Link Search Menu Expand Document
Senanu's Bio class

Note that there is an additional drawing assignment with this reading guide. You can download it here

Chapter 13 – Meiosis and sexual life cycle

OBJECTIVE: Understand the concept of life cycles and this relates to meiosis and mitosis. OBJECTIVE: Be familiar with differentiating between sister chromatids and homologous chromosomes

  1. Make a sketch of the human life cycle. Show babies, adults, and our gametes (eggs and sperm). Make sure to label which steps involve cell division by mitosis and which involve cell division by meiosis. Finally, for each life stage, indicate whether it is haploid (n) or diploid (2n).
  2. In humans, 2n=46. This indicates that humans are haploid / diploid (circle one). What is the haploid number? _________. How many pairs of homologous chromosomes do we have? _________.
  3. Look at the top cell of Fig 13.3. Which (sister chromatids or homologous chromosomes) are the result of DNA synthesis during S-phase?
  4. Which (sister chromatids or homologous chromosomes) are similar, but not identical?
  5. Which (sister chromatids or homologous chromosomes) are inherited from different parents?
  6. Draw a chromosome in its replicated and unreplicated forms. Label the centromere and sister chromatids.
  7. What is a diploid cell?
  8. Are your somatic cells (all cells that are not sex cells) diploid or haploid?

  9. Compare a pair of homologous chromosomes with a pair of sister chromatids? In particular, what is the origin of each?

    OBJECTIVE: Discuss the major features that differentiate meiosis and mitosis.

    For each of the features listed below, indicate if and how they occur in mitosis and meiosis

    Feature Mitosis Meiosis
    Condensation of chromosomes Yes, sister chromatids attached Yes, sister chromatids attached but also synapsed with homolog
    Chromosomes separated in anaphase (which ones?) (which ones in anaphase I and anaphase II?)
    Nuclear envelope breaks down
    Crossing over
    Number of daughter cells
    Number of cell divisions
    Haploid or diploid daughter cells
  10. Describe how cells in metaphase of mitosis and metaphase I of meiosis are similar and how they are different (pay attention to HOW the chromosomes are aligned and the NUMBER of chromosomes/cell).

  11. Describe how cells in metaphase of mitosis and metaphase II of meiosis are similar and how they are different (pay attention to HOW the chromosomes are aligned and the NUMBER of chromosomes/cell).

    OBJECTIVE: Describe how independent assortment and crossing over generate diversity among gametes.

  12. According to Fig 13.8, how many types of gametes are possible for an organism with 2 chromosomes? (notice that only the final products of meiosis I are shown, not intermediate steps and not the final cells resulting from meiosis II. Also, this diagram simplifies things by showing only 2 of the 4 cells that result from meiosis. The 2 that are not shown are identical to the two shown.)
  13. Which type of gamete in Fig 13.8 (if any) is most likely?
  14. If the paternal copy of the long chromosome is present in a gamete, what is the likelihood that the paternal copy of the short chromosome will also be present? How does this relate to the term “independent assortment”?
  15. At what stage of meiosis does crossing over occur?

  16. Fig 13.8 does not show crossing over, with the result that there are 8 gametes of 4 different types (again, half of them are omitted from the drawing). For this organism with 2 chromosomes, how many types of gamete would there be if the original cells shown in the figure divided by meiosis but included crossing over?

    OBJECTIVE: Describe aneuploidy and how it is caused by nondisjunction

  17. Describe nondisjunction. When does it happen?
  18. What is the consequence of nondisjunction on the gamete?

  19. What is the consequence of nondisjunction on the individual?

    OBJECTIVE: Understand hypotheses about why biologists think sexual species exist despite the costs of sexual reproduction.

  20. Why is there a ‘paradox of sex’?
  21. In the ‘purifying selection’ hypothesis, are all offspring of sexual species the same genetically? What about for asexual species?
  22. What do you think happens (will it become more or less common?) to a deleterious (bad) mutation when it appears in a sexual species? What would happen to it among offspring of an asexual species?
  23. Why might asexual species be more likely to go extinct as their environment changes?