Topic 5 Cell Division: Meiosis

Question 1

• Occurs in two steps: meiosis I and meiosis II
• Meiosis I: reduction division, homologous chromosomes pair at plate, migrate to opposite poles, no separation of sister chromatids
• Meiosis II: analogous to mitosis
• Occurs in gametes (egg, sperm, pollen)

Answer 1

Meiosis I

Question 2

• Nucleus disassembles, nucleolus disappears, nuclear envelope breaks down, chromatin condenses, mitotic spindle develops
• Microtubules begin attaching to kinetochores, crossing over occurs, which allows for genetic recombination
  a. Synapsis occurs, which involves homologous chromosomes pairing up, forming a tetrad (4 chromatids) or bivalents
  b. Chiasmata: where crossing over of non-sister chromatids occurs
  c. Synaptonemal complex: protein structure that forms between homologous chromosomes, gives ride to tetrad with chiasmata and crossing over
  d. 5 steps: leptotene (chromosomes start condensing) -> zygotene (synapsis begins, synaptonemal complex forms) -> pachytene (synapsis complete, crossing over) -> diplotene (synaptonemal complex disappears, chiasma still present) -> diakineses (nuclear envelope fragments, chromosomes complete condensing and tetrads ready for metaphase)

Answer 2

1. Prophase I

Question 3

• Homologous pairs line up along plat, microtubules attach to kinetochores of one member of each homologous pair

Answer 3

2. Metaphase I

Question 4

• Homologues within tetrads uncouple and are pulled to opposite sides (disjunction)

Answer 4

3. Anaphase I

Question 5

• Nuclear membrane develops
• Each pole forms a new nucleus that has half the number of chromosomes (from homologous pair to each chromosomes = 2 sister chromatids)
• Cell is now haploid
• Interphase may occur after telophase depending on species
• Now have 2 cells with 23 chromosomes and 46 chromatids

Answer 5

4. Telophase I

Question 6

• Involves chromosomes lining up on plate and sister chromatids separating and migrating to opposite poles

Answer 6

Meiosis II

Question 7

• Nuclear envelope disappears and spindle develops, no chiasmata of crossing over

Answer 7

1. Prophase II

Question 8

• Chromosomes align on plat but now with half the number of chromosomes

Answer 8

2. Metaphase II

Question 9

• each chromosome is pulled into 2 separate chromatids and migrate to opposite poles of cell

Answer 9

3. Anaphase II

Question 10

• Nuclear envelope reappears and cytokinesis occurs -> 4 haploid cells from, each chromosome consisting of one chromatid
• Now have 4 cells with 23 chromosomes and 23 chromatids

Answer 10

4. Telophase II