Meiosis and Sources of Genetic Variation (1C) - Bio & Biochem

1. What is the key structural difference between homologous chromosomes and sister chromatids?

   Homologs: maternal vs paternal; sisters: identical copies after S phase. Homologs differ in parental origin and may have allelic variations, while sisters are replication products.

2. What is nondisjunction in meiosis?

   Failure of homologs (I) or sister chromatids (II) to separate properly. Nondisjunction leads to aneuploid gametes by improper segregation during meiotic divisions.

3. What is the primary purpose of meiosis in sexually reproducing organisms?

   Produce haploid gametes and increase genetic variation. Meiosis ensures genetic diversity in offspring by halving chromosome number for fertilization and introducing variation through recombination.

4. What is the immediate product of meiosis in humans (starting from one primary germ cell)?

   Four haploid cells (gametes or gamete precursors). Meiosis yields four genetically distinct haploid cells from one diploid precursor for reproduction.

5. Which stage of meiosis separates sister chromatids?

   Anaphase II. Sister chromatids separate in this phase, similar to mitosis, to distribute identical DNA copies.

6. Which specific prophase I substage is characterized by crossing over?

   Pachytene. In pachytene, homologs are fully synapsed, enabling enzymes to mediate recombination events.

7. If nondisjunction occurs in meiosis II, what gamete types result in terms of $n$?

   Two normal $n$, one $n+1$, and one $n-1$ gamete. Meiosis II nondisjunction impacts only one cell, yielding two normal and two aneuploid gametes.

8. Which meiotic division is equational, and what does that mean?

   Meiosis II; separates sister chromatids without changing ploidy. Meiosis II divides chromatids equally, maintaining the haploid state established in meiosis I.

9. What is the formula for the number of chromosome combinations from independent assortment?

   $2^n$ (where $n$ is the haploid chromosome number). The formula accounts for each homologous pair having two possible orientations, yielding exponential combinations.

10. If a species has $n=3$, what is the number of possible gamete chromosome combinations from assortment alone?

    $2^3=8$. With three pairs, each can assort independently, resulting in $2^3$ unique gamete genotypes from assortment.

11. Which meiotic division is reductional, and what does that mean?

    Meiosis I; reduces ploidy by separating homologs. Meiosis I halves the chromosome number by segregating homologous pairs into daughter cells.

12. What is synapsis in prophase I of meiosis?

    Pairing of homologous chromosomes via the synaptonemal complex. Synapsis facilitates close alignment of homologs, allowing crossing over to exchange genetic material.

13. What is trisomy in terms of chromosome copy number?

    Three copies of one chromosome (a $2n+1$ condition). Trisomy results from gaining an extra chromosome, often due to nondisjunction, altering the diploid complement.

14. If nondisjunction occurs in meiosis I, what gamete types result in terms of $n$?

    Two gametes $n+1$ and two gametes $n-1$. Meiosis I nondisjunction affects both daughter cells, producing equal numbers of hyper- and hypoploid gametes.

15. Which two meiotic mechanisms are the major sources of genetic variation in gametes?

    Crossing over and independent assortment. These processes shuffle alleles and chromosome combinations, ensuring genetic diversity in offspring.

16. What is the term for paired homologous chromosomes during prophase I?

    Tetrad (bivalent). This structure forms when homologs pair, enabling crossing over for genetic recombination.

17. What is independent assortment in meiosis?

    Random orientation of homologous pairs at metaphase I. This random alignment ensures each gamete receives a unique combination of maternal and paternal chromosomes.

18. What is the DNA replication status between meiosis I and meiosis II?

    No S phase; DNA is not replicated between divisions. Absence of interphase replication preserves the reduced DNA content for haploid gamete production.

19. What is the ploidy change when a diploid cell completes meiosis?

    Diploid $2n$ to haploid $n$. Meiosis reduces the chromosome number by half to produce gametes that can restore diploidy upon fusion.

20. Which stage of meiosis separates homologous chromosomes?

    Anaphase I. During this phase, homologous chromosomes are pulled apart to opposite poles, reducing ploidy.

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