Cell Division - Meiosis

Chapter 10, sections 10.1 – 10.5

I. Basic Principles of MeiosisA. meiosis is a special kind of cell division

1. occurs only in certain reproductive organs/cells• cells undergoing meiosis = germ cells

2. gametes (egg or sperm) result from meiosis

a. haploid

b. not genetically identical to parent cell or each other

3. meiosis is a 1:4 cell division• one 2n cell divides to produce 4, non-identical haploid cells

B. role and functions of meiosis1. meiosis does not take place in all organisms

• sexual reproduction requires meiosis

2. basic functions:

a. produces haploid gametes

b. ensures offspring receive full set and correct # of chrom.

c. introduces genetic variability into gametes and offspring

II. Meiotic Cell DivisionA. more complicated than mitosis

1. basic cell cycle is same

2. interphase with G1, S, and G2 is same

B. meiosis I (reduction division)

1. recall that chromosomes have already been replicated

2. recall that chromosomes occur in pairs

Fig. 10.1 Homologous chromosomes

A karyotype of a human male, showing all pairs of homologous chromosomes

3. prophase Ia. nuclear envelope and nucleolus disappear

b. centrioles migrate to opposite poles

c. spindle fibers appear as asters and attach to centromeres

d. synapsis• homologous chromosomes pair up and physically attach to each other

o resulting structures are called tetrads

e. crossing over (crossover)• non-sister chromatids exchange segments of themselves with each other

o chiasmatao introduces genetic variability into gametes and then offspring

Fig. 10.3 Crossing over occurs in meiosis I

4. metaphase Ia. tetrads line up along equatorial plane

b. homol. chrom. in each tetrad line up on random sides of each other• independent assortment

o promotes genetic diversity

Fig. 10.4 Independent assortment

5. anaphase I• homologous chrom. separate and move to opposite poles

6. telophase I 1. nuclear envelope and nucleolus reform

2. centrioles return to normal position

3. spindle fibers disappear

7. cytokinesis1. cell cleaves into two daughter cells

2. have two non-identical, haploid cells at end of meiosis I

C. meiotic interphase (interkinesis)1. cells grow slightly and synthesize some protein

2. no replication of DNA

Fig. 10.6 Meiosis I

D. meiosis II1. essentially the same as mitosis

2. each haploid daughter cell divides again

3. sister chromatids separate and move to opposite poles

4. prophase II metaphase II anaphase II telophase II cytokinesis

5. have 4 non-identical, haploid gametes at end of meiosis IIa. these 4 came from one diploid germ cell

b. diversity comes from crossing over & independent assortment

Fig. 10.6 Meiosis II

Fig. 10.6 Meiosis II, continued