Chapter 5.3 Meiosis

Chromosomes –Contain instructions for traits –Found on Genes Segments of DNA Code for specific proteins –46 found in the human cells 23 from each parent Chromosomes and Chromosome Number

Chromosomes and Chromosome Number

Homologous Chromosomes –Have same length –Carry same genes Hair color, eye color, height, etc. –23 homologous pairs One of the pair from each parent Example: each shoe is like a homologous chromosome in a pair of shoes Chromosomes and Chromosome Number

Sex Cells In order to perform sexual reproduction sex cells are used Sex cells have 23 chromosomes-half the usual number Sex cells have only one “shoe” of the pair

Haploid and Diploid cells –Haploid (n) cells have one set of chromosomes 23 chromosomes in humans Sex cells –Diploid (2n) cells have two sets Contain homologous pairs 46 chromosomes Body cells Chromosomes and Chromosome Number

Human Life Cycle Haploid and Diploid stages of the human life cycle.

Formation of Haploid Cells Meiosis reduces the number of chromosomes in a cell Cells end with half the number of chromosomes than the original cell. Creates leads to four haploid cells (gametes-sex cells) During meiosis chromosomes are copied once and then the nucleus divides twice during the two stages of meiosis

Meiosis I Meiosis II Meiosis I includes Prophase I Metaphase I Anaphase I Telophase I Produces two haploid cells Meiosis II includes prophase II metaphase II anaphase II telophase II Four new haploid cells result.

Meiosis I: reductive division. –Chromosome number from diploid to haploid Meiosis II: produces gamete cells –individual chromosomes

Meiosis I Prophase I: homologous pairs come together to form tetrads Tetrads Two homologous pairs that come together (4 chromatids) Nucleus disapears Centrioles move toward poles and spindle fibers begin to form

Meiosis I Metaphase I: Tetrads line up in middle Spindle fibers attach to the tetrads

Meiosis I Anaphase I: –Tetrads separate, centromeres odo not separate. –homologous chromosomes move toward the poles –Spindle fibers shorted

Meiosis I Telophase I: two haploid daughter cells doubled stranded homologous chromosomes Nucleus reforms

Meiosis II Prophase II: begins shortly after Telophase I. Nucleus disapears New spindle fibers forms around the homologous chromosomes. Centrioles move to poles

Meiosis II Metaphase II: –double stranded homologous chromosomes line up in middle –Spindle fibers attach to centromeres –Just like Mitosis!

Meiosis II Anaphase II: –centromeres separate, –individual chromatids move toward the poles

Meiosis II Telophase II: –nuclear membrane reforms –chromosomes unwind and become chromatin –four haploid daughter cells form

Mitosis and Meiosis

Summary

Meiosis Variability

Development of Gametes Spermatogenesis is the process by which sperm cells are produced. Oogenesis is the process that produces mature egg cells.