AP Biology Unit 5 Cheatsheet: Heredity

Prophase I: Homologous chromosomes pair (synapsis), chiasmata form, spindle begins, nuclear envelope breaks down.

Metaphase I: Homologous pairs align at metaphase plate.

Anaphase I: Homologous chromosomes separate; sister chromatids stay attached.

Telophase I: Spindle breaks down, nuclear envelope reforms, cytokinesis occurs → 2 haploid cells.

Key: Homologs separate, not sister chromatids.

Prophase II: The spindle forms; sister chromatids remain joined at their centromeres.

Metaphase II: Chromosomes align at metaphase plate; kinetochores attach to microtubules.

Anaphase II: Separase cleaves the centromeric cohesin that was protected during meiosis I, so sister chromatids separate and are pulled to opposite poles.

Telophase II: Spindle breaks down, nuclear envelope reforms, cytokinesis occurs → 4 haploid cells.

Key: Sister chromatids separate (anaphase of mitosis/anaphase II), and each separated chromatid now counts as its own individual, unduplicated chromosome.

Similarity: Both use spindle apparatus to move chromosomes.

1. Crossing over (recombination): Non-sister chromatids exchange genetic material during Prophase I → new allele combinations on single chromosome.
2. Random assortment: Homologous pairs align randomly at metaphase plate → each gamete receives different mix of maternal and paternal chromosomes.
3. Fertilization: Random fusion of two haploid gametes creates new allele combinations in zygote.

Result: Sexual reproduction generates far more variation than asexual reproduction.