Reproduction and Individual Development
What you must understand
Mitosis produces two genetically identical diploid cells; its phases are prophase, metaphase, anaphase, telophase. Key events: chromosomes condense, the spindle forms, chromosomes align at the equator, sister chromatids separate to opposite poles, nuclei reform. Mitosis underlies growth, repair, and asexual reproduction.
Meiosis halves the chromosome number through two divisions after one replication. In prophase I, homologous chromosomes pair (synapsis) and may cross over, creating recombination; in anaphase I homologues separate, in anaphase II chromatids separate. Result: four haploid cells, genetically distinct.
Gametogenesis: spermatogenesis yields four sperm; oogenesis yields one ovum and polar bodies, with meiosis arrested until fertilization in many animals. Ontogenesis is the individual development from zygote to death; embryogenesis includes cleavage, gastrulation, and organogenesis, followed by postembryonic development (direct or with metamorphosis).
Traps
- Crossing over happens in prophase I, not in mitosis.
- In anaphase of mitosis chromatids separate; in anaphase I homologues separate.
- Meiosis has two divisions but only one DNA replication.
- Cleavage does not increase the embryo's mass, only cell number.
Rule of thumb
Count DNA replications versus divisions: mitosis = 1 replication, 1 division; meiosis = 1 replication, 2 divisions.
Worked example: A diploid cell with 2n = 8 divides mitotically: each daughter has 8 chromosomes. After meiosis: each gamete has 4 chromosomes, and crossing over makes them non-identical.