Meiosis & Sexual Life Cycle Chapter 13

Slide 2 of 27 Definitions  Genetics – scientific study of heredity and hereditary variation  H eredity – transmission of traits from one generation to the next one  Genes – Hereditary units that code for proteins  Gametes – Actual mechanism for hereditary transmission  Fertilization – Combining gametes  Locus – gene’s location on a chromosome

Slide 3 of 27 Asexual Reproduction  Single Parent  Reproduction occurs by mitosis, binary fission, budding, etc.  Offspring is exact copy (genetically)  Can be called a clone or a “Mini-me  Can get genetic variation, but rarely  Due to mutations  Common among unicellular organisms, but also found in multicellular organisms as well  Budding

Slide 4 of 27 Sexual reproduction  It takes 2 to tango = 2 parents  Unique combination of genes  Vary genetically from both parents and their siblings  May exhibit similarities to parents

Slide 5 of 27 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 6 of 27 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 7 of 27 Karyotype  Ordered display of chromosomes used to distinguish the number and size of homologous chromosomes

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Slide 9 of 27 Unnecessary Censorship

Slide 10 of 27 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 11 of 27 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 12 of 27 Repeat the Diagram but with 4 Chromosomes  The cell before interphase has 4 chromosomes and is diploid  Indicate how many chromosomes are present: 1. After interphase but before Meiosis 2. After Meiosis I 3. After Meiosis II

Slide 13 of 27 Questions?  If a cell has 10 chromosomes and is diploid, how many chromosomes (include what the book calls chromatids) are found at: A) the end of Meoisis I B) the end of Meiosis II

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Slide 15 of 27 What is different in Prophase I? What is different in Anaphase I?

Slide 16 of 27 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 17 of 27 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 18 of 27 What is different between Meiosis I & II? This division is sometimes called the Mitotic division, why?

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Slide 20 of 27 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 21 of 27 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 22 of 27 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 23 of 27  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 24 of 27  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 25 of 27 Random Fertilization  Egg + sperm cells are genetically different from parent cells  Their combination (fertilization) increases variation even more