Class Notes Sexual Reproduction Meiosis

Interphase (not part of mitosis) Cell goes about its regular business Copies are made of all the cells organelles In the S stage of Interphase: Each DNA Strand makes an exact copy of itself The two identical strands are joined together by a centomere. The joined strands are called sister chromatids

Prophase I Sister chromatids become visible in the nucleus The nuclear membrane disappears Sister chromatids form homologous pairs. Each pair is made of two sets of sister chromatids. The pairs swap genetic material. Homologous pair

Metaphase I Spindle fibers pull the homologous pairs to the center of the cell. Each homologous pair is made of two sets of sister chromatids The pairs line up opposite each other.

Anaphase The cell elongates The spindle fibers pull apart the homologous pairs and drag them to opposite sides of the cell.

Telophase I The spindle fibers disappear The nuclear membrane forms around the chromatids A cell membrane/ cell wall forms down the center of the cell Two similar cells are created.

The cell makes copies of all its organelles. Interphase The cell makes copies of all its organelles. The chromosomes DO NOT replicate.

Meiosis II

Prophase II Nuclear membrane breaks apart.

Metaphase II Spindle fiber appear at each end of the cell and pull the sister chromatids to line up along the center.

Anaphase II Spindle fibers pull apart the sister chromatids and pull the chromosomes toward each side of the cell.

Telophase II Spindle fibers disappear. A nuclear membrane forms around the chromatids.

Cytokinesis The cytoplasm divides. 4 haploid cells are produced. Each contains ½ the original number of chromosome. These are sex cells.(egg/sperm)

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Fertilization A sperm and an egg cell each have ½ the number of chromosomes as the parent cell. When an egg and a sperm are combined, fertilization occurs. The homologous chromosomes from each parent combine together to create one new cell called a zygote. Once the egg is fertilized, the two haploid sex cells return to being one diploid cell.

Compare Mitosis and Meiosis

Comparing Meiosis and Mitosis Characteristic Mitosis Meiosis Chromosomes Sister chromatids line up (2) Homologous pairs of chromosomes line up (4) Crossover occurs No Yes Number of Cell Divisions 1 2 Number of Daughter cells 4 Comparison to parent cell Diploid (same number of chromosomes as original) Haploid (1/2 the number of chromosomes as the original Type of cells that result All other cells Sex cells (sperm/egg)

Vocabulary Homologous pairs Egg Sperm Zygote Diploid Haploid Meiosis