Section 10.2 Summary – pages 263-273

• After replication, each chromosome consists of two identical sister chromatids, held together by a centromere.

• During interphase, the cell replicates its chromosomes.

Interphase

Interphase

Section 10.2 Summary – pages 263-273

• The chromosomes coil up and a spindle forms.

Prophase I

• As the chromosomes coil, homologous chromosomes line up with each other gene by gene along their length, to form a four-part structure called a tetrad.

Prophase I

Section 10.2 Summary – pages 263-273

• The chromatids in a tetrad pair tightly.

• In fact, they pair so tightly that non-sister chromatids from homologous chromosomes can actually break and exchange genetic material in a process known as crossing over.

Prophase I

Prophase I

Section 10.2 Summary – pages 263-273

• Crossing over can occur at any location on a chromosome, and it can occur at several locations at the same time.

Prophase I

Prophase I

Section 10.2 Summary – pages 263-273

• It is estimated that during prophase I of meiosis in humans, there is an average of two to three crossovers for each pair of homologous chromosomes.

Sister chromatids

Nonsister chromatids

Tetrad

Homologous chromosomes

Crossing over in tetrad

Gametes

Prophase I

Section 10.2 Summary – pages 263-273

Sister chromatids

Nonsister chromatids

Tetrad

Homologous chromosomes

Crossing over in tetrad

Gametes

• Crossing over results in new combinations of alleles on a chromosome.

Prophase I

Section 10.2 Summary – pages 263-273

• During metaphase I, the centromere of each chromosome becomes attached to a spindle fiber.

Metaphase I

• The spindle fibers pull the tetrads into the middle, or equator, of the spindle.

Metaphase I

Section 10.2 Summary – pages 263-273

• Anaphase I begins as homologous chromosomes, each with its two chromatids, separate and move to opposite ends of the cell.

• This critical step ensures that each new cell will receive only one chromosome from each homologous pair.

Anaphase I

Anaphase I

Section 10.2 Summary – pages 263-273

• Events occur in the reverse order from the events of prophase I.

Telophase I

• The spindle is broken down, the chromosomes uncoil, and the cytoplasm divides to yield two new cells.

Telophase I

Section 10.2 Summary – pages 263-273

Telophase I

• Each cell has half the genetic information of the original cell because it has only one chromosome from each homologous pair.

Telophase I

Section 10.2 Summary – pages 263-273

• The second division in meiosis is simply a mitotic division of the products of meiosis I.

The phases of meiosis II

• Meiosis II consists of prophase II, metaphase II, anaphase II, and telophase II.

Meiosis II

Section 10.2 Summary – pages 263-273

• During prophase II, a spindle forms in each of the two new cells and the spindle fibers attach to the chromosomes.

Prophase II

The phases of meiosis II

Section 10.2 Summary – pages 263-273

• The chromosomes, still made up of sister chromatids, are pulled to the center of the cell and line up randomly at the equator during metaphase II.

Metaphase II

The phases of meiosis II

Section 10.2 Summary – pages 263-273

• Anaphase II begins as the centromere of each chromosome splits, allowing the sister chromatids to separate and move to opposite poles.

Anaphase II

The phases of meiosis II

Section 10.2 Summary – pages 263-273

• Finally nuclei, reform, the spindles break down, and the cytoplasm divides during telophase II.

Telophase II

The phases of meiosis II

Section 10.2 Summary – pages 263-273

• At the end of meiosis II, four haploid cells have been formed from one diploid cell.

• These haploid cells will become gametes, transmitting the genes they contain to offspring.

The phases of meiosis II

Section 10.2 Summary – pages 263-273

• Cells that are formed by mitosis are identical to each other and to the parent cell.

Meiosis Provides for Genetic Variation

• Crossing over during meiosis, however, provides a way to rearrange allele combinations.

• Thus, variability is increased.