Unit 3

1. Unit 3 Chapter 9: Meiosis and Sexual Reproduction

2. Impact, Issues: Why Sex? • Asexual Reproduction: quick and efficient • Does not require the participation of a partner • BUT the offspring are all clones – no variation • Sexual Reproduction: most costly, but also most responsive to changing conditions • Male and female partners must find each other and exchange genetic material • The variation introduced by sex has selective advantages

3. Asexual Reproduction • One parent passes a duplicate of its genes (stored in DNA molecules) to its offspring • Offspring can only be genetically identical __________ of the parent • Bacteria:

4. Sexual Reproduction • Two Parents • Each parent contributes one gene for each trait • Offspring has a pair of genes on a pair of chromosomes • One chromosome of a pair is maternal, and the other is paternal • Offspring differ from its parents and each other • Sexual reproduction includes: meiosis, formation of mature reproductive cells called gametes, and fertilization

5. Alleles • Genes for each trait come in slightly different forms called ___________ • Alleles are unique molecular forms of the same gene; they specify different versions of a trait • Originally produced by mutations • Example: gene for eye color • Different alleles would include:

6. Sexual Reproduction • Meiosis shuffles the alleles during gamete formation • Fertilization produces offspring with unique combination of alleles • The variation generated by sexual reproduction allows for natural selection to occur and is the basis for evolutionary change

7. Meiosis • Meiosis is a nuclear division process that divides the parental chromosome number in half • In animals: gametes form by meiosis of germ cells • Plants: spores • Begin with diploid (2n) germ cells and produces ____________ gametes (n) • Humans: • In 2n cells, there are TWO chromosomes of each type, which are called _______________ chromosomes • Homologous chromosomes line up during meiosis, including sex chromosomes • Each gamete produced by meiosis has one of each pair of homologous chromosomes

8. Homologous Chromosomes The same length, size, and genes except for the non-identical sex chromosomes (X and Y)

9. Where Gametes Form Figure 9.3, pg 140

10. ______________ • In most multicelled species, gametes form from cells in reproductive structures or organs • Fertilization is the fusion of two gamete nuclei • Restores the parental chromosome number • Forms a ____________ , the first cell of a new individual

11. Two Divisions, Not One! • Mitosis and meiosis are similar, but different • Similarities between meiosis and mitosis: • Chromosomes are duplicated during interphase to form sister chromatids held together at the centromere • Chromosomes are moved by spindle fibers • Meiosis is different in that it has two series of divisions: ______________ and _______________

12. Meiosis I • Meiosis I is the ______________________________ • Each duplicated chromosome lines up with its homologous partner • Homologous chromosomes are separated • The two homologous chromosomes move apart toward opposite spindle poles • Each of the two daughter cells receives a haploid number of chromosomes • After meiosis I, each chromosome is ________ ____________________

13. Meiosis II • The _______________________________ • In meiosis II, the sister chromatids of each chromosome separate and the cytoplasm divides again • Once pulled away from each other, each sister chromtid is now an individual chromosome • Results in FOUR _________ cells (n) • Each have one unduplicated chromosome

14. Meiosis I and Meiosis II Figure 9.12, pg 150

15. Prophase I • Chromosomes condense and align tightly with their homologues • Each homologous pair undergoes ____________ _____ • Microtubules form the bipolar spindle • One pair of centrioles moves to the other side of the nucleus • Nuclear envelope breaks up • Microtubules growing from each spindle pole enter the nuclear region • Microtubules tether one or the other chromosome of each homologous pair

16. Prophase I Figure 9.5, pg 142

17. Metaphase I • Microtubules from both poles position all pairs of homologous chromosomes at the spindle equator Figure 9.5, pg 142

18. Anaphase I • Microtubules separate each chromosome from its homologue • As in mitosis, other microtubules that overlap at the equator slide past each other to push the poles farther apart • At the end of anaphase I, one set of duplicated chromosomes nears each spindle pole Figure 9.5, pg 142

19. Telophase I • Two nuclei form • In most species, the cytoplasm divides Each chromosome still consists of TWO _________ ______________ (remains duplicated) Figure 9.5, pg 142

20. Prophase II • There is no DNA replication between the two nuclear division • Prophase II is the beginning of meiosis II • One of the two centrioles moves to the opposite side of the cell Figure 9.5, pg 143

21. Metaphase II • Chromosomes are aligned at the equator • Sister chromatids are attached to spindle fibers from opposite poles Figure 9.5, pg 143

22. Anaphase II • One chromosome of each type is moved toward opposite spindle poles Figure 9.5, pg 143

23. Telophase II • Each step of Meiosis II occurs in BOTH nuclei formed in meiosis I • By the end of telophase II, there are four haploid nuclei, each with unduplicated chromosomes Figure 9.5, pg 143

24. Haploid Daughter Cells • Once the cytoplasm divides after meiosis, • One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies