11 4 Meiosis Lesson Overview Meiosis Chromosome Number

11. 4 Meiosis

Lesson Overview Meiosis Chromosome Number • Chromosomes carry genes. • Genes are located in specific positions on chromosomes.

Lesson Overview Meiosis Diploid Cells • Fruit flies have eight chromosomes. 1 2 2 1 3 3 • Four come from the father, four come from the mother. • Homologous pair – The two sets of chromosomes that match. Always one from each parent. 4 4

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

Lesson Overview Meiosis 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 2 N with 8 chromosomes. How many chromosomes do the gametes have? Four

Lesson Overview Meiosis 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. Meiosis I Interphase Prophase I Metaphase I Anaphase I Telophase I Cytokinesis Prophase II Metaphase II Anaphase II Telophase II Cytokinesis Meiosis II

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

Lesson Overview Meiosis 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).

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

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

Lesson Overview 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.

Lesson Overview 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.

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

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

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

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

Lesson Overview Meiosis 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.

Lesson Overview Meiosis Comparing Meiosis and Mitosis Meiosis 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

Lesson Overview Meiosis

Lesson Overview Meiosis I 23 Meiosis II

Lesson Overview Meiosis 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.

Lesson Overview Meiosis 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.

Lesson Overview Meiosis 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.

Lesson Overview Meiosis 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.

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