BIOB 160 Principles of Living Systems · Unit 3: Cell division and inheritance · on Exam 3
3.2 Meiosis and life cycles
Study notes for Meiosis and life cycles in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, key formulas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in Mathub.
Big ideas
- Homologous chromosomes carry the same genes (one from each parent). Diploid (2n) cells have two sets; haploid (n) gametes have one. Humans have 22 pairs of autosomes plus the sex chromosomes.
- Meiosis I separates homologs (reduces 2n to n): in prophase I homologs pair (synapsis) and exchange segments by crossing over at chiasmata; in anaphase I homologs move apart while sister chromatids stay together. Meiosis II separates sister chromatids, like mitosis. One cell becomes four genetically different haploid cells.
- Life cycles: animals make gametes by meiosis; plants and some algae show alternation of generations, with a multicellular diploid sporophyte making haploid spores by meiosis, which grow into a multicellular haploid gametophyte that makes gametes by mitosis; fungi and many protists have a haploid adult and a diploid zygote only.
- Three sources of genetic variation (the “meiotic superpowers”): independent assortment of homologs (2ⁿ combinations; 2²³ ≈ 8.4 million in humans), crossing over, and random fertilization (about 70 trillion zygote combinations before crossing over).
- Errors: nondisjunction (homologs or sisters fail to separate) produces aneuploid gametes, as in trisomy 21.
Key formulas
Combinations from independent assortment
$$2^n \ \text{(n = haploid number)}$$
Worked example
An organism has 2n = 6. How many chromosome combinations can its gametes have from independent assortment alone? How many chromosomes are in each gamete?
Show the solution
The haploid number is n = 3, so 2³ = 8 combinations. Each gamete has 3 chromosomes.
Common mistakes
- Saying sister chromatids separate in meiosis I. Homologs separate in meiosis I; sisters separate in meiosis II.
- Using 2n instead of n in the 2ⁿ formula.
- Thinking the gametophyte makes gametes by meiosis. In alternation of generations, spores come from meiosis and gametes from mitosis.
Exam tipReading: Campbell ch. 13 (pp. 254–268) and p. 831 on alternation of generations. Draw meiosis for 2n = 4 once with colors; it answers most questions.
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