1. AP Biology Chapter 10
Meiosis

2. Key Concepts:
Sexual reproduction entails meiosis, gamete formation, and fertilization
Germ cells of male and female animals produce gametes
Diploid cells have two of each type of chromosome, one from each parent
Meiosis divides the chromosome number in half

3. Key Concepts: Gametes are haploid (n)
The union of gametes at fertilization results in a diploid number (2n)
During meiosis, chromosomes may exchange random segments by crossing over
Meiosis leads to variations in traits among offspring

4. Comparison of Asexual and Sexual Reproduction
Single parent  offspring inherit same genes as parent (number and type)
Sexual
Produces genetic variation among offspring
Involves
Meiosis
Gamete formation
Fertilization

5. Chromosomes come in pairs, like socks
One homolog
Homologous chromosomes (unreplicated)

6. Homologous Chromosomes
Cell has two of each chromosome
One chromosome in each pair from ♀ (mom) and one from ♂ (dad)
Paternal and maternal chromosomes carry the same genes but different alleles.

7. Sexual Reproduction Shuffles Alleles
Through sexual reproduction, offspring inherit new combinations of alleles, which leads to variations in traits.
This variation is traits is the basis for evolutionary change

8. Gamete Formation Gametes are sex cells (sperm, eggs)
Arise from germ cells
anther
ovary
Germ cells are diploid (2n)
Gametes are haploid (n)
Meiosis halves chromosome number

9. Chromosome Number Sum total of chromosomes in a cell
Germ cells are diploid (2n)
Gametes are haploid (n)
Meiosis halves chromosome number
Human Karyotype

10. Meiosis: Two Divisions
Two consecutive nuclear divisions
Meiosis I
Meiosis II
DNA is not duplicated between divisions
Four haploid nuclei form

11. A Chromosome Refresher…

12. Germ Cell at Interphase
A diploid cell (2n)
DNA duplicated
Ready for first division of meiosis

13. Meiosis I

14. Meiosis I - Overview newly forming microtubules Prophase I Meiosis I
Metaphase I
spindle equator
one pair of homologous chromosomes
Anaphase I
Telophase I
Stepped Art

15. Meiosis I Prophase I Metaphase I Homologs pair Crossing over occurs
Spindle forms
Nuclear envelope disappears
Metaphase I
Pairs of homologous chromosomes are at spindle equator
Random arrangement

16. Crossing Over
Crossing over

17. Effect of Crossing Over
After crossing over, each chromosome contains both maternal and paternal segments
Creates new allele combinations in offspring
Either the maternal of paternal member of a homologous pair can end up at either pole
The chromosome in a gamete are a mix of chromosomes from the two parents

18. Meiosis I – continued… Anaphase I Telophase I Duplicated chromosomes
pulled apart from homologue
toward opposite spindle poles
Sister chromatids remain attached
Telophase I
Chromosomes arrive at poles
Usually followed by cytokinesis
Cells divides into two cells

19. Meiosis II - overview NO DNA duplication between I and II
Sister chromatids are separated
Result is four haploid daughter cells

20. Meiosis II Prophase II Anaphase II Telophase II Metaphase II
New spindle formed
Chromosomes move towards equator
Metaphase II
Duplicated chromosomes positioned at the equator
Anaphase II
Sister chromatids are moved apart
Telophase II
Four nuclei with haploid chromosome number (n) resulting in four cells

21. combinations possible1 2 3
combinations possible
Possible outcomes of the random alignment of only three pairs of chromosomes at metaphase I (without crossing over!!) As a result of random alignment, the number of possible combinations in a gamete is 2n, where n=# of chromosome types or or or
Stepped Art

22. Results of Recombination
Random alignment

24. Oogenesis

25. Spermatogenesis

26. Fertilization Male and female gametes unite and nuclei fuse
Fusion of two haploid nuclei produces diploid nucleus in the zygote
Which two gametes unite is random
Adds to variation among offspring

27. In Conclusion
Life cycle of sexually reproduction includes meiosis, gamete formation, and fertilization
A germ cell (2n) has two of each type of chromosome inherited from each parent. Each pair is homologous.
Chromosomes are duplicated in interphase.
Meiosis consists of two divisions

28. In Conclusion Meiosis produces haploid cells
In meiosis I, each chromosome moves away from homologous partner. In meiosis II, sister chromatids are separated
Meiosis produces haploid cells
Crossing over, random allocation of chromosomes, and the chance of gametes meeting at fertilization contribute to variation in traits among offspring