Ch. 7 Meiosis & Sexual Reproduction

What if… What if… A HUMAN sperm & egg each had 46 chromosomes, like other human body cells, how many chromosomes would a zygote have? Why might an increase in the number of chromosomes a human cell has cause problems?

WHITEBOARDS Draw the gametes for males and females Inside each sex cell, give the number of chromosomes Below your drawing, write whether the sex cells are haploid or diploid If an organism’s sex cells have 12 chromosomes, how chromosomes will a body cell have?

Meiosis- Formation of Haploid Cells 1. DNA in the original cell replicated 2. Meiosis halves the number of chromosomes when forming gametes or spores Still occurs in the nucleus! Start out with diploid cell, replicate DNA – chromosomes are cut in half

Draw it out! Meiosis Look at your books (p. 144) and your notes for pictures of Meiosis I & II Draw your assigned stage on the board – be ready to explain it! Start out with diploid cell, replicate DNA – chromosomes are cut in half

Meiosis I Meiosis Start out with diploid cell, replicate DNA – chromosomes are cut in half

Meiosis II Meiosis Start out with diploid cell, replicate DNA – chromosomes are cut in half

Hold them up! Meiosis Hold up the number of cells produced at the end of Meiosis I Hold up the number of cells produced at the end of Meiosis II Hold up the number of sets of chromosomes in one haploid cell at the end of Meiosis II 2 Sets = 46 chromosomes 1 Set = 23 chromosomes

Meiosis- Formation of Haploid Cells 4 haploid cells result at the end of Meiosis II

Think – Pair - Share Genetic Variation What does the term “genetic variation” mean? How important is genetic variation to the survival of our species?

Importance of Genetic Variation 1. Meiosis and the joining of gametes are essential to evolution No genetic process generates variation more quickly 2. Evolution appears to increase with increases in genetic variation

Meiosis & Genetic Variation 1. Independent Assortment- In humans, each gamete receives 1 chromosome from each of 23 pairs of homologous chromosomes ( = 23 total chromosomes = 1n) Which of the 2 homologues each offspring receives is a matter of chance 8 million different gamete combinations can result

Meiosis & Genetic Variation 2. Crossing Over- DNA exchange during Prophase I in Meiosis Portions of a chromatid on 1 homologous chromosome are broken & exchanged with the chromatid portions of the other homologous chromosome

This recombination GREATLY increases genetic variation -How is this different from translocation? -Crossing over occurs between homologous chromosomes -Translocation is with non-homologous

Meiosis & Genetic Variation 3. Random Fertilization- two gametes are randomly joined to form a zygote 64 trillion possible outcomes

Genetic Variation - Importance Importance of genetic variation Breeding larger or faster animals can be limited until enough genetic variation is eventually generated to continue the breeding

Genetic Variation Importance Importance of genetic variation 4. Natural selection doesn’t always favor genetic change Existing conditions may be favorable, slowing evolution

Meiosis- Formation of Haploid Cells Pages 148 – 149 Boys do Spermatogensis Girls do Oogenisis

Meiosis- Formation of Haploid Cells Meiosis in Males SPERMATOGENESIS- produce sperm in the testes of male animals A. Diploid cell increases in size to germ cell B. Undergoes meiosis I C. Undergoes meiosis II to form 4 haploid cells D. 4 cells develop tail (sperm)

Meiosis- Gamete Production Meiosis in females OOGENESIS- produces eggs in the ovary of female animals A. Diploid cell increases in size to a germ cell B. Undergoes meiosis I- cytoplasm splits unevenly into each egg cell C. Undergoes meiosis II- more uneven cytoplasm distribution D. One large egg cell develops, 3 polar bodies that do not survive

Meiosis vs. Mitosis Meiosis vs. Mitosis Mitosis – Meiosis – Division of somatic (body) cells Diploid (2n) Creates identical cells Meiosis – Division of chromosomes to form reproductive cells Haploid (n) sex cells produced Genetic variation

Sexual Life Cycles in Eukaryotes Entire span in the life of an organism from one generation to the next

Sexual Life Cycles in Eukaryotes Sexual Haploid Life Cycle 1. Zygote is diploid but undergoes meiosis immediately to make new haploid cells (any DNA damage is repaired)

Sexual Life Cycles in Eukaryotes Sexual Haploid Life Cycle 2. Haploid cells create haploid individuals 3. Gametes are produced by mitosis

Sexual Life Cycles in Eukaryotes Sexual Haploid Life Cycle 4. Gametes fuse to produce diploid zygote, cycle continues Occurs in protists, fungi, algae, chlamydomonas

Sexual Life Cycles in Eukaryotes Sexual Diploid Life Cycle 1. Adults are diploid, each inheriting characteristics from both parents 2. Diploid reproductive cells undergo meiosis to produce gametes

Sexual Life Cycles in Eukaryotes Sexual Diploid Life Cycle 3. Fertilization- joining of gametes (sperm and egg) 4. Resulting zygote divides by mitosis

Sexual Life Cycles in Eukaryotes Sexual Diploid Life Cycle 5. Cells of adult also end up diploid 6. All cells involved are diploid, except gametes which are haploid Ex: Humans/ Animals

Sexual Life Cycles in Eukaryotes Alternation of generations 1. Often in plants, alternates between haploid and diploid phases

Sexual Life Cycles in Eukaryotes Alternation of generations 2. Sporophyte- diploid phase in plant life cycle that produces spores Spore forming cells undergo meiosis to produce spores (haploid reproductive cell that become an adult without fusing with another cell)

Sexual Life Cycles in Eukaryotes Alternation of generations 3. Gametophyte- haploid phase in plant life cycle that produces gametes through mitosis Gametes fuse and give rise to diploid phase

Sexual Life Cycles in Eukaryotes Alternation of generations 4. Ex: Moss Stalk tip’s capsule pops off scattering haploid spores Spores germinate by mitosis and form gametophytes Male gametophyte releases sperm that swim through film of moisture to eggs in female gametophyte Diploid zygote develops as sporophyte within the gametophyte Cycle continues

Cloning by parthenogenesis 1. A new individual develops form an unfertilized egg 2. No male involved, offspring is genetic clone of mother 3. Mother’s own chromosomes are copied in place of the male’s 4. Occurred in females without male companionship for long periods of time 5. Dandelions, hawkweeds, fish, lizards, frogs, snakes, male drone honeybees, NO MAMMALS

Research in Parthenogenesis

Assessment Identify the type of reproduction that results in offspring that are genetically identical to their parent Describe two different types of eukaryotic asexual reproduction Compare the haploid life cycle found in chlamydomonas with a diploid life cycle Summarize the process of alternation of generations Evaluate the significance of mutations and repair of mutations to the evolution of sexual reproduction