Lecture 9b

Question 1

Steps in meiosis

Answer 1

Interphase: DNA replication, precedes Meiosis I (not meiosis II)

Meiosis I
Prophase I
Prometaphase I
Metaphase I
Anaphase I
Telophase I
End of meiosis I: 2 haploid nuclei
Followed by
Cytokinesis
2 haploid daughter cells

Meiosis II
Prometaphase II
Metaphase II
Anaphase II
Telophase II
End of meiosis II: 2 daughter cells, each with 2 haploid nuclei
Cytokinesis
4 haploid daughter cells (sperm or egg)

Question 2

Prophase I

Answer 2

Replicated chromosomes condense (1 replicated chromosome made up of 2 identical sister chromatids)

Spindle fibers appear

Replicated homologous chromosomes pair up to form a tetrad
Crossing-over. Between homologous chromosomes

Question 3

Prometaphase I

Answer 3

Nuclear membrane breaks down
Spindle fibers attach to replicated condensed chromosomes
Tetrads still present

Question 4

Metaphase I

Answer 4

Random line up of replicated, condensed homologous pairs (in tetrads) at metaphase plate
Independent assortment of chromosomes: homologous pairs line up in random orientations at the metaphase plate in preparation of anaphase I, where they will be separated

Question 5

Independent assortment

Answer 5

Search

Question 6

Anaphase I

Answer 6

Homologous chromosomes separated.

Ends when replicated condensed chromosomes reach the poles of the cell

Question 7

Telophase I

Answer 7

Nuclear membrane reforms: 2 non-identical haploid nuclei are formed.
Meiosis I complete
Replicated chromosomes (ie sister chromatids still attached) decondense
Cleavage furrow formed
Followed by cytokinesis: get 2 haploid cells

Question 8

Prophase II:

Answer 8

Prophase II: replicated chromosomes condense

Any crossing over? No, crossing over only happens in homologous chromosomes

Question 9

Prometaphase II:

Answer 9

nuclear envelope breaks down, spindle fibers attach to condensed replicated chromosomes

Question 10

Metaphase II:

Answer 10

condensed replicated chromosomes line up

Compare with metaphase I and with metaphase of mitosis

Question 11

Anaphase II

Answer 11

sister chromatids separated → unreplicated condensed chromosomes
compare with anaphase I & with anaphase of mitosis

Question 12

Telophase II

Answer 12

nuclear envelope reforms, unreplicated chromosomes decondense,
4 haploid nuclei in 2 cells
Followed by cytokinesis: get 4 haploid cells

Question 13

Meiosis & Genetic variation

Answer 13

Each haploid cell has a different combination of genes

WHY ? 2 reasons

Maternal & paternal chromosomes of homologous pairs separate independently (independent assortment)

Crossing-over: DNA segments exchanged b/w maternal & paternal homologues