1. Two kinds of reproduction Asexual reproduction (1 parent) –Offspring inherit parent’s genes –Clones (identical copies of parent) Sexual reproduction (2 parents) –Offspring differ from parents and each another –Different combinations of alleles –Different details of shared traits –Slightly different forms of the same gene –Each specifies a different version of gene product

2. During sexual reproduction…. Meiosis, gamete formation, and fertilization All chromosomes are duplicated during interphase, before meiosis Two divisions, meiosis I and II, –divide parental chromosome number by two End result gamete is haploid Meiosis and fertilization shuffle parental alleles –Offspring inherit new combinations of alleles

3. What meiosis does? Meiosis –Nuclear division mechanism that precedes gamete formation in eukaryotic cells –Halves parental chromosome number Fertilization –Fusion of two gamete nuclei –Restores parental chromosome number –Forms zygote (first cell of new individual)

4. Prophase I is long Meiosis I is a continuous process with five distinct phases Chromosomes condense and align tightly with their homologues Each homologous pair undergoes crossing over Microtubules form the bipolar spindle

5. Prophase I (cont.) One pair of centrioles moves –to other side of nucleus Nuclear envelope breaks up –Microtubules growing from each spindle pole penetrate nuclear region Microtubules anchors homologous pair

6. Metaphase I homologous chromosomes aligns at equator region of the spindle

7. Anaphase I Microtubules separate each chromosome from its homologue, moving to opposite spindle poles As anaphase I ends, one set of duplicated chromosomes nears each spindle pole

8. Telophase I Two nuclei form –Typically, the cytoplasm divides All chromosomes are still duplicated –Each still consists of two sister chromatids

9. Meiosis II The second nuclear division Sister chromatids of each chromosome are pulled away from each other Each is now an individual chromosome Prophase II Metaphase II

10. Anaphase II and Telophase II anaphase II one chromosome of each type –moves toward opposite spindle poles end of telophase II, –four haploid nuclei, each with unduplicated chromosomes Anaphase IITelophase Il

11. Results in haploid daughter cells When cytoplasm divides, –four haploid cells Telophase IAnaphase IMetaphase IProphase I Meiosis I

12. Two events variation in traits in sexually reproducing species –Crossing over during prophase I of meiosis Chromosome shuffling during metaphase I of meiosis Nonsister chromatids of homologous chromosomes undergo crossing over –They exchange segments at the same place along their length Each ends up with new combinations of alleles not present in either parental chromosome prophase I metaphase I

13. Life cycles of plants and animals Multicelled diploid and haploid bodies Plants –Sporophyte: A multicelled plant body (diploid) that makes haploid spores –Spores give rise to gametophytes (multicelled plant bodies in which haploid gametes form) Animals –Germ cells in the reproductive organs give rise to sperm or eggs –Fusion of a sperm and egg at fertilization results in a zygote

14. Introducing Variation in Offspring Three events cause new combinations of alleles in offspring: –Crossing over during prophase I (meiosis) –Random alignment of maternal and paternal chromosomes at metaphase I (meiosis) –Chance meeting of gametes at fertilization All three contribute to variation in traits

15. Differences in mitosis and meiosis Both require bipolar spindle to move and sort duplicated chromosomes –mitosis, and may have evolved from meiosis Mitosis maintains parental chromosome number –Duplicates genetic information –Occurs in body cells Meiosis halves chromosome number –Introduces new combinations of alleles in offspring –Occurs only in cells for sexual reproduction