7.3 - Meiosis I

Meiosis (reductive division):

Nuclear division that

produces four haploid gametes that are not genetically

identical to the original cell.

• Gametes =

An organism’s reproductive cells (sperm

in males, eggs in females).

• Occurs in germ cells (specialized cells that produce

gametes).

Meiosis is comprised of two stages

separates homologous chromosomes.

• Meiosis I:

genetic recombination occurs and the cell

Meiosis II:

sister chromatids separate to form

gametes.

Prophase I

• Nucleolus and nucleus disappear.

• Chromatin condenses into chromosomes.

• Meiotic spindle begin to form.

• Centrosomes begin to move towards opposite ends

of the cell.

• Homologous chromosomes undergo synapsis and

crossing over.

- Synapsis: Process by which homologous

chromosomes pair up, forming tetrads.

‣ Tetrad = structure formed from paired

homologous chromosomes. Must be present

for genetic recombination to occur.

- Crossing over: exchange of chromosome

segments between paired-up chromosomes

(genetic recombination). Produces genetic

variation in gametes.

‣ Chiasmata = Locations where homologous

chromosomes meet to swap segments.

- Spindle fibers attach to kinetochores of

homologous chromosomes.

Metaphase I

• Homologous pairs are lined up across the

metaphase plate.

• Microtubules are attached to kinetochores.

• Meiotic spindle is fully developed.

Anaphase I

• Microtubules shorten.

• Homologous pairs uncouple and are pulled to

opposite poles (disjunction).

Telophase I and Cytokinesis

• Two nuclear envelopes develop (nuclei re-form).

• Chromosomes decondense back into chromatin.

• Spindle fibers disappear.

• Cytokinesis splits the cell into two daughter cells.

Final result of meiosis I: Two daughter cells with half the

number of chromosomes as the parent cell (2n parent

cell  n daughter cells).