
Meiosis and Genetic Variation
- Meiosis produces four genetically non-identical haploid gametes (sperm or eggs).
- Interphase occurs only once before meiosis; no interphase between meiosis I and II.
- Meiosis I: homologous chromosomes separate (reduction division, diploid to haploid).
- Meiosis II: sister chromatids separate (equational division, haploid to haploid).
- Prophase I: synapsis and crossing over create genetic diversity.
- Metaphase I: homologous pairs (tetrads) line up randomly (independent assortment).
- Consequences: crossing over and independent assortment yield genetic variation; humans have ~8.4 million combinations from independent assortment alone.
Gametogenesis: Spermatogenesis and Oogenesis
- Spermatogenesis: spermatogonia (2n) → primary spermatocyte (2n) → two secondary spermatocytes (n) → four spermatids (n) → four sperm.
- Oogenesis: oogonia (2n) → primary oocyte (2n) → one secondary oocyte (n) + polar body → ovum (n) + second polar body.
- Only one mature egg forms from each oogonium; all four spermatids become sperm.
Key Terms & Definitions
- Prokaryote — cell lacking a nucleus and membrane-bound organelles.
- Eukaryote — cell with a nucleus and membrane-bound organelles.
- Chromatin — complex of DNA and histone proteins.
- Homologous chromosomes — pairs of similar chromosomes (one from each parent).
- Sister chromatids — identical DNA copies of a chromosome after replication.
- Centromere — region joining sister chromatids, attachment site for spindle fibers.
- Kinetochore — protein complex at centromere where spindle fibers attach.
- Diploid (2n) — two homologous sets of chromosomes.
- Haploid (n) — single set of chromosomes.
- Synapsis — pairing of homologous chromosomes in prophase I of meiosis.
- Crossing over — exchange of genetic material between homologous chromosomes.
- Independent assortment — random alignment of homologous pairs during metaphase I.
- Gamete — haploid reproductive cell (sperm or egg).
Action Items / Next Steps
- Review textbook Chapter 2 on chromosomes and cellular reproduction.
- Watch linked videos for in-depth explanations of mitosis, meiosis, and chromosome structure.
- Clarify differences between homologous chromosomes and sister chromatids.
- Practice concept check questions on cell cycle stages and chromosomal behavior.