Meiosis Meiosis is a form of nuclear division that divides a diploid cell into haploid cells. This process is essential for sexual reproduction. Only takes place in your gametes (sperm and egg cell) This process ensures that the offspring receives the correct number of chromosomes. Why is it important that gametes are haploid cells?

Meiosis vs. Mitosis

Process of Meiosis Objectives: Compare and Contrast the two round of division in meiosis. Describe how haploid cells develop into mature gametes. Warm Up: In order to make 1 + 1 = 1, what needs to happen to the DNA of the parents? Words to Know: Tetrad, Crossing-Over, Spermatogenesis, Oogenesis, Polar body, Non-disjunction, Down syndrome, Karyotype, Pedigree

Cells go through Two Round of Division in Meiosis Meiosis is a form of nuclear division that creates four haploid cells from one diploid cell. Meiosis creates genetic diversity.

Homologous Chromosomes and Sister Chromatids Homologous chromosomes are two separate chromosomes: one from mom, one from dad. They are NOT copies of each other. Together the two chromosomes are called sister chromatids. Sister chromatids refer to the duplicated chromosomes that remain attached by the Centromere. Homologous chromosomes are divided in meiosis I. Sister Chromatids are divided in meiosis II.

Meiosis I START WITH ONE CELL Begins with interphase (identical to The Cell Cycle before mitosis). G1, S and G2 phase Before Meiosis I begins, DNA has already been copied. Meiosis I divides homologous chromosomes. Dad Mom Homologous Chromosomes (one from mom and one from dad) are the same size, shape, and carry the genes for the same traits. The choices may be different… one may code for brown eyes, the other for blue.

Meiosis I 1. Prophase I – nuclear membrane breaks down, centrosomes and Centrioles move to opposite sides of the cell and spindle fibers begin to form. The homologous chromosomes pair with their replicate to form a tetrad. Tetrad-a pair of homologous chromosomes which contain 4 chromatids. The Homologous chromosomes break and exchange genetic material in a process called crossing over. Crossing over results in new combinations of alleles on a chromosome. This causes genetic variability to increase. This is why we all look different

Meiosis I Metaphase I – Homologous chromosome pairs line up in the middle of the cell. Anaphase I – paired homologous chromosomes separate from each other and move towards opposite ends of the cell. 4. Telophase I – nuclear membrane reforms in some species, spindle fibers disassemble and the cell undergoes Cytokinesis.

Meiosis I

Meiosis II Meiosis II divides sister chromatids and results in undoubled chromosomes. DNA is NOT copied before this stage. 5. Prophase II – The nuclear membrane breaks down, centrosomes and Centrioles move to opposite side of the cell, spindle fibers form. 6. Metaphase II – Spindle fibers align the 23 chromosomes at the center of the cells. Each chromosome still has two sister chromatids. 7. Anaphase II – Sister chromatids are pulled apart and move to opposite ends of the cell. 8. Telophase II – Nuclear membranes form around each set of chromosomes and the cell undergoes Cytokinesis. END RESULT WITH: 4 HAPLOID SEX CELLS (N)

Meiosis II

Comparing Mitosis and Meiosis One cell division Two cell divisions Homologous chromosomes never pair up. Homologous chromosomes line up at the equator. Sister chromatids separate in anaphase. In anaphase I sister chromatids remain together and homologous chromosomes separate. Diploid Cells made. Haploid cells made. 46 chromosomes (Diploid 2N) 23 chromosomes (Haploid N) Occurs ONLY in Somatic Cells (body cells) Occurs ONLY in your Sex cells (gametes/sperm and egg)

Comparing Mitosis and Meiosis Figure 13.9 MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6

So, why don’t we look exactly like our parents or our siblings? Crossing over occurs during Prophase 1 A chromosome may cross over 1-3 times per chromosome pair. The reshuffling of genetic material during crossing over and independent assortment in meiosis produces genetic variation Figure 13.11 Prophase I of meiosis Nonsister chromatids Tetrad Chiasma, site of crossing over Metaphase I Metaphase II Daughter cells Recombinant chromosomes

The human life cycle Key Figure 13.5 Haploid gametes (n = 23) Haploid (n) Diploid (2n) Haploid gametes (n = 23) Ovum (n) Sperm Cell (n) MEIOSIS FERTILIZATION Ovary Testis Diploid zygote (2n = 46) Mitosis and development Multicellular diploid adults (2n = 46)

Haploid Cells Develop into Mature Gametes Haploid cells are the end result of meiosis. Gametogenesis is the production of gametes. Briefly explain how a sperm cell’s structure is related to its function.

Spermatogenesis: the process of forming sperm cells by meiosis. * The sperm cell, the male gamete, is much smaller that the egg, the female gamete. * Sperm formation starts with a round cell and ends by making a streamlined cell that can move rapidly.

Oogenesis: the process of forming an ovum (egg) by meiosis. * The formation of the egg is complicated. * The end result is one egg and 3 polar bodies, cells with little more than DNA that are broken down.

Mistakes in Meiosis Although the events of meiosis usually proceed accurately, sometimes chromosomes fail to separate correctly. The failure of homologous chromosomes to separate properly during meiosis is called Nondisjunction. In humans, if a gamete with an extra chromosome number is fertilized by a normal gamete the resulting zygote has 47 chromosomes instead of 46. This zygote will develop into a baby with Down Syndrome. Most human abnormal chromosome numbers result in embryo death often before a woman even realizes she’s pregnant.

KARYOTYPE: is a test to identify and evaluate the size, shape, and number of chromosomes in a sample of body cells.

Mistakes in Meiosis However, organisms with more than the usual number of chromosomes called polyploids are rare in animals and almost always cause death of the zygote this is not the case in plants. Often, the flowers and fruits of these plants are larger than normal, and the plants are healthier. Some examples of abnormalities: A woman with only one X chromosome (XO) instead of 2 is called Turner syndrome. There may be an extra one XXX or XYY. A male with an extra X chromosome XXY is called Klinefelter’s syndrome and is sterile. Most of these individuals lead normal lives, but they cannot have children and some have varying degrees of mental retardation.

Pedigree Chart A Pedigree is a Chart for Tracing Genes in a Family. A Pedigree chart can help trace the phenotypes and genotypes in a family to determine whether people carry recessive alleles. When enough family phenotypes are known, genotypes can be figured out.

Stages of Meiosis http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages of Meiosis Specific view of crossing over during Prophase I of Meiosis http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio16.swf::Unique Features of Meiosis

Comparison of Meiosis and Mitosis

Sexual Reproduction Creates Unique Gene Combinations The major advantage of sexual reproduction is that is gives rise to a great deal of genetic variation within a species. This variation results largely from: 1. The independent assortment of chromosomes during meiosis. 2. The random fertilization of gamete. Independent assortment and fertilization play key roles in creating and maintaining genetic diversity in all sexually reproducing organisms. The possible combinations vary from species to species. Fruit fly gametes each have four chromosomes, representing 24, or 16 possible chromosome combinations. How many chromosome combinations could result from fertilization between a fruit fly egg and a sperm cell?

Crossing Over During Meiosis Increases Genetic Diversity Crossing over is the exchange of chromosome segments between homologous chromosomes during prophase I of meiosis I. Part of one chromatid form each chromosome breaks off and reattaches to the other chromosome. Crossing over happens any time a germ cell divides. Crossing over is also known as genetic recombination.