1. Meiosis and Sexual Reproduction

2. Genes and Alleles Genes Alleles
Sequences of DNA that encode heritable traits
Alleles
Slightly different forms of the same gene
Each specifies a different version of gene product

3. Sexual and Asexual 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

4. Sexual Reproduction
Meiosis, gamete formation, and fertilization occur in sexual reproduction
Meiosis and fertilization shuffle parental alleles
Offspring inherit new combinations of alleles

5. Where Gametes Form

6. Key Concepts: SEXUAL VS. ASEXUAL REPRODUCTION
By asexual reproduction, one parent alone transmits its genetic information to offspring
By sexual reproduction, offspring typically inherit information from two parents that differ in their alleles
Alleles are different forms of the same gene; they specify different versions of a trait

7. What Meiosis Does Meiosis Fertilization
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)

8. Meiosis and Fertilization

9. Homologues Sexual reproducers inherit pairs of chromosomes
1 from maternal parent, 1 from paternal parent
The pairs are homologous (“the same”)
Except nonidentical sex chromosomes (X and Y)
Same length, shape, genes
All pairs interact at meiosis
One chromosome of each type sorts into gametes

10. Tour of Meiosis
All chromosomes are duplicated during interphase, before meiosis
Two divisions, meiosis I and II, divide the parental chromosome number by two
Each forthcoming gamete is haploid (n)

11. Prophase I

12. Metaphase I

13. Anaphase I

14. Telophase I

15. Prophase II

16. Metaphase II

17. Anaphase II

18. Telophase II

19. Anaphase I Telophase I Prophase I Metaphase I
Meiosis I
Stepped Art
Fig. 9-5a, p.142

20. Anaphase II Telophase II Prophase II Metaphase II
Meiosis II
Stepped Art
Fig. 9-5b, p.142

21. Haploid Daughter Cells
When cytoplasm divides, four haploid cells result
One or all may serve as gametes or, in plants, as spores that lead to gamete-producing bodies

22. Key Concepts: STAGES OF MEIOSIS
Diploid cells have a pair of each type of chromosome, one maternal and one paternal
Meiosis, a nuclear division mechanism, reduces the chromosome number
Meiosis occurs only in cells set aside for sexual reproduction

23. Prophase I: Crossing Over
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

24. Crossing Over

25. Fig. 9.6d, p.144

26. Fig. 9.6e, p.144

27. Fig. 9.6f, p.144

28. Comparing Plant And Animal Life Cycles

29. 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

30. secondary spermatocytes (haploid) primary spermatocyte (diploid)
sperm (mature,
haploid male
gametes)
secondary spermatocytes (haploid)
primary spermatocyte (diploid)
diploid male
germ cell
spermatids (haploid)
a Growth
b Meiosis I and
cytoplasmic division
c Meiosis II and cytoplasmic division
Fig. 9.9, p.147

31. Egg Formation in Animals

32. Differences in Mitosis and 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