Meiosis and Sexual Life Cycles

Page
6

attached to

kinetochore

Sister chromatids

remain attached

Homologous

chromosomes

separate

Cleavage

furrow

Slide 41

Prophase I

Prophase I typically occupies more than 90% of the time required for meiosis

Chromosomes begin to condense

In synapsis, homologous chromosomes loosely pair up, aligned gene by gene

Slide 42

In crossing over, nonsister chromatids exchange DNA segments

Each pair of chromosomes forms a tetrad, a group of four chromatids

Each tetrad usually has one or more chiasmata, X-shaped regions where crossing over occurred

Slide 43

Metaphase I

In metaphase I, tetrads line up at the metaphase plate, with one chromosome facing each pole

Microtubules from one pole are attached to the kinetochore of one chromosome of each tetrad

Microtubules from the other pole are attached to the kinetochore of the other chromosome

Slide 44

Fig. 13-8b

Prophase I

Metaphase I

Centrosome

(with centriole pair)

Sister

chromatids

Chiasmata

Spindle

Centromere

(with kinetochore)

Metaphase

plate

Homologous

chromosomes

Fragments

of nuclear

envelope

Microtubule

attached to

kinetochore

Slide 45

Anaphase I

In anaphase I, pairs of homologous chromosomes separate

One chromosome moves toward each pole, guided by the spindle apparatus

Sister chromatids remain attached at the centromere and move as one unit toward the pole

Slide 46

Telophase I and Cytokinesis

In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids

Cytokinesis usually occurs simultaneously, forming two haploid daughter cells

Slide 47

In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms

No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated

Slide 48

Fig. 13-8c

Anaphase I

Telophase I and

Cytokinesis

Sister chromatids

remain attached

Homologous

chromosomes

separate

Cleavage

furrow

Slide 49