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Meiosis

Meiosis. Chapter 11 Page 247. Section 11.1: Reproduction. What is reproduction? Process of producing offspring There are two types: Asexual reproduction A single parent passes a complete copy of its genetic information to each of its offspring Sexual reproduction

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Meiosis

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  1. Meiosis Chapter 11 Page 247

  2. Section 11.1: Reproduction • What is reproduction? • Process of producing offspring • There are two types: • Asexual reproduction • A single parent passes a complete copy of its genetic information to each of its offspring • Sexual reproduction • Two parents give genetic material to produce offspring that are genetically different from their parents

  3. Types of asexual reproduction Remember: The offspring of asexual reproduction are identical to the parent!

  4. Budding by a hydra

  5. Budding by a Sponge…

  6. Parthenogenesis • Page 255

  7. Sexual Reproduction • Each parent produces a sex cell (germ cell) or gamete through a process called meiosis. • All other cells in the body are called somatic cells. • Each gamete is haploid, or n,containing one set of chromosomes • (n = number of chromosomes)

  8. The two gametes fuse to form the zygote, or fertilized egg. • This fusion is called fertilization.

  9. The zygote contains two sets of chromosomes one from each parent and is diploid, or 2n. • These two sets of chromosomes are homologous, each chromosome from the male has a corresponding one from the female. They are similar in size, shape and genetic content

  10. Chromosome number • Genes are located on chromosomes • Each chromosome contains thousands of genes that control how an organism develops and functions • Different species have a specific number of chromosomes • If an organism has too many or too few chromosomes, the organism may not develop and function properly • Humans have 46 chromosomes total

  11. Autosomes vs Sex chromosomes • Autosomes are chromosome that do not determine sex • Sex chromosomes have genes that determine sex • The last pair of chromosomes are commonly referred to as X and Y chromosomes • Males are XY • Females are XX

  12. What is meiosis? • A form of cell division that produces daughter cells with half the number of chromosomes that are in the parent cell • Involves two divisions of a diploid (2N) eukaryotic cell that result in four haploid (N) daughter cells

  13. Meiosis continued… • Meiosis is divided into meiosis I and meiosis II each subdivided into prophase, metaphase, anaphase, and telophase. • Before meiosis begins, the DNA has already been replicated; meiosis begins with homologous chromosomes

  14. Meiosis • Many pictures were used from the Prentice Hall Biology Coach

  15. Meiosis I • Meiosis 1 happens after interphase in which the chromosomes have replicated and each chromosome consists of two sister chromatids • The chromosomes will only replicate once during meiosis

  16. Prophase I • Homologous chromosomes pair along their entire length • Chromosomes coil, becoming compact and visible. • The nucleolus and nuclear envelope disappears and the spindle forms

  17. Metaphase I • Spindle moves the pairs of homologous chromosomes to the equator of the cell • Homologous pairs remain together

  18. Anaphase I • Spindle fibers pull homologous chromosomes apart but each chromosome still consists of two chromatids

  19. Telophase I • Cytoplasm divides • Cytokinesis • Two new cells are formed • Both cells have one chromosome from each pair of homologous chromosomes • animation

  20. Meiosis II • No further replication of chromosomes • Begins with the two cells formed at the end of telophase I of meiosis I

  21. Prophase II • Chromosomes condense • Nuclear envelope breaks down • Spindle apparatus forms

  22. Metaphase II • Chromosome line up at equator • Spindle fibers attach to centromeres

  23. Anaphase II • Centromeres split • Chromatids separate and are now chromosomes • Pulled to opposite poles

  24. Telophase II • The nuclear membrane forms • The spindle breaks down • Cytokinesis occurs and the cells divide. • The result of meiosis is four haploid cells • Animation

  25. Comparison of mitosis and meiosis • http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter12/animations.html#

  26. Meiosis contributes to genetic variability • Three key contributions to genetic variation: • Chromosome pairs separate independently - called independent assortment • Crossing over • Random fertilization

  27. Independent assortment is the random distribution of homologous chromosomes during meiosis

  28. Crossing Over • During Prophase I, when homologous chromosomes line up next to each other, one arm of a chromatid crosses over the arm of the other chromatid and exchange pieces

  29. Crossing over a closer look

  30. Random fertilization • Because fertilization of an egg by a sperm is random, the number of possible outcomes is squared • In humans, the possibility is 223 x 223 or about 70 trillion different combinations!

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