BI108 Chapter 11 Notes: The Cell Cycle and Cell Division
11.5: What Happens During Meiosis?
Two nuclear divisions that reduce the number of chromosomes to haploid number.
Although the nucleus divides twice, the DNAis only replicated once.
Reduce chromosome number from diploid to haploid
Ensure that each of the haploid products has a complete set of chromosomes
Generate genetic diversity among products
Meiotic division reduces the chromosome number
Homologous chromosomes come together and then separate (but remember that
the sister chromatids remain intact)
Preceded by S phase—each chromosome is replicatedtwo sister chromatids held
together by cohesin proteins
At the end, two nuclei form, and each has half of the original chromosomes (one
member of each homologous pair)
Sister chromatids will then be separated during Meiosis II
Early Prophase Mid-Prophase Late Prophase-Prometaphase
Metaphase Anaphase Telophase
Prophase II Prometaphase II Anaphase II
Telophase II Products
Chromatid exchanges during meiosis I generate genetic diversity
Synapsis: homologous chromosomes pair by adhering along their lengths. This process lasts
from prophase I to the end of metaphase I.
Note: the four chromatids of each pair of homologous chromosomes form a tetrad.At the end of
prophase one the human cell contains 23 tetrads (each has four chromatids)
Chiasmata: regions where homologous chromosomes are attached, they have an X-shaped
appearance. BI108 Chapter 11 Notes: The Cell Cycle and Cell Division
The chiasmata reflects an exchange of genetic material between nonsister chromatids on
homologous chromosomes. This process is called crossing over.
Material is exchanged and later the homologs repel each other
Crossing over results in recombinant chromosomes. It also increases genetic variation by
shuffling info among the homologous pairs.
During meiosis homologous chromosomes separate by independent assortment
Diploid: (2n) = 2 sets of chromosomes; one from male parent, one from female parent.As the
organism grows, its cel