- Comparison of Asexual and Sexual Reproduction
- In asexual reproduction, one parent passes a duplicate of
its genetic information to its offspring, which can only be genetically identical clones
of the parent.
- In sexual reproduction, each parent contributes [one gene]
for each trait.
- Genes for each trait come in slightly different forms called alleles, originally produced by mutations.
- Meiosis shuffles the alleles during gamete formation, and fertilization produces
offspring with unique combinations of alleles.
- The variation generated by sexual reproduction is the testing ground for natural selection and is the basis for evolutionary change.
- How Meiosis Halves the Chromosome Number
- Think "Homologues"
- Meiosis begins with diploid (2n = 46 chromosomes
in humans) germ cells and produces haploid gametes (n =
- In 2n cells there are two chromosomes of each type, called homologous
- Homologous chromosomes line up (even unequally matched sex chromosomes!) during meiosis.
- Meiosis produces gametes that have a one chromatid genome.
- Two Divisions, Not One
- In some ways meiosis resembles mitosis:
- Unlike mitosis, meiosis has two series of divisions MI & MII.
- During MI, homologous chromosomes (92 chromatids / 46 chromosomes pair and cytokinesis
- In MII, the sister chromatids of each chromosome separate, cytokinesis follows resulting
in four haploid cells.
- Key Events of Meiosis I
- Prophase I Activities
- Homologous chromosomes pair up in a process called synapsis.
- Non-sister chromatids exchange segments in a process called crossing
- Because alleles for the same trait can vary, new combinations of genes in each
chromosome can result; this is one source of genetic variation (8,388,608 combinations).
- After crossing over, the non-sister chromatids begin to partially separate but remain
attached by chiasmata.
- Metaphase I Alignments
- During metaphase I, homologous chromosomes randomly line up at the spindle equator.
- During anaphase I, homologous chromosomes (still duplicated) separate into two haploid
cells each of which has a random mix of maternal and paternal chromosomes.
- From Gametes to Offspring
- Gamete Formation in Plants
- Germ cells within plant tissues produce haploid spores by meiosis.
- Each spore undergoes mitosis to produce a haploid gametophyte.
- Gametophytes produce haploid cells eggs or sperm.
- Fertilization results in a diploid sporophyte (example: pine tree).
- Gamete Formation in Animals
- The life cycle of multicelled animals proceeds from meiosis to gamete formation Þ fertilization Þ growth by mitosis.
- In males, meiosis and gamete formation is called spermatogenesis.
- In females, meiosis and gamete formation is called oogenesis.
- More Gene Shufflings at Fertilization
- The diploid chromosome number is restored at fertilization when two very different
gamete nuclei fuse to form the zygote.
- The variation present at fertilization is from three sources:
- Crossing over occurs during prophase I.
- Random alignments at metaphase I lead to millions of combinations of maternal and
paternal chromosomes in each gamete.
- Sexual selection: of all the genetically diverse gametes produced, chance will
determine which two will meet.
- Meiosis and Mitosis Compared
- Both are eukaryotic processes.
- Mitotic cell division produces clones; this type of
division is common in asexually reproducing organisms and in the growth process.
- Meiosis occurs only in the germ cells used in sexual reproduction; it gives rise to