Overview
This lecture covers the three main types of life cycles in multicellular organisms, focusing on the roles of meiosis, gamete formation, and genetic variation in sexual reproduction.
Importance of Sexual Reproduction
- Sexual reproduction increases genetic variation through crossing over, independent assortment, and random fertilization.
- Genetic variation is crucial for survival and adaptation.
- Asexual organisms mainly rely on mutations for genetic diversity.
Types of Life Cycles
- There are three main life cycle categories: diploid dominant, haploid dominant, and alternation of generations.
Diploid Dominant Life Cycle
- Most animals, including humans, have a diploid dominant life cycle.
- Somatic (body) cells are diploid (contain two sets of chromosomes).
- Gametes (sperm and egg) are haploid (contain one set of chromosomes) and produced by meiosis.
- Fertilization produces a diploid zygote.
- There is no multicellular haploid stage; only gametes are haploid.
Haploid Dominant Life Cycle
- Many fungi and some algae exhibit a haploid dominant life cycle.
- The main body is haploid and grows by mitosis from haploid spores.
- Sexual reproduction involves two haploid cells (different mating types) uniting to form a diploid zygote.
- The zygote immediately undergoes meiosis to produce genetically varied haploid spores.
Alternation of Generations
- Occurs in all plants and some algae.
- Organisms alternate between multicellular haploid (gametophyte) and multicellular diploid (sporophyte) stages.
- Gametophyte produces gametes via mitosis; fertilization forms a diploid zygote.
- Sporophyte produces haploid spores via meiosis; spores undergo mitosis to regenerate gametophytes.
Key Terms & Definitions
- Diploid (2n) — Cell with two sets of chromosomes.
- Haploid (n) — Cell with one set of chromosomes.
- Gametes — Haploid reproductive cells (sperm and egg).
- Germ cells — Cells that create gametes, found in gonads.
- Zygote — Diploid cell formed from the union of two gametes.
- Gonads — Sex organs where gametes are produced (testes and ovaries).
- Gametophyte — Multicellular haploid stage in plants and some algae.
- Sporophyte — Multicellular diploid stage in plants and some algae.
Action Items / Next Steps
- Review and understand the three main life cycles (diploid dominant, haploid dominant, alternation of generations).
- Prepare for Chapter 12: Mendelian Genetics.