AP Biology Meiosis Part 2

Interphase

Important concepts from previous units: Evolution is “change over time”. Sexual reproduction involves haploid sperm and egg gametes. The DNA within the egg and sperm will create the next generation organism.

Mom and Dad’s DNA contribution The egg surrounded by sperm.

Meiosis - means “The process of gamete formation” This process occurs in the cells of the sex organs of the organism. These organs are called Gonads. This process has 2 divisions in the process after the S and G2 phases. Remember , that the S phase doubles the number of chromosomes. In humans 46  92. Meiosis I - This division is the separation of chromosome pairs. In humans, 92  46 Meiosis II - This division is the separation of sister chromatids. In humans, 46  23

(First Division)

(Second Division)

TWO Divisions

In this process, males produce 4 haploid sperm; each having 23 chromosomes. In this process, females produce 1 haploid egg with 23 chromosomes. The other three cells degrade into structures called polar bodies during the process. These can be seen on the nucleus membrane in female cells, not males. Stages to the process of Meiosis These stages are very similar to the stages of Mitosis. Three major differences, from Mitosis, are present to increase variation. (Remember, Mitosis is normal cell division. It basically makes clones of the adult. No variation.) Crossover (“genetic swapping”) occurs in Prophase I. (Creates variation.) Chromosome pairs separate in Anaphase I. (Creates Variation.) Sister Chromatids separate in Anaphase II. (Creates Variation.)

Spermatogenesis Epididymis Seminiferous tubule Testis Cross section of seminiferous tubule Spermatogonium Mitotic division, producing large numbers of spermatogonia Sertoli cell nucleus Differentiation and onset of meiosis I Primary spermatocyte (in prophase of meiosis I) Meiosis I completed Secondary spermatocyte Meiosis II Lumen of Seminiferous tubule Early spermatids Spermatids (at two stages of differentiation) Differentiation Sperm cells Neck Head Midpiece Tail Plasma membrane Acrosome Nucleus Mitochondria

Oogenesis in the ovaries Ovary Primary germ cell in embryo Differentiation Oogonium in ovary Oogonium Mitotic division Primary oocyte within follicle Primary oocyte Completion of meiosis I and onset of meiosis II Growing follicle Secondary oocyte First polar body Ovulation Entry of sperm triggers completion of meiosis II Mature follicle Ruptured follicle Second polar body Ovum Ovulated secondary oocyte Corpus luteum Degen- erating corpus luteum

Crossover (“genetic swapping”) between homologous chromosomes. This creates variation from the parent’s genome. They are then called Recombinant Chromosomes. Synapsis – Chromosomes that are in a state of being intertwined together. (“syn” means “together”; “sis” means “process of”) Tetrad - Four chromosomes twisted together (“tetra” means “four”… Like the game Tetris has four different shapes.) Chiasmata – Where the chromosomes physically overlap making an “x”. (Chi is the Greek letter for X.)

Crossover in Prophase I

Major differences between Mitosis and Meiosis: The number of divisions (Mitosis has 1; Meiosis has 2) The final products of each process (Mitosis – “cloned” daughter cells; Meiosis – haploid gametes) Crossover, in Prophase I, creates variation (No crossover in Mitosis) Chromosome pairs vs. sister chromatids separating in the second division to reduce DNA to haploid state.

Mitosis vs. Meiosis

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

Sources of variation creation Independent assortment of chromosomes. ( This happens 2x in Anaphase I and II.) 2 = Total number of possibilities (One goes one way; the other the other way in separation.) n = number of variables; 23 = number needed to make a haploid set in humans. 2n = 223 For humans the total is about 8 Million possibilities for each parent with each division. 8 Million possible outcomes X 2divisions X 2 parents = 4,096,000,000 possible combinations for just 46 chromosomes!

Independent Assortment Key Maternal set of chromosomes Possibility 1 Possibility 2 Paternal set of chromosomes Two equally probable arrangements of chromosomes at metaphase I Metaphase II Daughter cells Combination 1 Combination 2 Combination 3 Combination 4

Now add in Crossover (“genetic swapping”) Amount of crossing over varies from tetrad to tetrad. If little crossover occurs, the offspring looks very much like the adult parent. If lots of crossover occurs, the offspring looks very different from the adult parent. Random fertilization by a sperm. (There are millions released by the male. Which one will make it to the finish line?) That makes you a 1 in 70 trillion possibility – YOU ARE PRETTY DARN SPECIAL!

2^23 8388608

Evolution? As organisms became more complex, a more complex and more survival oriented way of reproducing came into existence over millions of years. The addition of a second division with a couple of slight changes in the same four steps (Prophase, Metaphase, Anaphase, and Telophase) creates the variation. The variation helps with survival and this would be beneficial in changing environments. Those that survive long enough get to reproduce and keep the species going. Those that don’t do not pass on those defective traits for surviving in that environment.

Volume of a sphere V= 4/3 π r3 Volume calculation, teach them this approach: Step 1: Take the radius and cube it (For example 33 = 3 X 3 X 3 = 27) Step 2: Take the cube answer and the multiply it by 4. Step 3: Take the answer from step 2 and divide it by 3. Step 4: Take the answer from step 3 and multiply it by 3.14