Meiosis & Sexual Life Cycle Chapter 13

Slide 2 of 20 AP Essential Knowledge Essential knowledge 3.A.2: In eukaryotes, heritable information is passed to the next generation via processes that include the cell cycle and mitosis or meiosis plus fertilization. b. Mitosis passes a complete genome from the parent cell to daughter cells. 4. Mitosis is a continuous process with observable structural features along the mitotic process.  Students must know the order of the processes (replication, alignment, separation).

Slide 3 of 20 AP Essential Knowledge  c. Meiosis, a reduction division, followed by fertilization ensures genetic diversity in sexually reproducing organisms.  Gametes have a haploid set of chromosomes  Homologous chromosomes are paired and then separated ensuring haploid set of chromosomes  During meiosis (Prophase I), homologous chromatids exchange genetic material via a process called “crossing over” which increases genetic variation in the resultant gametes.

Slide 4 of 20 Homologous Chromosomes  2 chromosomes that have same length, centromere position, and staining pattern  Autosomes  Non-sex chromosomes  Chromosomes that do not determine gender  Sex Chromosomes  Chromosomes that determine gender

Slide 5 of 20 Chromosomes  Human somatic cell = 44 autosomes + 2 sex chromosomes  Human gamete = 22 autosomes + 1 sex chromosome  Sex Chromosomes  Can be XX or XY  XX = Homologous chromosomes  XY = Not homologous chromosomes  Egg must contain X, sperm may contain X & Y  Hence, males determine the gender of offspring

Slide 6 of 20 Unnecessary Censorship

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Slide 8 of 20 Meiosis  2 Stages of Meiosis  Meiosis I & Meiosis II  Much of Meiosis resembles Mitosis  Chromosomes are replicated only once  B efore Meiosis I  4 daughter cells are produced

Slide 9 of 20 Meiosis: An Overview Assume that an organism has: 1 Homologous Pair = 2 Chromosomes (Diploid cell – 2n) STEP 1: Each of the chromosomes is replicated in Interphase STEP 2: Chromosome pairs of copies separate in Meiosis I (Haploid cell – n) BUT 2 copies of each one STEP 3: Each of the copies (sister chromatids) in a cell separates creating 4 haploid cells (Haploid cell with only 1 copy)

Slide 10 of 20 What is different in Prophase I? What is different in Anaphase I?

Slide 11 of 20 Meiosis I  Prophase I  Longest phase  Homologous pairs align  Crossing-Over may occur  Synapsis – pairing of homologous pairs tied tightly together  Tetrads form (4 chromosomes = 2 pairs)  Each tetrad has 1 or more chiasmata  Criss-crossed regions where crossing over has occurred

Slide 12 of 20 Meiosis I (Page 2)  Metaphase I  Tetrads are aligned at the metaphase plate  Each chromosome pair faces a pole  Anaphase I  Homologous chromosomes (composed of 2 copies of each chromosome called chromatids) are pulled apart

Slide 13 of 20 What is different between Meiosis I & II? This division is sometimes called the Mitotic division, why?

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Slide 15 of 20 Meiosis vs. Mitosis  Tetrads align in Prophase I,  Chromosomes align in Prophase mitosis  Chromosomes metaphase plate (Mitosis)  Tetrads metaphase plate (Meiosis)  Homologues separate in Meiosis I  Sister chromatids separate in Meiosis II & Mitosis  Crossing over = Meiosis NOT mitosis

Slide 16 of 20 MitosisMeiosis DNA replicates in interphase 1 division No synapsis 2 Diploid cells Genetically identical cells Responsible for: -- Zygote growth into multicellular organism DNA only replicates in Pre- meiotic interphase 2 divisions Synapsis occurs during prophase I forming tetrads Crossing over occurs now 4 haploid cells Genetically different cells Responsible for: -- Gamete production -- Genetic variation

Slide 17 of 20 Genetic Diversity  The reason for meiosis + sexual reproduction  Mutations are the original source of genetic diversity  3 main sources of Genetic Diversity 1. Independent Assortment of Chromosomes 2. Crossing Over 3. Random Fertilization

Slide 18 of 20  Each daughter cell has a 50% chance of getting maternal chromosome (or its copy)  Similarly, 50% chance of getting paternal chromosome (or its copy)  Independent assortment - each chromosome is positioned independently of the other chromosomes

Slide 19 of 20  When homologous pairs are formed in Prophase I, a recombinant chromosome can be formed -- A chromosome that has DNA from 2 different parents  2 chromosome segments trade places (cross over) producing chromosomes with new combos of maternal & paternal genes  1-3 times per chromosome in humans  Increases genetic variation

Slide 20 of 20 Random Fertilization  Egg + sperm cells are genetically different from parent cells  Their combination (fertilization) increases variation even more