Overview
This lecture reviews cell types, chromosomes, the processes of mitosis and meiosis, compares sexual and asexual reproduction, details the steps of meiosis, explains genetic diversity, covers errors in meiosis, and describes how gametes form in humans.
Cell Types and Chromosomes
- All living things are made of cells: prokaryotic (no nucleus) or eukaryotic (with nucleus).
- Prokaryotes have one circular chromosome; eukaryotes have several linear chromosomes.
- Chromosomes carry genes that provide instructions for cell functions.
- Humans have 46 chromosomes (23 pairs): 22 pairs autosomes, 1 pair sex chromosomes (XX = female, XY = male).
- Homologous chromosomes are matching pairs; karyotype shows all chromosomes arranged in pairs.
Cell Division in Prokaryotes and Eukaryotes
- Prokaryotic cells divide by copying DNA and splitting, producing identical single-celled organisms.
- Eukaryotic cells divide by mitosis (nucleus divides) followed by cytokinesis (cytoplasm splits).
- Eukaryotic cell cycle: interphase (G1 – growth, S – DNA synthesis, G2 – prep for mitosis), then mitosis.
Sexual vs. Asexual Reproduction
- Asexual reproduction uses one parent to create genetically identical offspring (clones).
- Sexual reproduction requires two parents, producing gametes (sperm, eggs) that combine via fertilization to form a zygote with traits from both parents.
Meiosis Overview and Purpose
- Meiosis occurs in testes (males) and ovaries (females), producing gametes.
- Meiosis halves chromosome number: gametes are haploid (1n), somatic cells are diploid (2n).
- Fertilization restores diploid state in zygote.
Steps of Meiosis
- Meiosis has two divisions: Meiosis I and Meiosis II.
- Meiosis I: homologous chromosomes pair (synapsis), tetrads form, crossing over creates genetic diversity.
- Metaphase I: tetrads align at metaphase plate; Anaphase I: homologous chromosomes separate; Telophase I and cytokinesis form two haploid cells.
- Interkinesis: brief pause with no DNA replication.
- Meiosis II: similar to mitosis; separates sister chromatids, resulting in four haploid cells.
Mistakes in Meiosis: Nondisjunction and Aneuploidy
- Nondisjunction: chromosomes or chromatids fail to separate, causing abnormal chromosome numbers (aneuploidy).
- Aneuploid gametes can cause disorders like Down syndrome (extra chromosome 21).
Gametogenesis (Formation of Gametes)
- Spermatogenesis (males): spermatogonia → primary spermatocyte → meiosis I → secondary spermatocytes → meiosis II → spermatids → sperm.
- Oogenesis (females): oogonia → primary oocytes halted at prophase I until puberty; after meiosis I and II, one large ovum and up to three small polar bodies form.
Key Terms & Definitions
- Prokaryotic cell — cell without a nucleus; DNA in cytoplasm.
- Eukaryotic cell — cell with DNA inside a nucleus.
- Chromosome — DNA molecule carrying genetic information.
- Homologous chromosomes — chromosome pairs with genes for the same traits.
- Mitosis — cell division producing two identical cells.
- Meiosis — cell division producing four genetically unique haploid cells.
- Diploid (2n) — cell with two sets of chromosomes.
- Haploid (1n) — cell with one set of chromosomes.
- Synapsis — pairing of homologous chromosomes during meiosis.
- Crossing over — exchange of DNA between non-sister chromatids.
- Nondisjunction — failure of chromosomes to separate during meiosis.
- Gametogenesis — process of forming gametes (sperm or eggs).
- Oogenesis — formation of egg cells.
- Spermatogenesis — formation of sperm cells.
- Karyotype — image of all chromosomes in a cell.
Action Items / Next Steps
- Review the diagrams/graphics illustrating meiosis and chromosome behavior.
- Complete practice quizzes and comprehensive exam for self-assessment.
- Study glossary for full understanding of key terms.