Biology Now, 2e

Toxic Plastic ■ 109

Each former

chromatid is sorted

into a different

daughter cell. Four

genetically different

gametes are

generated by just this

one crossover event.

Nonsister chromatids exchange

identical DNA segments.

Recombined chromosomes

Maternal

chromosome

Paternal

chromosome

Sister

chromatids

Site of

crossing-over

Prophase I

Metaphase I

Metaphase II

Gametes

The maternal and paternal members

of each homologous pair are

positioned parallel to each other.

1

2

3

4

5

Crossing-over creates variation in

chromosomes at the end of prophase I.

In metaphase I, homologous chromosomes

align and are pulled to the poles during

anaphase I and telophase I, then cytokinesis

separates the cell into two haploid cells.

Prophase II and metaphase II

prepare and align chromosomes

on the metaphase plate. Sister

chromatids are segregated to

opposite poles during anaphase

of meiosis II. Telophase II and

cytokinesis create four haploid

cells.

Figure 6.9

Crossing-over produces chromosomes with new combinations of DNA

Only one maternal and one paternal chromosome are depicted here, rather than the 23 pairs of

homologous chromosomes found in humans. Only the resulting cells of prophase I, metaphase I,

metaphase II, and the resulting gametes of the process of meiosis are depicted here.

Q1: Why is the term “crossing-over” appropriate for the exchange of DNA segments between

homologous chromosomes?

Q2: At what stage of meiosis (I or II) does crossing-over occur?

Q3: What would be the effect of crossing-over between two sister chromatids?