11.4 Meiosis

Chromosome Number Chromosomes carry genes. Genes are located in specific positions on chromosomes.

Diploid Cells 2 1 1 2 Fruit flies have eight chromosomes. Four come from the father, four come from the mother. Homologous pair – The two sets of chromosomes that match. Always one from each parent. 3 3 Why might chromosome 4 be different? X and Y chromosome (Larger X, smaller Y) 4 4

Diploid Cells Diploid cell – Cell containing both sets of homologous chromosomes.  Represented by 2N. For the fruit fly, the diploid number is 8, which can be written as 2N = 8 All cells are diploid except the sperm and egg.

Haploid Cells Haploid (N) - cells containing a single set of chromosomes, and therefore a single set of genes. Gametes (sperm and egg) are haploid. Fruit flies are 2N with 8 chromosomes. How many chromosomes do the gametes have? Four

Phases of Meiosis Meiosis - process in which the number of chromosomes in a diploid cell is cut in half. Made up of meiosis I and meiosis II. By the end of meiosis II, the diploid cell becomes four haploid cells. Interphase Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis Meiosis II Meiosis I

Interphase Meiosis I - Prophase I Chromosomes replicate Homologous chromosomes pair up. Form tetrads – four chromatids Go through crossing-over. Example cell: 2N (4 chromosomes) 

Prophase I – Crossing-Over First, homologous chromosomes cross over one another. Crossed sections of the chromatids are exchanged. Important because it produces new combinations of alleles (genes).

Metaphase I Anaphase I Homologous chromosomes line up across the middle of the cell. Homologous chromosomes are pulled apart and move towards opposite ends of the cell.

Cytokinesis Telophase I Nuclear membrane reforms around each cluster of chromosomes. Cells splits, forming two new cells.

Meiosis I Concludes Results in two haploid (N) daughter cells Each has 2 chromosomes, 4 chromatids. Each cell has different chromosomes than when it started because of crossing-over.

Meiosis II After meiosis I comes meiosis II. No interphase The final four phases of meiosis II are similar to those in meiosis I. However, the result is four haploid daughter cells.

Metaphase II Prophase II Chromosomes become visible. No tetrads Chromosomes line up in the middle of each cell.

Telophase II, and Cytokinesis Anaphase II Paired chromatids separate. Splits into four haploid (N) daughter cells Each has 2 chromosomes, 2 chromatids.

Interphase Prophase I Prophase II Metaphase I Metaphase II Meiosis II Anaphase I Telophase I Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis Meiosis II Meiosis I 4 cells 2 chromosomes 2 chromatids 2 cells 2 chromosomes 4 chromatids

Gametes to Zygotes The haploid cells produced by meiosis II are gametes. In males, gametes are called sperm. In females, gametes are called eggs.

Gametes to Zygotes Fertilization — the fusion of the male and female gametes Makes a zygote. The zygote undergoes mitosis and eventually forms a new organism. Zygote has a new combinations of alleles

Comparing Meiosis and Mitosis One division Two divisions Daughter cells have the same number of chromosomes as first cell Daughter cells have half the number of chromosomes as first cell Results in two genetically identical diploid cells Results in four genetically different haploid cells

Meiosis I 23 Meiosis II

Gene Linkage and Gene Maps How can two alleles from different genes be inherited together? Alleles from different genes tend to be inherited together when those genes are located on the same chromosome.

Gene Linkage A scientist used a fly with reddish-orange eyes and miniature wings in a series of test crosses. His results showed that the genes for those two traits were almost always inherited together. Morgan discovered that many of them appeared to be “linked” together in ways that seemed to violate the principle of independent assortment.

Gene Linkage Findings led to two conclusions: First, each chromosome has groups of linked genes. Second, it is the chromosomes that assort independently, not individual genes. Alleles of different genes tend to be inherited together when those genes are located on the same chromosome.

Gene Mapping Sturtevant wondered could gene linkage be a clue to the genes’ locations? Sturtevant reasoned if two genes are close together, then crossovers between them should be rare. If two genes are far apart, then crossovers between them should be common.

Gene Mapping Used the frequency of cross-overs between genes to determine their distances from each other. Method still used today.